ELECTRIC FIELD BASED FABRICATION METHODS FOR MULTI-SCALE STRUCTURED SURFACES MASSACHUSETTS INSTITUTE OF TECIHNOLOGY BY AUG 15 2014 YOUNG SOO JOUNG LiBRA RIES B.S., MECHANICAL ENGINEERING, YONSEI UNIVERSITY, 2004 M.S., MECHANICAL & AEROSPACE ENGINEERING, SEOUL NATIONAL UNIVERSITY, 2006 SUBMITTED TO THE DEPARTMENT OF MECHANICAL ENGINEERING IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY IN MECHANICAL ENGINEERING AT THE MASSACHUSETTS ISTTITUTE OF TECHNOLOGY JUNE 2014 C MASSACHUSETTS INSTITUTE OF TECHNOLOGY, 2014. ALL RIGHTS RESERVED. Signature redacted SIGNATURE OF A UTHOR..................................................................... YOUNG SOO JOUNG DEPARTMENT OF MECHANICAL ENGINEERING MAY 21, 2014 Signature redacted CERTIFIED BY ............................. CULLEN R. BUIE ASSISTANT PROFESSOR, MECHANICAL ENGINEERING THESIS SUPERVISOR Signature redacted ACCEPTED BY ....... ....................... DAVID E. HARDT PROFESSOR, MECHANICAL ENGINEERING CHAIRMAN, DEPARTMENT COMMITTEE ON GRADUATE STUDENTS 1 2 ELECTRIC FIELD BASED FABRICATION METHODS FOR MULTI-SCALE STRUCTURED SURFACES BY YOUNG Soo JOUNG SUBMITTED TO THE DEPARTMENT OF MECHANICAL ENGINEERING ON MAY 23, 2014 IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY IN MECHANICAL ENGINEERING ABSTRACT Control of micro/nano scale surface structures and properties is crucial to developing novel functional materials. From an engineering point of view, the development of scalable and economical micro/nano-fabrication methods has been in high demand. In this dissertation, electrophoretic deposition (EPD) and breakdown anodization (BDA) are examined for their potential to produce multi-scale structured surfaces. EPD uses electrophoresis to deposit thin films of nanoparticles, dispersed in suspension, onto charged or porous substrates. Depending upon the dispersion stability, the surface roughness can be modulated in order to affect the resulting wettability. BDA can be utilized to alter surface features by employing instabilities during high voltage anodization, which lead to micro scale topography. Different microporous structures are generated depending on electric potential and electrolyte temperature during BDA. A hybrid method employing EPD and BDA results in hierarchical surface structures with both nano/micro scale features. In this work EPD and BDA are utilized for the development of superhydrophobic and superhydrophilic surfaces; sample applications include anti-wetting fabric, capillarity driven flow design, and critical heat flux enhancement. In many applications it is critical to understand how moving liquid water droplets will behave when they encounter these modified surfaces. We investigate drop impingement on porous thin films produced by BDA and EPD in order to understand the effects of surface structure and chemical properties on droplet dynamics. Using dimensional analysis we've discovered a novel dimensionless parameter, named the WashburnReynolds number, which can predict the droplet impingement modes. Intriguingly we've also discovered that under certain conditions drop impingement results in gas trapped in the spreading droplet, leading to the generation of aerosol above the droplet when the gas bubbles burst. The Washburn-Reynolds number also largely dictates the aerosol generation process. Our results inform the understanding of dynamic interactions between porous surfaces and liquid drops for applications ranging from droplet microfluidics to aerosol generators. In summary, EPD and BDA provide promising micro and nano-scale fabrication technologies with reasonable control of surface morphology and properties in a cost-effective and time-effective and scalable. Thesis Supervisor: Cullen R. Buie Title: Assistant Professor of Mechanical Engineering 3 4 Acknowledgements Five years ago, Professor Buie told me "let's work together." At that time, my wife was expecting our child and I had a good job in Korea. Without his early confirmation of support, I would not have decided to come to MIT for a Ph.D. I also remember the moment when he brought a celebration cake to our group meeting after I passed the doctoral qualification exam. He has always sincerely supported me throughout my Ph.D. program. I am most grateful to him for all his advice and support toward my successful achievement of my Ph.D. degree. I was happy to have Professor Daniel Blankschtein, Professor Gareth McKinley, and Professor Evelyn Wang as my thesis committee members. I gained deep knowledge of colloid and interface sciences from Professor Blankschtein. His unique explanation and brilliant understanding gave me new insights into my scientific findings. Professor McKinley has impressed me with his keen insight in fluid mechanics. His advice helped me to make firm bridges between my understandings of liquid transportation on porous media. Professor Wang showed me how a junior faculty member can successfully develop a new laboratory for new research areas. She was passionate about teaching students and managing her group, and I learned about effective collaboration while I worked with her group. I sincerely thank Professor Sang-Gook Kim for his teaching about micro/nano systems and comments on my research. I am also deeply thankful for the continuous support and advice from Professor Yoon Young Kim, who is my former adviser during my M.S. program. I must also thank Dr. Hyuk Chang, who is my former supervisor at Samsung Advanced Institute of Technology, for his concern and support for my future career. My research achievements have been earned with great collaborators. Bruno provided me with a new idea of developing a theoretical model of capillary flows. Dr. Kuang-Han demonstrated great use for our surfaces in pool boiling. I am grateful to Professor Shao-Horn, Dr. Byon, and Dongkyu Lee for help with the AFM and XRD measurements. I thank Professor Sangbae Kim and Dr. Sangok Seok for help with high speed imaging. I am grateful to Dr. Jim Bales and Sandra Lipnoski of MIT Edgerton Center for sharing their high speed camera. It was my pleasure to have smart and sincere interns and UROP students; especially Robert McMeeking worked very hard to make the progress of producing conductive hydrogel films. I was also happy to work with my other labmates: Dr. William Braff, Zhifei Ge, Andrew Jones, Alisha Schor, Naga Dingari, Qianru Wang, Laura Gilson, Dr. Paulo Garcia, Dr. Pei Zhang, and Dr. Jeffrey Moran. Their fruitful comments helped me to improve my research output. I especially enjoyed working with Soon-ju Choi toward developing practical applications of breakdown anodization. I must also thank all of my Korean friends at MIT for their help in my successful achievement of Ph.D. I was only able to attain my Ph.D. degree with the endless trust and support from my family. My parents, Mr. Chang-ho Joung and Mrs. Jun-sook Hong, and my siblings and their families, Nana, Hangu, Soo-bin, You-ree, Byung-gook, Ji-hyun, Sung-enu and Young-chan are always standing behind me. My mother-in-law, father-in-law, and my wife's sister, Mrs. Sun-nyu Lim, Mr. Hak-joo Kim and Taeun, filled my position and took care of my daughter and wife during my absence from Korea. Lastly, my wife, daughter and my soon-in-be son, Taejin, Yehwi and Woo-Ju, are all the reasons for overcoming any challenges in my life. Without the sacrifice and support of all my family members, I could not have reached this position. 5 6 Table of Contents Acknowledgements 5 Table of Contents 7 List of Figures 13 List of tables 29 Chapter 1. Introduction: Electrophoretic Deposition and Breakdown Anodization 31 1.1. Introductory remarks 32 1.2. Fundamental of electrophoretic deposition 35 1.2.1. Electrophoresis 35 1.2.2. Suspension stability 36 1.2.3. Stability ratio 37 Fundamental of breakdown anodization 39 1.3.1. Anodization process 40 1.3.2. Breakdown potential 41 1.3.3. Effect of electrolyte temperature on oxide films 42 Thesis objective and outline 43 1.3. 1.4. Chapter 2. Electrophoretic Deposition of Unstable Colloidal Suspensions for Superhydrophobic Surfaces 49 2.1. Introduction 50 2.2. Experimental 50 2.2.1. Suspension preparation 51 2.2.2. Surface characterization 53 Results and Discussion 53 2.3.1. Zeta potential and particle size 53 2.3. 2.3.2. Optical absorbance and stability ratio 54 2.3.3. Contact angle 59 61 2.3.4. Surface roughness 2.3.5. 2.4. 63 Mechanical durability 67 Conclusions Chapter 3. Anti-wetting Fabric Coatings Produced by Electrophoretic Deposition of Polymer Stabilized Hydrophobic Nanoparticles 7 69 3.1. Introduction 70 3.2. Experimental 72 3.2.1 72 3.3. 3.4. Preparation of materials and suspensions 3.2.2. Electrophoretic deposition for fabric coating 74 Result and discussion 75 3.3.1. Suspension Zeta potential 75 3.3.2. Wettability of coated fabric 77 3.3.3. Scanning Electron Microscopy 79 3.3.4. Color change test 80 3.3.5. Air permeability testing 81 3.3.6. Coating durability 83 3.3.7. Patterned anti-wetting fabric 84 Conclusions 86 Chapter 4. A Hybrid Method Employing Breakdown Anodization and Electrophoretic Deposition for Superhydrophilic Surfaces 89 4.1. Introduction 90 4.2. Theory 91 4.2.1. Capillary flow 91 4.2.2. Electrophoretic deposition 93 Experimental 94 4.3.1. Suspension and electrolyte preparation 94 4.3.2. Capillary rise measurement (CRM) 95 4.3.3. Characterization 96 Results and discussion 97 4.4.1. Surface morphology 97 4.42. Capillary pressure and hydraulic diffusivity 99 4.3. 4.4. 4.5. 4.4.3. Relationship between wetting properties and surface structure 103 4.4.4. Surface chemical composition and atomic structure 105 4.4.5. Relationship between the effective pore radius and surface roughness 109 4.4.6. Modification of capillary flows 111 Conclusions 113 8 Chapte r 5. Hybrid Electrophoretic Deposition with Anodization Process for Superhydrophilic Surfaces to Enhance Critical Heat Flux 115 5.1. Introduction 116 5.2. Experimental 117 5.2.1. Hybrid method of electrophoretic deposition of TiO 2 nanoparticles and 117 breakdown anodization of titanium plates 118 5.2.2. Capillary rise measurement 5.2.3. 118 Critical heat flux in pool-boiling 5.3. Results and discussion 119 5.4. Summary 124 Chapte r 6. Design of Capillary Flows with Functionally Graded Porous Films Controlled by Anodization Instability 125 6.1. Introduction 126 6.2. Theory 128 6.3. Experimental 136 6.3.1. Fabrication method 136 6.3.2. 6.4. 137 Sample characterization 137 Results and discussion 6.4.1. Case 1: Capillary flows with constant capillary properties and varying surface 141 width 6.4.2. Case 2: Capillary flows with constant surface width and varying capillary 144 properties 6.5. 152 Conclusions Chapte r 7. Scaling Laws for Drop Impingement on Thin Porous Media 153 7.1. Introduction 154 7.2. Experimental 155 7.2.1. Preparation of micro and nanoporous films 155 7.2.2. Four impingement modes on wettable porous films 157 Theory 158 7.3.1. Energy conservation of a cylindrical model of droplets 158 7.3.2. Kinetic energy 160 7.3.3. Gravitational energy 161 7.3. 9 7.4. 7.3.4. Droplet surface energy 161 7.3.5. Line dissipation 162 7.3.6. Viscous dissipation inside a droplet 163 7.3.7. Matrix potential 164 73.8. Viscous dissipation within the porous media 165 7.3.9. 167 Dimensionless energy equation with the capillary-Weber number Results and discussion 169 7.4.1. Simplified energy equation at high capillary-Weber number 169 7.4.2. Simplified energy equation at low capillary-Weber number 172 7.4.3. The Washburn-Reynolds number 176 7.4.4. Impingement mode transitions correlate with the Washburn-Reynolds number 177 7.5. Conclusions 180 Chapte r 8. Sparkling Droplets: Evidence of Aerosol Generation from Drop Impingement on Porous Media 181 8.1. Introduction 182 8.2. Theory 186 8.2.1. Measurements of hydraulic diffusivity 186 8.2.2. 187 8.3. -ydraulic diffusivity from the capillary rise measurement 8.2.3. Hydraulic diffusivity from Darcy's law 189 Experimental 190 8.3.1. Preparations and characterizations of porous media 190 8.3.2. Characteristics of typical soil and rainfall 194 8.3.3. Characteristics of droplets and bubbles with respect to drop heights 194 8.4. Results and discussion 197 8.5. Conclusions 202 Chapte r 9. Thesis Summary and Future Work 203 9.1. Thesis Summary 204 9.2. Future Research 207 9.2.1. Porous Thin Film Microfluidic Devices Designed by Breakdown Anodization 207 10 9.2.2. Conductive Hydrogel Produced by Electrophoretic Deposition at the Interface of Two Immiscible Liquids 211 9.2.3. Dispersion of Carbon Nanotubes in Aqueous Solutions of Ionic Surfactants 212 Bibliography 217 11 12 List of Figures Figure 2-1. Schematic illustration of the EPD cell. The cell consists of two titanium electrodes with a 15 mm of gap, a DC power supply (10 V applied), and a suspension consisting of hydrophobic SiO 2 particles dispersed in a mixture of 90% methanol and 10% DI water 52 by volum e... .................................................................................................... Figure 2-2. Characterization of PDMS coated SiO 2 particles as a function of pH. (a) Average zeta potential showing the isoelectric point (IEP) at pH 3. (b) Average agglomerate diameter, indicating smaller agglomerates at high pH. The error bars shown in (a) and (b) represent two-standard deviations of three measurements.................... 55 Figure 2-3. Change in suspension optical absorbance as a function of time for ten different pH suspensions with 0.1 g/L PDMS coated Si0 2 particle concentration. The inset shows absorbance curves for higher pH values (7.4 to 8.3). The low pH curves show fast ............ change of absorbance, which is attributed to rapid coagulation..... 56 Figure 2-4. Stability ratio as a function of suspension pH. Wabs and Wsize are the absorbance based and size based stability ratios, respectively, calculated by equations (1-8) and (1-11), respectively. The measured particle size of 304 nm was used as the initial particle size, ........... 57 Dh.,, to obtain Wsize with equation (1-11). ................................................. Figure 2-5. Contact angles on films deposited via EPD. (a) Static contact angle as a function of deposition time for four suspensions with varying pH. (b) Advancing and receding contact angles on the surfaces created at suspension pH 7.6. The inset in (b) shows rolloff angles calculated from the advancing and receding contact angles. The error bars shown in (a) represent two-standard deviations of twelve measurements at each point. The maximum contact angle was achieved at pH 7.6, corresponding to a stability ratio of Wab, = 8 and deposition time of 60 s..................................... ......... ................... 58 Figure 2-6. Patterns of EPD films produced by the pH 7.9 suspension with different deposition times. All images have the same scale bar in the top and most left picture. Deposition features grow with deposition time, resulting in a decrease in contact angle when deposition tim e exceeds 60 sec....................................................... .... ..................... 61 Figure 2-7. Contact angle as a function of RMS surface roughness. In surface roughness measurements, the measured area was 40x40 pm 2 and three point values were averaged 13 in each sample. The contact angle is roughly proportional to the surfaces roughness, as can be expected by the Cassie mode. ........................................................................ 62 Figure 2-8. Qualitative schematic of deposition behavior with respect to suspension stability. In unstable suspensions, particles flocculate and thick deposition layers cannot be formed due to sedimentation. In contrast, well dispersed (i.e. stable) particles form uniform films with low surface roughness. At moderate stability ('quasistable') EPD of particle agglomerates yields high surface roughness. ...................................... I.......................... 63 Figure 2-9. SEM images at two different magnifications of deposited films yielding maximum contact angles at pH 7.4 (a), 7.6 (b), 7.9 (c), and 8.3 (d). As pH increases the film become increasingly uniform, consistent with the stability measurements of Figure 2-4. ............ ............... . . . .......................................................................... .. ..................... 64 Figure 2-10. Characteristics of EPD films with suspensions including conductive epoxy. (a) Image of liquid water droplet on EPD modified surface with contact angle of 168*. (b) 40x40 pm2 AFM image of the deposition surface and SEM images [(c)-(e)] at three magnifications. In (c) Several ten micron scale features composed of flocculated SiO 2 particles were observed under the SEM. In image (d), micron scale porous structures are visible on the surface, and (e) shows ten nanometer scale SiO 2 particles mechanically connected by the epoxy additive. ............................................................................... 65 Figure 2-11. Contact angle on EPD surfaces obtained with epoxy (circles) and without epoxy (squares), after successive 'peel' tests. The static contact angle produced without epoxy decreases far faster due to the destruction of the deposition layer after each test. However, the surface obtained with epoxy maintains its superhydrophobicity after successive peel tests. .................................................................................................. 66 Figure 3-1. Schematic illustrations of the fabrication process suggested for anti-wetting fabric coating. (a) Step 1: multi-layered polymers are electrostatically deposited on the fabric. Step 2: nanoparticle are electrostatically deposited on the polymer layers of the fabric. Step 3: the polymer and nanoparticle assemblies are deposited by electrophoretic deposition (EPD). Step 4: heat treatment is conducted on the fabric for enhancing mechanical durability. (b) The EPD cell for Step 3 in (a). The cell consists of two stainless steel electrodes and a fabric sheet is wrapped around the cathode electrode. . 74 Figure 3-2. Zeta potentials of the polymers and the Si0 2 nanoparticles. (a) Zeta potentials of poly(sodium 4-styrenesulfonate) (PSS), poly(dimethyl-diammonium chloride) (PDDA), 14 , SiO 2 nanoparticles with respect to pH. (b) Digital image of the suspensions of SiO 2 polyvinylidene fluoride (PVDF), PDDA, and PSS. (c) The table summarizes the zeta potentials at pH 8.5. The zeta potential of non-treated polyester fiber was obtained from a reference.[163] The solvent for the Si0 2 nanoparticle and PVDF suspensions is composed of methanol 90 vol. % and water 10 vol. %. The others are deionized water. The concentrations are 8 g/L PSS and PDDA and 1 g/L Si0 2 and 0.5 g/L PVDF with and without 3.75x 10- g/L PDDA polymers. The error bars indicate + one standard deviation from three measurements.................................... 76 Figure 3-3. Images of water droplets on polyester fabrics modified by the fabrication process shown in Figure 3-1. Two different fabrics, which color red and yellow, were used to produce anti-wetting fabrics. Electrophoretic deposition was conducted for 90 sec in Step 3. (a) The modified fabric does not lose its original color but the droplets show high contact angles and low roll-off angles. Flexibility of the original fabric is maintained after modification while wettability changes dramatically from hydrophilic to superhydrophobic. (b) A water droplet is rapidly absorbed into the original polyester fabric. The scale bars indicate 1 mm. (c) The brown fabrics do not show significant difference in their colors before and after the modification. Water droplets show contact angles higher than 1500 and roll-off angles lower than 50......................................... 78 Figure 3-4. Static contact angle and roll-off angle of fabric produced with different deposition times for EPD. Contact angles were measured after heat treatment. Water droplets instantly absorbed into the original fabric resulting in and effective static contact angle of zero degrees. The static contact angle is 1360 on the fabric coated with polymer and thin layers of hydrophobic Si0 2 nanoparticles (after Step 2). Static contact angle is dramatically enhanced after EPD process (after Steps 4). The maximum contact angle of 1590 and the minimum roll-off angle of 20 are achieved at 90 sec deposition time. The error bars indicate one standard deviation from five measurements................ 79 Figure 3-5. SEM images of fabrics produced by electrophoretic deposition (EPD) with different deposition times. (a) Original polyester fabric has twill weave patterns. The inset shows a fabric fiber with Si0 2 nanoparticles deposited on the surface (after Step 2 in Figure 3-1). (b) After 30 sec of EPD, Si0 2 nanoparticle layers are observed on the fabric but do not fully cover the surface. (c) After 60 sec of EPD, the fabric fibers are uniformly and 15 densely coated with SiO 2 nanoparticles. This surface exhibits the maximum contact angle of 1570 and minimum roll-off angle less than 5 ................... 80 Figure 3-6. Color difference tests using RGB comparison and real images of the modified fabrics. (a) R G, and B*, which are the relative RGB values to the original fabric, with respect to deposition time in electrophoretic deposition (EPD). The error bars indicate one standard deviation from five measurements. (b) RGB values are obtained by using image software (Adobe Photoshop CS4) with captured images of fabric produced with varying EPD time. (c)-(e) Captured images of fabrics produced with three different fabrication processes and with the same EPD time of 90 sec: (c) Fabric produced with Steps 1-4 in Figure 3-1, (d) Fabric produced without Steps 1-2. (e) Fabric without polymeric stabilization of the particles in Step 3. ...................................................... 81 Figure 3-7. Water vapor transmission (WVT) rate tests for the original fabric (box symbols) and the anti-wetting fabric (circle symbols). The anti-wetting fabric was produced with 90 sec of EPD at 120 V. The WVT rate experiments were conducted with water temperatures ranging from 35 *C to 65 0C. The error bars indicate one standard deviation from four m easurements................................................................................................................. 83 Figure 3-8. Contact angles on the anti-wetting fabric with respect to test time. The durability test was conducted with a customized skin friction generator. The skin friction was induced by boundary layers on the fabric with flows circulated by a water pump. The flow speed was 10 rm/s. The contact angles were measured after the fabric was fully dried on a hot plate. Each data point is the average contact angle. The error bars indicate one standard deviation from five measurements.............................................................. 84 Figure 3-9. Patterned anti-wetting fabric using a patterned electrode. (a) An electrode with honeycomb shaped holes (left) was used to coat hydrophobic nanoparticles on polyester fabric (right). The fabric coated by the patterned electrode does not show significant color difference relative to the original fabric. (b)-(e) Water was continuously sprayed on two different fabrics, which are uniform anti-wetting fabric and patterned antiwetting fabric, and the resulting droplets on the surfaces were imaged with respect to time. (a) Droplets grow irregularly on the uniform anti-wetting fabric. (c) Droplets grow with regular patterns and ultimately show similar patterns as the electrode. (d) The pattern of coalescent droplets on the patterned anti-wetting fabric is similar to the dew 16 drops on green Oregon grape leaves shown in the inset of (e) photography by Brent Vanfossen. ..................................... Copyright and 86 Figure 4-1. Schematic illustration of the experimental setups for breakdown anodization (BDA) (a), and electrophoretic deposition (EPD) (b). The water temperature is controlled to maintain constant electrolyte conditions during BDA. TiO 2 nanoparticles are dispersed in acetic acid solvents with 1 wt% concentration for EPD. The solvents of both methods are stirred with magnetic bars..................................................................................... 95 Figure 4-2. Schematic illustration of the capillary rise measurement (CRM) system. Capillary height is recorded by a digital camera and estimated with customized image processing software (Labview). Evaporation from the sample surface is prevented by encapsulating the sample in a glass colum n.................................................................................... 96 Figure 4-3. SEM images of superhydrophilic surfaces fabricated by breakdown anodization (BDA) (a)-(c), electrophoretic deposition (EPD) (d)-(f), and the hybrid BDA/EPD method (g)(i). BDA was conducted at the electric potential 90 V for 30 min with an acidic electrolyte (pH 3) at 10 'C. EPD was conducted with the electric potential 30 V for 90 sec with 1 wt.% TiO 2 in an acetic acid solvent at ambient temperature. The surface produced by BDA consists of several entangled micro-porous structures while the surface deposited with TiO 2 nanoparticles by EPD shows uniformly distributed nanoporous structures. The hybrid BDA/EPD method yields hierarchical micro- and nanoporous structures................................................................................. ....... . ......... 98 Figure 4-4. Square of experimentally derived capillary rise height is linearly proportional to time, as predicted by Washburn's equation (a). The samples produced by BDA and EPD show different capillary pressures and spreading speeds (b). The sample produced by the hybrid BDA/EPD method shows slightly lower spreading speed than the sample produced by BDA at the same conditions. In BDA, electrolyte temperatures were maintained at 10 'C and electric potentials were supplied for 30 min. EPD was conducted at the electric potential 30 V for 90 sec. Hydraulic diffusivity, Dcap, of the surface can be obtained from the slope of each curve as shown in equation (4-2). The capillary pressure is calculated from equation (4-3) using the maximum rise height.. 100 Figure 4-5. Changes in capillary pressure (a) and spreading speed constant (b) with respect to electric potential and electrolyte temperature during BDA. Four different electric potentials (30 V, 60 V, 90 V, and 120 V) and four different electrolyte temperatures 17 (10 *C, 25 *C, 50 'C, and 75 "C) were used to obtain the surfaces. The error bars indicate one standard deviation resulting from four measurements.......................... 101 Figure 4-6. Sample SEM images of surfaces produced by BDA. At low electrolyte temperature and low electric potential (a), initiation of BDA is not facile; therefore, partially burst structures are observed on the surface produced at 30 V and 10 "C. When the electrolyte temperature was maintained at 75 "C during BDA, the highly interconnected porous structures shown in Figure 4-3a-c dissolve in the electrolyte, leaving smooth surfaces w ith large pores (b)......................................................................................................102 Figure 4-7. Capillary rise measurements of surfaces modified by EPD. EPD processes was conducted on bare titanium substrates (a) and on surfaces augmented by BDA (b). Pcap/P cap and Deap/D*cap are the ratios of capillary pressure and spreading speed constant before and after EPD, respectively. EPD was conducted at 30 V and the BDA conditions were 120 V, 30 min, and 25 'C. Porosity of the deposits by EPD was calculated using the deposition weight per unit area and the deposition height (c). The error bars indicate one standard deviation resulting from three measurements........ 104 Figure 4-8. XRD patterns of the surfaces produced by EPD, BDA, and the hybrid BDA/EPD method. A bare titanium plate and the TiO 2 deposit surface were used as control XRD substrates showing the spectra of titanium (a) and titanium dioxide (b). Surfaces produced by variations of EPD and BDA shows aspects of both Ti and TiO 2 spectra. Sediments obtained after BDA shows very weak peaks (j), which are indicative of amorphous titanium oxide [174, 175]. After heat treatment, the surface used in (i) and the sediments used in (j) show the spectra of anatase TiO 2 as shown in (k) and (1).... 108 Figure 4-9. Plots of the pore radius (a), the capillary pressure (Pcap) (b), and hydraulic diffusivity (Dcap) (c) with respect to surface roughness. The pore radius obtained by equation (4-4) with Pcap and Dcap shown in (b) and (c) is linearly proportional to the surface roughness (a). The box, circle, triangle, and diamond symbols indicate the electric potentials of 30 V, 60 V, 90 V, and 120 V, respectively, used in the BDA processes. The fill colors indicate the electrolyte temperature, 10 "C (white) and 25 "C (red). Morphological data was obtained with a profilometer (Measurement system: Tencor P-16 Surface Profilometer (TM)). The error bars indicate one standard deviation resulting from four m easurem ents. ......... ................................................................................................... 18 110 Figure 4-10. Schematic illustrations of electrode gap-profiles and electrode shapes used to demonstrate capillary transport modification with BDA and EPD. Case 1 was used as a reference with a constant gap-distance (10 mm) and a rectangular electrode shape (10 mm x 70 mm). In Cases 2 and 3 the electric potentials vary with gap distance between anode and cathode electrodes, with identical electrode cross sectional area as Case 1. In Case 3, TiO 2 nanoparticles were partially deposited on the surface produced by BDA using EPD. In Cases 4 and Case 5, triangular electrodes were used with the same electrode gap-distance as Case 1 (10 mm). .................................. 112 Figure 4-11. Square of measured capillary rise height with respect to time for superhydrophilic surfaces produced by the BDA and EPD methods presented in Figure 4-10. In the BDA processes, two different electrolyte temperatures (10 'C and 25 'C) and one electric potential (90 V) were used. In the EPD process, 30 V was supplied for 5 min to deposit TiO 2 nanoparticles. In Cases 2-4 the square of capillary height is nonlinear in time, unlike conventional capillary flows. With augmentation via EPD, the spreading speed can be discontinuously changed, as shown in Case 3. ....................... 113 Figure 5-1. SEM images of prepared surfaces; (a) superhydrophobic surface by EPD, (b) superhydrophilic surface by BDA, and (c) mixed wetting surface by the combined B D A /EPD process....................................................................................................... 120 Figure 5-2. Boiling behavior on the prepared surfaces. Images of the superhydrophobic surface of sample 1 at the saturation temperature (a), the boiling inception temperature (b), and the nucleate boiling region (c). Images of the superhydrophilic surface of sample 2 at the saturation temperature (d), the boiling inception temperature (e), and the nucleate boiling region (f). Images of the hybrid wetting surface of sample 3 at the saturation temperature (g), the boiling inception temperature (h), and in the nucleate boiling region .... .................. 12 3 (i). ........................................................................................................ Figure 6-1. Schematic illustration of the conceptual model for wettable porous films with varying cross sectional area. The porous film has the constant height, He, and varying half width, R. The porous film can be modeled as multi-layered capillary tubes vertically oriented in the capillary height, y. The number of the capillary tubes in x-direction and ydirection is varied according to the radius of a single capillary tube, re, and the width of R .. . ............................................................................................................................... 19 132 Figure 6-2. Schematic illustrations of control volumes depending on the shapes of wicking surfaces. The wicking surfaces can be conceptually modeled as vertically packed capillary tubes. (a) The control volume covers the whole wetted area when the capillary width increases with respect to the capillary height. (b) The wet area that has the same column width as the top wetting line is considered as the control volume when the wicking surface has decreasing width............................................. 136 Figure 6-3. Droplet spreading on the surface produced by breakdown anodization (BDA). BDA was conducted with 90 V for 30 min in pH 3.0 electrolytes at 25 *C. The droplet suspended from the needle is immediately separated into two parts after contacting to the surface, finally showing almost zero contact angles. The scale bars in the images indicate 1 mm length. ............................................................................. ..................... 138 Figure 6-4. SEM images of surfaces fabricated by breakdown anodization (BDA). BDA was conducted at the electric potential 120 V for 30 min with an acidic electrolyte (pH 3.0) at 10 'C. The surface shows highly irregular micro porous structures (a)-(b) that have submicron scale-like structures on the surface. The cross sectional area of the surface is composed of multi-layers of amorphous titanium oxide, which have micro-scale w icking channels (c)-(e) ............................................................................................... 139 Figure 6-5. Results of the capillary rise measurements with highly wettable porous films produced by breakdown anodization (BDA). As expected from Washburn's equation, the square of capillary height is linear to time at initial region of rising time. The maximum capillary diffusivity is 155 mm2 /s and the maximum capillary pressure is 1.75 kPa. These values are ten times and three times higher than the commercial wicking sheet composed of micron Si0 2 particles (200 tm thickness of 25 pm silica particles). Samples were produced by BDA with titanium plates (10 mm x 250 mm) in pH 3.0 electrolytes. The gap distance between anode and cathode electrodes was 10 mm. The error bars indicate one standard deviation resulting from four measurements. ........ 140 Figure 6-6. Shapes of the BDA surfaces and the chromatography papers (Genesee Scientific) used for the capillary rise measurements. Type A and B have linearly changed shape functions, and Type C and D have exponentially changed shape functions. All BDA processes had the same condition of electrolyte (pH 3.0 and 25 *C), electrode gap distance (10 mm), and electric potential (90 V for 30 min)...................... 142 20 Figure 6-7. Capillary flows on the BDA surfaces and the chromatography papers with the surface shapes shown in Figure 6-6 and Table 6-1. The symbols and the lines indicate the experimental data and the theoretical expectations calculated by equation (6-10). The gray small dots and solid-line show the reference capillary rise on a constant width surface.......................................................................................................................... 144 Figure 6-8. Capillary pressure (Pcap) and capillary diffusivity (Dap) with respect to surface roughness (Ra) and electric potential (V) of breakdown anodization (BDA) at the electrolyte temperature of 25 'C. The box, circle, triangle, and diamond symbols indicate the electric potentials of 30 V, 60 V, 90 V, and 120 V, respectively. The red and the white colors inside the symbols correspond to the capillary pressure and the capillary diffusivity, respectively. When the electrolyte temperature is 25 *C, the capillary pressure and the capillary diffusivity are linearly proportional to the surface roughness, which is linearly enhanced by the electric potential of BDA. The error bars indicate one standard deviation..................................... 145 Figure 6-9. Capillary pressure and surface roughness produced by breakdown anodization (BDA) with varying electrode gap distance, x. (a) Two parallel electrodes were used with varying the electrode gap distance (x) in BDA processes. (b) The capillary pressure and the surface roughness are inversely proportional to the electrode gap distance. The electric potential of 120 V and an electrolyte temperature of 25 0C were used to produce the BDA surfaces. The error bars indicate one standard deviation. .......................... 147 Figure 6-10. Electrode distance profiles used in breakdown anodization (BDA) processes. The constant gap distance profile was used to produce a reference surface. Four different gap profiles, which linearly change (a)-(b) and quadratically change (c)-(d), were used in BDA processes. All BDA processes had the same condition of electrolyte (pH 3.0 and 25 *C), electrode width (10 mm), and electric potential (120 V and 30 min). The left side electrodes (the red lines) in each image indicate the anode electrodes converted to highly wetting porous surfaces. The anode electrodes have different surface heights (H)....................................................................................... .............. .... .......... ... 14 8 Figure 6-11. Capillary flows on surfaces produced by breakdown anodization (BDA) with the electrode gap profiles shown in Figure 6-10 and Table 6-2. The solid lines and the symbols indicate the theoretical data and the experimental data, respectively. The solidcircle, solid-box, circle, and box symbols are obtained with the electrode configurations 21 shown in Figure 6-1 0a, (b), (c) and (d), respectively. BDA surfaces were produced with different electrode gap profiles but the same BDA conditions: 120 V for 30 min in pH 3.0 electrolyte at 25 0C. ............................................ 150 Figure 6-12. Surface roughness changes with respect to the non-dimensional height of surfaces produced by the electrode profiles shown in Figure 6-10 and Table 6-2. The surface height (H) shown in Figure 6-10 was used to obtain the non-dimensional capillary height in the x-axis. The surface roughness (Ra) is fimctionally varied according to the profiles of electrode gap distance. The symbols and the dashed-lines indicate the measured data and the predictive data, respectively. The predictive data were obtained from the relationship between the surface roughness and the electrode gap distance. 151 Figure 7-1. SEM images of as-prepared surfaces show micro-scale and nano-scale porous structures produced by breakdown anodization (BDA) (a) and electrophoretic deposition (EPD) (b), respectively [198]. The combined BDA/EPD process yields highly wetting micro/nano scale hierarchical porous structures...................... ............................... 156 Figure 7-2. Droplets show four different impingement modes depending on the WashburnReynolds number after impacting highly wetting porous surfaces. The WashburnReynolds number can be expressed as Rew = U0precos9/p, where U is the impact velocity, p is the liquid density, r, is the effective capillary radius, 0 is the surface contact angle, and p is the liquid viscosity. Mode-Al: compressing-oscillating, ModeA2: necking and spreading, Mode-B: spreading, and Mode-C: radial jetting and spreading. A high-speed camera (Photron) was used to record the behavior of liquid droplets on the surfaces. The scale bar in each image is 1 mm..................... 157 Figure 7-3. Schematic diagram of the cylindrical droplet impact model with time dependent radius, R(t), and height, H(t). The model cylindrical droplet has the same volume and impact velocity as the real droplet...................................... .... ........................................ . 160 Figure 7-4. Schematic diagram of capillary flow for the assumed cylindrical droplet geometry. The porous region can be considered as several cylindrical capillaries in parallel, each having a height, 2rc. The porous region has height, H,, and the flow velocity, vr, at a distance, R(t), from the center, assuming the ratio of the height of the porous region to the initial droplet diameter, He/D, is small. , is the sliding contact angle, the contact angle of the droplet on the top surface of the porous layer during spreading. ............. 166 22 Figure 7-5. Schematic illustrations of droplet spreading on highly wetting porous surfaces. At high capillary-Weber numbers (Wee > 103), the kinetic energy determines droplet expansion, while the contact line and viscous effects dissipate mechanical energy (a). At low capillary-Weber numbers (Wee < 10), the capillary energy governs droplet spreading, and viscous dissipation is most prominent inside the porous region (b)...................... 169 Figure 7-6. Theoretical and experimental droplet spreading for multiple liquids after impact on highly wetting porous surfaces. Solid lines represent spreading radii obtained from equation (7-9) while circles are experimental data. The impact velocities are selected such that Wec exceeds 10 3 . The table provides the surface and liquid properties and the dimensionless parameters of the energy equation for each data set. Viscous dissipation inside the porous layer, which is the column highlighted in gray in the table, is not considered when Wec > 103. Here we only considered impingement Mode-Al and Mode-B because the droplet mass is not conserved in Mode-A2 and Mode-C due to necking and jetting behavior, respectively. .................................. 174 Figure 7-7. Experimental results for droplet spreading at low capillary Weber number for the liquids shown in Table 7-1. The symbols and the black solid line indicate the experimental data when Wec < 100 and the theoretical data obtained from equation (7-13), respectively. Viscous dissipation inside the droplet and line dissipation, which are the columns highlighted in gray in the table, are not considered in the energy equation because Wec is low. The square of the dimensionless radius change is linearly proportional to the dimensionless time. The table provides the surface and liquid properties and the dimensionless parameters of the energy equation for each data set. The impingement modes are confined to Mode-Al and Mode-A2 because Rew < 0.2 when W ec < 100..................................... .......................... ..................... 175 Figure 7-8. Impingement mode transitions correlate with the Washburn-Reynolds number, as shown in the log-log plot of Rew vs. cos/Oh2 , where Oh is the Ohnesorge number and , 9 is the Young's contact angle of the surface. Dcap/u has the same form of cos/0h 2 where o is the kinematic viscosity of liquids used. Both dimensionless parameters can be used to distinguish wetting properties of the surfaces. Droplets fall from heights ranging from 0.003 m to 1 m prior to impact. Symbols: , LI, A and 0 indicate Mode-Al, Mode-A2, Mode-B, and Mode-C, respectively. The capital letter under each 23 vertical line indicates the surface name shown in Table 7-1. The inset shows the mode transitions in a narrow Dca,/lu region of 1 to 100. ........................................................ 179 Figure 8-1. Aerosol generation from droplets hitting soils and porous surfaces. a, The clay soil has irregularly rough surfaces and a droplet falls with an impact speed of 2.0 m/s. b, Tiny bubbles are formed under the droplet. The bubbles are inside the white circles in the image. c, Tiny water-jets are ejected from the droplet during impact. The white circles and arrows in the image highlight aerosols and jets ejected from the droplet. d-g, Highspeed images of the process of generating aerosol when a single bubble breaks inside the droplet. d, A bubble is trapped inside the droplet. e, The bubble breaks at the interface of water and air. f, A water pillar is jetted from the droplet. g, The water pillar breaks up into tiny droplets. h, Many simultaneous water-jets and aerosols are generated on a thin layer chromatography plate within a specific range of drop impact speeds.. 185 Figure 8-2. Bubble generation, growth, and breaking inside droplets hitting porous surfaces. a-c, High-speed images of bubble formation inside water-droplets on a thin layer chromatography plate with respect to impact velocity. Bubble size and distribution become uniform with impact velocity, resulting in simultaneous water-jets on the droplet surface. d-g, The schematic illustration of the process of water-jet generation. d, A droplet impacts a porous layer and the droplet radius is expanded. e, After it reaches the maximum radius, tiny bubbles are inflated at the interface of the droplet and the porous surface. f, The droplet height thins due to the absorption of water into the porous layer and the bubble size grows due to the inflation with air squeezed from the porous layer. g, Finally, the tops of the bubbles meet the interface of air and water, and they are ruptured, generating tiny water jets. The maximum number of bubbles inside the droplet increases with impact velocity. The scale bars in a-c indicate 1 mm........................... 186 Figure 8-3 Characteristics of droplets and bubbles generated after the droplets hit the three TLC plates. The hydraulic diffusivities of TLC-A, TLC-B and TLC-C are 9.5, 20.4, and 22.4 MM /s respectively. (a) The average bubble diameter does not change significantly by impact velocity U 2 but the average droplet film thickness is inversely proportional to U. (b) The maximum number of bubbles inside the droplet linearly increases with respect to U0 . (c) The time it takes to observe the first jet is affected by the surface wettability at low impact velocity but by the droplet film thickness at high impact velocity since the droplet film thickness governs the maximum bubble size at high 24 impact velocity. (d) The droplet film thickness h,,, after hitting the surfaces is the highest on TL C -B ................................................................................... ................. 196 Figure 8-4. Characterization of aerosol generation when droplets hit soils and porous surfaces. The x-axis indicates a modified Peclet number (Pe (U -Do)/Dcap), where U and D, are the impact velocity and the diameter of the droplet. The y-axis indicates the Weber number (We pD0 U 2/a), mainly varied by the impact velocity. We and Pe represent the impact condition and the surface property, respectively. The Da, values of the soils were measured by the capillary rise experiments. The red symbols indicate the observation of aerosol dispersion from the droplets at the corresponding We and Pe. The blue circle named "aerosol generation region" highlights the group of the data points where aerosols are generated. The yellow symbols inside the aerosol generation region show the data points of frenetic aerosol generation. Rainfalls on soils are placed on the upper part of the characterization map, and clay and sandy-clay soils have concentrated regions in the sparling zone. a-c, the reprehensive images of drop impingements in the regions classified with We........................... 199 Figure 9-1. Fabrication processes for micro/nano hierarchically structured surfaces and Scanning Electron Microscopy (SEM) images (a)-(f). For micro-porous structures, breakdown anodization (BDA) is used with different electric fields, (a)-(c). For nano porous structures, electrophoretic deposition (EPD) is used with different nanoparticles, (d)-(f). The hierachical micro- and nano-porous structures are produced by a series of BDA and EPD, (g)-(i). The electrolyte temperature and the nanoparticles affect the surface morphology and the surface energy, respectively .......................... 205 Figure 9-2. Functional surfaces produced by electro-fabrication. (a) Anti-wetting fabric with biomimicked coating-layers for water harvesting. Each fiber of the fabric was uniformly coated with hydrophobic silica nanoparticles, resulting in high static contact angle and low roll-off angle. The fabric maintains its original flexibility and breathability after the modification. The water droplets were growing with a regular pattern similar to tlhose on a real leaf during condensation. (b) Hierarchical surfaces for critical heat flux enhancement. The micropillars, created on a silicon wafer, were coated with hydrophilic silica nanoparticles, enhancing hydrophilicity. The coatings prevent vapor-blankets on 25 the heater surface and dramatically enhance critical heat flux and heat transfer efficiency in pool boiling [255]...................................... 206 Figure 9-3. Aerosol generation from droplets hitting porous surfaces. Frenzied aerosol dispersion can be observed in a specific region of the Weber number of droplets and the hydraulic diffusivity of surfaces......................................... 207 Figure 9-4 Schematic comparison between (a) a conventional microfluidic device and (b) the porous thin film m icrofluidic device. ................................................. ...................... 208 Figure 9-5. Fabrication process. A photoresist layer is patterned using photolithography. Breakdown anodization is conducted with the patterned Ti electrode in an acidic electrolyte .................................................................................................................... 209 Figure 9-6. Porous thin film microfluidic channels produced by BDA. (a)-(c) Patterned Ti sheets after BDA with different electric potentials. (d)-(f) Thin channels generated by BDA. .................... ...... . ........................................................................................................ 2 09 Figure 9-7. Wettable area and wicking speeds of different porous media with a one microliter sample of 1 g/L Rhodamine B in deionized water. The wetted area of the BDA surface is significantly larger due to its high permeability and low thickness. ....................... 210 Figure 9-8. Fabrication procedure for the carbon nanotube and hydrogel composite layer and the experiment setup for electrophoretic deposition with two immiscible liquids. Both water and oil are added to a container and the electrode assembly is placed into the container. The oil-water interface must be positioned at the middle of both electrodes. During an electric potential is applied to the electrodes, the crosslink solution is added to the top of the oil phase. After the polymerization, the oil phase is extracted and then the composite hydrogel film is gently lifted up. Experiment setup for electrophoretic deposition at the oil-water interface................................................ 212 Figure 9-9. Schematic illustrations of carbon nanotube and surfactant assemblies. The surfactant is composed of a hydrophilic head, which has the cross sectional area of A, (the head radius is R,) and a hydrophobic tail, which has length L,. (a) In reality, multiple surfactants are attached on the surface of the CNT. (b) In the simplified model, the surfactant heads are considered a single cylinder....................................................... 213 Figure 9-10. Total interaction potentials between two CNT-surfactant assemblies. The total interaction potential can be obtained by the summation of van der Waals force, electrostatic force, and osmotic pressure. The total interaction potentials of (a) and (b) 26 are obtained with the surfactant coverage ratio of 30 % and the tail stretching ratio of 100 %. Different bulk surfactant concentrations of CMC (8.7xO1 mol/L) and 2xCMC are used in (a) and (b) to compare the effect of micelle formation .................. 214 27 28 List of tables Table 4-1. Changes in capillary pressure for five superhydrophilic surfaces 30 days after fabrication. ...... . . ................................................................................................. ................. .......... 10 7 Table 5-1. Evaluation of prepared surfaces with BDA and/or EPD methods................................. 120 Table 5-2. CHF results for modified surfaces. Reference: an untreated titanium surface. Sample 1: a superhydrophobic surface produced with EPD. Sample 2: a superhydrophilic surface produced by BDA. Sample 3: a hybrid surface produced by the coupled BDA and EPD m ethod ......................................................................................................................... 12 1 Table 6-1. Shape functions of the BDA surfaces and the chromatography papers used to verify the alternation of capillary flows. As a reference, a BDA surface was produced with two parallel electrodes, which have a constant width of 10 mm and an electrode gap distance o f 10 mm ................................................................................................ ..................... 143 Table 6-2. Profile functions of electrode gap distance shown in Figure 6-10. In Type A and B, linearly changed gap distance was employed. Circular shape electrodes were used in Tape C and D. The reference BDA surface was produced with a constant electrode gap distance of 10 mm . ............................................................................. ......................... 149 Table 7-1. Liquid and surface properties used in the drop impingement experiments and numerical simulations. The properties of the glycerol and water mixture were obtained from the literature [208]. Highly wetting porous titania (TiO 2) surfaces were produced by an electrochemical fabrication method [198]. The porous titania surfaces showed wide variations in capillary pressure and spreading speed. The thin layer chromatography (TLC) plates were obtained from Sigma-Aldrich. The Gel-Blotting paper is a chromatography paper (grade 230) obtained from Genesee Scientific. The coffee filter is a commercial product made by Melitta USA, Inc. The symbols correspond to the symbols in the plot of drop impingement modes (Figure 7-8) ................. ... 158 Table 7-2. Simplified energy conservation equation and dimensionless droplet radius at different physical regimes. The line dissipation in equation (7-9) is considered dominant and the other dissipation terms are ignored to obtain simplified energy conservation equations as functions of common dimensionless parameters. The dimensionless droplet radius can be expressed as a function of the Reynolds number and another dimensionless 29 parameter; therefore, we can characterize the droplet radius change with respect to the relative influence of the Reynolds number................................................................... 171 Table 8-1. Characteristics and wetting properties of the media used to examine aerosol generation. .. . ......... ..... ................................................................................................................ 30 19 3 Chapter 1. Introduction: Electrophoretic Deposition and Breakdown Anodization Reproduced in part with permission from Young Soo Joung, Cullen R. Buie, "Electrophoretic Deposition of Unstable Colloidal Suspensions for Superhydrophobic Surfaces," Langmuir, 2011. 27(7): pp. 4156-4163. Copyright 2011 American Chemical Society. Young Soo Joung, Cullen R. Buie, "A Hybrid Method Employing Breakdown Anodization and Electrophoretic Deposition for Superhydrophilic Surfaces," JournalofPhysical Chemistry B, 2013, 117 (6), pp. 1714-1723. Copyright 2013 American Chemical Society. 31 1.1. Introductory remarks Over the past decade, superhydrophobic surfaces have been explored due to their promising applications in diverse areas such as self-cleaning surfaces [1, 2], drag reduction [3, 4], microfluidics [4, 5], heat transfer [6, 7], and anti-wetting textiles [8, 9]. To date, dozens of fabrication methods have been investigated to produce superhydrophobicity. The key properties to achieve superhydrophobicity are low surface energy and high surface roughness. The techniques to make high surface roughness can be classified as either top-down (machining) or bottom-top (molding) methods. Top-down methods transform a flat surface to a roughened surface. Lithography and etching [10-191, soft-lithography [3, 20-22], and micro-machining [23-25] can be classified as topdown. These methods typically provide precise control of surface roughness, and consequently, wettability. However, these processes tend to be complicated, toxic, or relatively expensive. In addition, top down techniques are often difficult to scale to large areas (i.e. order m 2 ). Alternatively, bottom-up approaches add materials to a flat substrate in order to increase surface roughness. Coating methods such as dip-coating [26-28], layer-by-layer [29-32], sol-gel processes [8, 33], chemical vapor deposition electrohydrodynainics [6, 7, 34-36], electrochemical deposition [37-41], and [42] are considered bottom-up techniques. Advantages of bottom-up approaches include cost, and scalability but they often suffer from reduced control of surface roughness. Methods to reduce surface energy can be categorized in two ways; those that use lowsurface-energy materials during the roughening process [42-48], and those that reduce surface energy after surface roughening such as via chemical vapor deposition [19, 20, 49-52], or the application of self-assembled monolayers [16, 39, 53, 54]. Regardless of the type of surface roughening process, most fabrication methods use additional surface treatments to reduce surface 32 energy since the raw materials used for the fabrication rarely have sufficiently low surface energy. This tends to add additional complexity to the fabrication process. In addition to superhydrophobic surfaces, superhydrophilic surfaces offer great opportunities to provide unique surface functionality for practical applications such as anti-fogging glass, heat sinks in pool boiling, heat pipes, evaporators, fuel spreaders, and micro-fluidic devices [55-59]. Classically, superhydrophilic surfaces have been defined as surfaces with contact angles less than five degrees [60]. However, this criterion is not sufficient to precisely characterize superhydrophilicity since superhydrophilic surfaces can show diverse behaviors in the above applications, even though all have similar static contact angles. Indeed, a definition of superhydrophilic surfaces has remained elusive [61]. Therefore, appropriate parameters to characterize superhydrophilic surfaces and fabrication methods to control those parameters are necessary. To realize superhydrophilicity two key features, highly porous (rough) structures and high surface energy, are necessary [60]. From Wenzel's equation [62], it is well known that contact angles on a surface decrease with increasing surface roughness when the flat surface has a static contact angle less than ninety degrees. Thus, it is necessary to achieve highly rough structures with nanomaterials or micro-fabrication techniques to produce superhydrophilic surfaces [58, 63-67]. In some cases, additional surface modifications with high surface energy materials are necessary [41]. In most studies of superhydrophilic surfaces, the surfaces show nearly zero contact angle, but they are rarely evaluated in terms of transport properties such as capillary pressure or spreading speed. Given the importance of transport properties it is impossible to distinguish between superhydrophilic surfaces utilizing contact angle alone. Manufacturing demands for superhydrophobic and superhydrophilic surfaces include process simplicity, low manufacturing cost, environmental compatibility (i.e. non-toxic), scalability, and 33 potential for mass production. With these considerations, this thesis presents electrophoretic deposition (EPD) and breakdown anodization (BDA) as potential tools to produce superhydrophobic and superhydrophilic surfaces. EPD employs electrophoresis of charged particles in dielectric solvents to created dense porous films and structures [68]. When a sufficient electric field is supplied to a colloidal suspension, charged particles are attracted to and deposit upon the oppositely charged electrode. Among many applications, EPD has been investigated to fabricate microscale and nanoscale structures [69, 70]. EPD has also been explored to develop novel electrodes and catalyst layers for electrochemical systems [71, 72], since EPD is considered as an effective technique to control porosity, surface area, and density of porous films. Superhydrophobic surfaces, composed of coated SiO 2 nanoparticles, have been produced using electrophoretic deposition (EPD) [73]. In this study low surface energy PDMS (Polydimethylsiloxane) coated silicon dioxide (SiO 2) nanoparticles were employed while controlling suspension stability to produce highly rough surfaces. To produce superhydrophilic surfaces, we can use the same mechanism to achieve high porosity/roughness but while using high surface energy particles. TiO 2 nanoparticles or nanotubes have been used to achieve high surface energy [63, 67, 74]. However, it is challenging to simultaneously obtain high capillary pressure and fast spreading speed due to high viscous resistance in small nano-structures, even though they show contact angles near zero. In this work, we suggest a novel fabrication method to produce superhydrophilic surfaces with high capillary pressure and fast spreading speed. We show that the method can be utilized to control capillary pressure and spreading speed independently. EPD is utilized to obtain nano-structures composed of TiO 2 nanoparticles, while breakdown anodization method (BDA) was employed to produce micro-porous structures. During anodization, the oxidization film thickness linearly increases with respect to applied electric potential [75]. However, when the electric potential is higher than a critical value, the film thickness abruptly increases, resulting in a number of micro- 34 scale irregularities. The critical electric potential is called the breakdown point [76, 77]. At electric potentials exceeding the breakdown point, micro-scale porous structures are created on the anode surface. 1.2. 1.2.1. Fundamental of electrophoretic deposition Electrophoresis When the surface of a particle in an electrolyte is electrically charged, the particle has an electrophoretic mobility p. Under an applied electric field, E, the charged particle moves towards an oppositely charged electrode with the velocity, v, expressed by, v = pE The mobility, p, is a function of zeta potential, (1-1) , permittivity, , and viscosity, , of the fluid as is shown in Henry's equation [78], (1-2) 317 This equation assumes spherically shaped particle with small r/AD, where AD- is the Debye-HUckel length and r is particle radius [78].Particles transported to the electrode agglomerate on the surface of the electrode if the electric field is sufficiently high to induce deposition [79]. 35 1.2.2. Suspension stability In EPD, suspension stability is critical since the morphology of the deposition layer is significantly affected by particle flocculation. A stable suspension results in well-dispersed particles, devoid of serious flocculation. In contrast, fast particle sedimentation is observed in unstable suspensions due to particle agglomeration. Interfacial forces between particles determine suspension stability. Two opposing forces are induced between particles in close proximity. The attraction force is commonly known as the van der Waals force and the repulsive force is due to the electrical double layer. The net interaction potential, D,,t, is the summation of the attractive potential, (DA, and the repulsive potential, (DR, between two particles. Assuming spherical particles of identical size, the interaction potential can be expressed as [80], DA + R=64kB ke A 2 12vd (1-3) where A is the Hamaker constant, d the distance between particles, kB the Boltzmann constant, T the absolute temperature, and n,, the bulk ionic concentration expressed as the number of ions per cubic meter. ;o is a function of the surface potential, defined as [80] ii, e zo 12 kBT -I 2 4; ezwIol kjT+ (1-4) where c is the elementary electric charge, and z the valence number. Surface potential is directly proportional to the zeta potential such that we can consider the electric repulsion a function of the 36 zeta potential. To evaluate suspension stability experimentally, the stability ratio, W, is employed and expressed by [811 net W K Kr KS 2 fo . oo e ^D S 0 Here, Kr (skBT oo e -ds is the rate constant for rapid coagulation, (1-5) kBT 2 K3 , the rate constant for slow coagulation, and s is the ratio of the particle radius to the distance between two-particle centers. Equation (1-5) assumes that fast coagulation occurs when the attraction force dominates and electric double layer repulsion is negligible. From equations (1-2) and (1-5), it is notable that the zeta potential, which can be determined experimentally, influences both deposition rate and stability. In general, higher zeta potential leads to higher deposition rate and improved stability. The effects of pH [82], ionic concentration [83], surfactants [84], and solvent composition [85] on suspension stability have been investigated by others. In this work, we chose to vary stability by varying suspension pH at a specified ionic concentration, since the zeta potential is a strong function of pH. 1.2.3. Stability ratio Suspension stability can be quantitatively evaluated from the change of optical absorbance with respect to time [86, 87]. A stable suspension shows moderate change of absorbance due to slow flocculation and sedimentation. In contrast, unstable suspensions feature fast sedimentation, attributable to the low net interaction potential between particles, as shown in the equation (1-3). 37 This results in a steep gradient in absorbance versus time. Change of absorbance, a, versus time is directly proportional to the initial rate constant, ,, for coagulation when time, t, is small, and can be expressed by [86, 87] KKNO2 2 (1+INt)2 da dt (1-6) Here No is the initial number of particles, A, the wavelength of incident light, and K, is a proportionality constant. Experimental stability ratio can be acquired by dividing the maximum rate of change of absorbance by the rate at a particular pH, as in the following equation, _[ dcd dtL l Kc afbs(17 S dt (-7 H As an alternative, changes in particle size can be measured using dynamic light scattering (DLS) to assess stability [83, 88]. The average hydrodynamic particle diameter, Dh, can be calculated by the Stokes-Einstein equation, D= kBT B 3,TriD (1-8) In equation (1-8), D is the average translational diffusion coefficient of colloidal particles in dilute suspension, which can be determined by the temporal evolution of intensity fluctuations in dynamic light scattering measurements. The change of particle size, Dh, with respect to time is a function of 38 the initial aggregation rate constant K and the initial particle concentration, Co, when time, t, is small, as h 'ArCo dt- (1-9) where, 8 is a constant that depends on scattering angle and material properties of particles. Noting the presence of coagulation rate in equation (1-7) and equation (1-9), the experimental stability ratio can be expressed in terms of the ratio of the fast coagulation rate to the slow coagulation rate, W - = (dDh I dt)max (dDh /dt)pH If we know the initial particle size, Dhj, and the maximum particle size, Dh..'ax at a specific time t, the stability ratio based on size, Wize, can be calculated as sD -h.i The size based stability ratio, Wsze, calculated by equation (1-11) should be comparable with the absorbance stability ratio, Wabs, obtained through the absorbance measurement as in equation (1-7). Notably, the initial particle size can be estimated by comparing both stability ratios. 1.3. Fundamental of breakdown anodization 39 1.3.1. Anodization process On the Pourbaix diagram of titanium-water systems [89], cathode and anode potentials during breakdown anodization correspond to immunity and passivity regimes, respectively. The chemical reaction processes on the anode and the cathode electrodes we consider are. as follows: <Anode> (1-12) Electrolysis: 2H 20 -+02() + 4H + 4e Corrosion: Ti -+ Ti43 + 3 Oxidation: Ti+ 2H 2 O(0 Oxidation: Ti +02 -+ -+ (1-13) Ti02+ 4H + 4e- (1-14) TiO2 (1-15) Dissolution: TiO 2 -x+ H 2 0(0 1-+ TiO 26xH 2 O(s) (1-16) Dissolution: TiO 2 + H 2 0 + H+ -+ Ti(OH) 3 (1-17) <Cathode> Electrolysis: 4H2 0+ 4e Electrodeposition:Ti + 3e 2 H2(g) + 40HW + (1-18) Ti (1-19) Electrolysis (1-12) and dissolution (1-16) are continuously observed during the anodization process. Corrosion and electrodeposition can be expected in the initial stage of the anodization process but these processes quickly terminate because the oxide layer prevents further corrosion (1-13). At 40 steady state, the electrolysis, oxidation, and dissolution processes continue. Therefore, the overall anodization reaction in aqueous electrolytes can be expressed as '/ 2Ti+ 2H 2 0(0 '/2Ti02+ 2 H2(g)+ 2O2(g) (1-20) considering both oxidation and electrolysis processes. Given this, two dissolution processes (1-16) and (1-17) can be taken into account on the anode electrode. Through the dissolution process (1-16), hydrated titanium oxide sedimentations are observed in the electrolyte after the anodization process. Then, the increase of electrolyte pH during the anodization process can be explained by the dissolution process (1-17). 1.3.2. Breakdown potential The general breakdown phenomenon was theoretically investigated by Sato [90]. He suggested that the surface pressure, P, of the electrode during the anodization process can be estimated by the summation of the electrostriction effect and the interfacial tension effect as follows: P=Pe+ 0 87r ha (1-21) with ambient pressure P0 , electric field E, surface tension of the anodic oxide film y, and the anodic oxide film thickness ha. When an electric field is applied to dielectric materials, the electrostriction stress is induced due to the effect of deformation on permittivity [91]. When the surface pressure is higher than the critical compressive stress of the film ao, the breakdown process takes place during 41 anodization. If surface tension effects are considered, the electrochemical properties of the electrolyte and electrode must be taken into account to estimate the breakdown conditions because ion adsorption under the film surface influences surface tension. The breakdown electric field, Ec, on the oxide layer can be expressed as a function of the electric potential, E= (# -#fz)/ha where # is the electrode potential and o is the equilibrium potential at the interface between the oxide layer and the electrolyte. Finally, when breakdown is initiated, the electrode potential, 0*, can be expressed by [90]. do* dIna 8xkBTPa -87ro -e (6+1)E (1-22) 1-22, where a is the anion activity in the electrolyte and pa is the anion adsorption density. From equation (1-22), we expect that high temperature electrolytes and high anion adsorption under the film surface lower the critical potential. 1.3.3. Effect of electrolyte temperature on oxide films For identical electrolytes, the maximum oxide film thickness is governed mainly by the electrolyte temperature and electric potential during the anodization process. The dissolution process (1-16) expresses the dissolution reaction governing the total oxide film thickness. This reaction is the hydration of the titanium oxide, which is soluble in acidic solutions. Electrolyte temperature affects the rate of this hydration step. In other studies, it has been generally accepted that with increasing electrolyte temperature, the hydration rate also increases [92, 93]. This behavior can be verified by investigating the oxide film thickness with respect to electrolyte temperature. The 42 oxide film thickness can be measured using the capacitance, C, of the oxide layer. The inverse capacitance, C is proportional to the oxide film thickness [94]. An empirical relationship between the capacitance and electrolyte temperature was suggested by Bockris et al. [95]: _A C7 1 AH -l -RT (1 -2 3 ) dIn d(C;) l-dt I10.5 kJ/mol where and the R isactivation the universal energy gas constant. AH 1From equation (1-23) we find that the rate at which the oxide film thickness increases is inversely proportional to the electrolyte temperature. 1.4. Thesis objective and outline The first objective of this thesis is to develop new fabrication methods employing electrophoretic deposition (EPD) and breakdown anodization (BDA) for producing micro and nano structures with various surface-functionality. The second objective is to understand liquid transport and droplet spreading on micro and nano porous surfaces produced by EPD, BDA, and combinations thereof. The third objective is to apply the electro-fabrication methods and the liquid transport properties to real world applications such as superhydrophobic and superhydrophilic surfaces, anti-wetting fabric, capillary flow devices, and anti-biofouling surfaces. The following paragraphs outline the organization of this thesis. In Chapter 2, a new method to fabricate superhydrophobic surfaces using electrophoretic deposition (EPD) is presented. Low surface energy materials with high surface roughness are achieved using EPD of unstable hydrophobic SiO2 particles suspensions. The effect of suspension stability on surface roughness is quantitatively explored with optical absorbance measurements (to 43 determine suspension stability) and atomic force microscopy (to measure surface roughness). Varying suspension pH modulates suspension stability. Contrary to most applications of EPD, superhydrophobic surfaces favor mildly unstable suspensions since they result in high surface roughness. Particle agglomerates formed in unstable suspensions lead to highly irregular films after EPD. After only one minute of EPD we obtain surfaces with low contact angle hysteresis and static contact angles exceeding 1600. This thesis also presents a technique to enhance the mechanical durability of the superhydrophobic surfaces via co-deposition of a polymeric binder added to the suspension prior to EPD. In Chapter 3, electrophoretic deposition is used for fast modification of polyester fabric with silica nanoparticles and polymer networks for highly durable coatings To date, electrophoretic deposition (EPD) has been utilized on electrically conductive substrates due to its dependence on electrical sources. Even though EPD on non-conductive materials such as ion conductive polymers has been attempted, weak adhesion, cracks, and irregular deposition have remained drawbacks [96, 97]. To resolve these problems, thin polymer layers were coated first on a polyester fabric using an electrostatic self-assembly technique. Then, the silica nanoparticles were uniformly seeded on the polymers. Finally, polymer stabilized silica nanoparticles were deposited by EPD on the silica nanoparticles, followed by heat treatment. As a result, the modified polyester fabric shows high static contact angle and low contact angle hysteresis, while keeping its original color, flexibility, and breathability. In Chapter 4, a new fabrication method is presented for superhydrophilic surfaces with high capillary pressure and fast spreading speed. The fabrication method consists of electrophoretic deposition (EPD) and breakdown anodization (BDA). Nanopores and micropores were produced on titanium plates by EPD and BDA, respectively. In EPD, TiO 2 nanoparticles were used to enhance the surface energy and create nano-porous structures; while BDA was employed to produce micro- 44 porous structures. Capillary rise measurements (CRM) were utilized to characterize superhydrophilic surfaces in terms of capillary pressure and spreading speed. From CRM, it was revealed that micro-porous structures play a dominant role in determining transport properties, and nano-porous structures affect local wettability without significantly reducing spreading speed. By combining BDA and EPD into a hybrid method, dual-scale (nano and micro) porous structures were produced on titanium plates. The methods presented offer the potential to vary the transport characteristics of superhydrophilic surfaces by altering the nano-scale and micro-scale features independently. As an example, surfaces with unconventional capillary flows were produced by the hybrid method. This method provides additional opportunities to investigate wetting phenomena, while offering a potentially low cost process for industrial applications. In Chapter 5, superhydrophilic surfaces with hydrophobic layers are used to enhance critical heat flux (CHF) and to reduce boiling inception temperatures (BIT). The novel surfaces were fabricated by a hybrid electrophoretic deposition (EPD) method coupled with break down anodization (BDA). With the BDA process, microporous superhydrophilic surfaces were created on titanium substrates. Subsequently, nanoporous hydrophobic particles were deposited with EPD on the superhydrophilic surfaces. The hydrophobic layers provide numerous nucleation sites, lowering BIT while the superhydrophilic layers prevent film boiling, resulting in increased CHF. The resulting surfaces exhibit higher CHF with lower BIT than untreated titanium surfaces. In Chapter 6, breakdown anodization (BDA) is employed to present various capillary flows that can be expressed as functions of capillary height. Capillary transport properties can be controlled by varying the electric field and electrolyte temperature. Furthermore, they can be expressed as functions of capillary height when customized electric fields are used in BDA. To predict capillary flows on BDA surfaces, a conceptual model of highly wettable porous films is developed, the model assumes the porous films consist of multiple layers of capillary tubes oriented 45 in the flow direction. From the model, a general capillary flow equation of motion was derived in terms of capillary pressure and capillary diffusivity, which can be expressed as functions of capillary height. The theoretical model was verified by comparison with experimental capillary flows and showed good agreement. From the investigation of the surface morphologies, it is found that the surface structures were also functionally graded with respect to the capillary height. The fabrication method and the theoretical model suggested offer novel design methodologies for devices requiring passive control over liquid propagation speeds. In Chapter 7, drop impingement is investigated on highly wetting porous films and paper. Experiments reveal previously unexplored impingement modes on porous surfaces designated as necking, spreading and jetting. Dimensional analysis yields a new non-dimensional parameter, denoted the Washburn-Reynolds number, relating droplet kinetic energy and surface energy. The impingement modes correlate with Washburn-Reynolds number variations spanning four orders of magnitude and a corresponding energy conservation analysis for droplet spreading shows good agreement with the experimental results. The simple scaling laws presented inform the investigation of dynamic interactions between porous surfaces and liquid drops. In Chapter 8, it is shown that artificial aerosol particles can be generated on porous surfaces. Aerosols have their significant impact on the environmental and global warming. To date, bubbles breaking on the sea and fossil fuel combustion have been considered the main origins of aerosol dispersion. However, this thesis reports that rainfall hitting soil can also generate aerosol. Use of high-speed imaging technologies provides visual evidence of aerosol generation. Interesting droplet behaviors are observed when simulated rainfall impinges upon engineered porous surfaces. Within a specific range of impact velocity, we observe frenzied ejection of tiny droplets, which produce aerosol clouds above the surface. Knowledge of surface properties and impact conditions can be used to predict when frenzied aerosol generation will occur. 46 In Chapter 9, we will show on-going efforts to develop additional applications using electric field based fabrication methods. First, highly wettable thin porous films, produced by breakdown anodization, are utilized to produce thin porous film based microfluidic devices. Second, conductive hydrogel membranes are produced by electrophoretic deposition of carbon nanotubes at the oil/water interface. Third, dispersion of carbon nanotubes in aqueous solutions is investigated to understand the role of surfactants on the dispersion. 47 48 Chapter 2. Electrophoretic Deposition of Unstable Colloidal Suspensions for Superhydrophobic Surfaces Reproduced in part with permission from Young Soo Joung, Cullen R. Buie, "Electrophoretic Deposition of Unstable Colloidal Suspensions for Superhydrophobic Surfaces," Langmuir, 2011. 27(7): pp. 4156-4163. Copyright 2011 American Chemical Society. 49 2.1. Introduction EPD is a well-established process, but wettability of structures fabricated with EPD has largely been overlooked. In one particular study, the wettability of thin films produced by EPD with titanate nanotubes was investigated [74]. In their work, the surface of the titanate deposition layer was switched from superhydrophilic to superhydrophobic after a surface modification with 1H,lH,2H,2H-perfluorooctyltriethoxysilane. The porous structure of the titanate deposition layer was considered the primary factor to produce superhydrophobicity, but EPD itself was not investigated as a tool to control wettability. Recently, Ogihara et al. demonstrated the possibility of using EPD to fabricate superhydrophobic surfaces [98]. Several hydrophobic particles including carbon black, activated carbon, vapor-grown carbon nanofibers, titanium dioxide, beta-type copper phthalocyanine, and phthalocyanine green were used to produce superhydrophobic surfaces. However, they did not explore mechanisms to control wettability with EPD or address the relatively weak adhesion of EPD surfaces. The purpose of this work is to illuminate how EPD can be utilized to control surface roughness and achieve superhydrophobicity. Prior EPD studies have used suspension stability, the electric field, and deposition time as variables to control surface roughness of deposited films [99, 100]. This work is a significant extension beyond the study of Ogihara et al. as we elucidate the role of suspension stability and deposition time to enhance surface roughness for the purposes of antiwetting. The effect of colloid stability on surface wettability is explored experimentally, resulting in superhydrophobic surfaces with static contact angles exceeding 1600. 2.2. Experimental 50 2.2.1. Suspension preparation For all experiments in this study, hydrophobic SiO 2 particles (polydimethylsiloxane (PDMS) coated, average particle diameter 14 nm [PlasmaChem, Berlin, Germany]) were used without additional purification or modification. A mixture of 90% methanol (ACS Reagent, Baker analyzed) and deionized (DI) water (by volume) was used as the solvent. The PDMS coated SiO 2 particles are hydrophobic, making dispersion in aqueous solvents impractical. Therefore, non-aqueous solvents were used and methanol was chosen since its refractive index (n = 1.33 at 25 *C) is similar to water. However, we note that our technique to create superhydrophobic surfaces is not limited to PDMS modified hydrophobic SiO 2 particles and methanol based solvents as presented here. We have also successfully produced superhydrophobic surfaces with both hydrophobic TiO 2 particles and SiO 2 particles modified by octylsilane (unpublished results). For our colloid stability, zeta potential, and particle size measurements we use particle concentrations of 0.1 g/L. Following initial dispersion, potassium nitrate (ACS reagent, > 99 %, Sigma-Aldrich) was added to the suspension in order to adjust the salt concentration to ~10-6 M. The purpose of the salt is to vary the gradient of the zeta potential curve [86]. After 5 minutes of sonication (all sonication was conducted with the amplitude 0.25 pm/mL, Sonicator 400, Qsonica, LLC.), the SiO 2 particles were well dispersed in the suspension. Next, the suspension pH was adjusted with acid (Nitric acid, 70 % ACS reagent, SigmaAldrich) or base (Potassium hydroxide, 45 wt.% solution in water, Sigma-Aldrich). Ten different pHs; 3.0, 4.0, 5.0, 6.0, 7.0, 7.4, 7.6, 7.9, 8.0, and 8.3, were prepared for the measurements. Five minutes after the pH was adjusted, the suspension was sonicated again for 5 min. After the suspension settled for 30 s, zeta potential and particle sizes were measured with a zeta potential meter (Zetasizer Nano ZS, Malvern Instruments, Inc.) and absorbance vs. time was recorded with a spectrophotometer (Wavelength 450 nm, Spectrophotometer 51 UV-1800, Shimadzu). Each measurement at a given pH was conducted three times with independently prepared suspensions in order to acquire average values. The suspensions used for EPD were prepared using the procedure indicated above except that the SiO 2 particle concentration was 1 g/L. Figure 2-1 shows a schematic of the EPD system. Two titanium plates (Purity > 99.6 %, Annealed, Goodfellow Corporation), with identical dimensions of thickness (0.5 mm), width (10 mm), and length (50 mm), were used as working and counter electrodes. In each experiment 10 V was applied between two electrodes separated by 15 mm. The EPD process was conducted at ambient temperature and without mechanical stirring. Deposition times were 30 s, 60 s, 90 s, 120 s, and 180 s. After completing EPD the sample was dried in ambient temperature air. DC Power Supply Substrate Particle Suspension Counter electrode <- Figure 2-1. Schematic illustration of the EPD cell. The cell consists of two titanium electrodes with a 15 mm of gap, a DC power supply (10 V applied), and a suspension consisting of hydrophobic SiO 2 particles dispersed in a mixture of 90% methanol and 10% DI water by volume. 52 To improve the adhesion of deposits, two part conductive epoxy (CW2400, Chemtronics) was added into the suspension. After the 1 g/L SiO 2 suspension was prepared with the same way mentioned above, the concentration 10 g/L of each epoxy was separately mixed into the suspension with 5 min sonication. After mixing both epoxies, the EPD process was instantly conducted. A stirrer (7 x 7 in ceramic top plate, 1xO.5 in Teflon magnetic stirring bar, VWR) was used at the rotating speed of 1000 rpm during the EPD process in order to maintain well dispersed epoxy in the suspension. The electric potential was 30 V/cm and the deposition time was 10 min. Other conditions for the EPD process were the same to the case of the non-epoxy suspensions. 2.2.2. Surface characterization The morphology of deposited films was characterized with a scanning electron microscope (SEM, JEOL 6320FV Field-Emission High-resolution SEM). Surface roughness was measured by atomic force microscopy (AFM, DI Nanoscope) at three distinct points on each sample. The area scanned for AFM measurements was 40 x 40 pm and the average root mean square (RMS) surface roughness was calculated using commercial software provided by the AFM manufacturer. A goniometer (Kyowa, DM-CE1) was used to dispense and image a 3 pL drop of DI water at four different points on each sample. Static contact angles (CA) were calculated using the tangential curvefitting method. 2.3. 2.3.1. Results and Discussion Zeta potential and particle size 53 The first suspension property measured was zeta potential since it is the key characteristic of a colloidal suspension. Zeta potential measurements were conducted at ten pH values: 3.0, 4.0, 5.0, 6.0, 7.0, 7.4, 7.6, 7.9, 8.0, and 8.3. Figure 2-2a shows zeta potential as a function of suspension pH. The isoelectric point (IEP) is roughly pH 3 for the PDMS coated SiO 2 particles, and zeta potential generally decreases with increasing pH. From equations (1-4) and (1-5), one would expect that particles in lower pH suspensions (< pH 7) show lower stability ratios. In addition, the deposition rate during EPD should be higher in the basic region than the acidic region due to increased zeta potential. It is worth noting that the maximum absolute value of zeta potential measured occurs at pH 8. Further, zeta potential directly affects the size of particle agglomerates in the suspension. Figure 2-2b shows the average particle diameter as a function of pH in the suspension after sonication. As expected, particle agglomerate size decreases with higher absolute value of zeta potential due to electric double layer repulsion. Particle size at pH 8.3 was larger than pH 8.0, indicating a slightly reduced stability ratio. 2.3.2. Optical absorbance and stability ratio Optical absorbance as a function of time was measured with the suspensions prepared in the zeta potential and size measurements. Figure 2-3 shows the change of absorbance for each pH. A moving average filter was used to remove random fluctuations. The window of the moving average filter was 300 seconds. The absorbance results qualitatively agree with the particle size measurements since suspensions with larger particle sizes showed steeper absorbance changes in time. With equation (1-4), this result can be explained by the high absolute zeta potential. High zeta potential leads to high electrostatic repulsion, (DR, and the net interaction potential, (D, between 54 particles becomes high, resulting in slow agglomeration. The slow agglomeration reduces both the rate of change of absorbance and the rate of particle coagulation. 0 - > E C-10 -20 0 > -30 2 3 4 5 3 4 5 67 pH 8 9 8 9 3.5 3 2.5 1.5 0.5 $1 pH 6 7 Figure 2-2. Characterization of PDMS coated SiO 2 particles as a function of pH. (a) Average zeta potential showing the isoelectric point (IEP) at pH 3. (b) Average agglomerate diameter, indicating smaller agglomerates at high pH. The error bars shown in (a) and (b) represent two-standard deviations of three measurements. 55 .0 - -pH 3.0 -QpH 4.0 -0-pH 5.0 -V-pH 6.0 ,-r-pH 7.0 -0-pH 7.4 CV) VII o -10a I -5 <-15 215 C 0 -0-pH 7.6 -)-pH 7.9 -V-pH 8.0 ,--V -ApH 8.3 -0.5 -0.75 ' -1.25 2,200 2,500 2,800 2000 Time (sec) 1000 3000 Figure 2-3. Change in suspension optical absorbance as a function of time for ten different pH suspensions with 0.1g/L PDMS coated SiO 2 particle concentration. The inset shows absorbance curves for higher pH values (7.4 to 8.3). The low pH curves show fast change of absorbance, which is attributed to rapid coagulation. From the absorbance curves, experimental stability ratios were calculated with equation (1-10). The maximum absorbance slope occurs at pH 4.0, which was used for the numerator (fast coagulation rate) of equation (1-8). By dividing the maximum absorbance slope by the slope at each pH, the stability ratio, Wabs, was determined, as shown in Figure 2-4. For comparison, a second stability ratio based on agglomerate size, Wsize, and calculated using equation (1-11) is given in Figure 2-4. The measured particle size of 304 nm was used as the initial particle size, Dhj, in equation (1-8). Both stability curves are in good agreement, as expected from equations (1-8) and (1-11). 56 16 14 *W bs .W. 12 size 0 10 422 3 4 5 pH 6 7 8 9 Figure 2-4. Stability ratio as a function of suspension pH. Wabs and Wsize are the absorbance based and size based stability ratios, respectively, calculated by equations (1-8) and (1-11), respectively. The measured particle size of 304 nm was used as the initial particle size, D.,, to obtain Wsize with equation (1-11). Films consisting of PDMS coated SiO 2 particles were produced by EPD using four suspensions with pH 7.4, 7.6, 7.9, and 8.3. The pH values were chosen to span a wide range of stability ratios (c.f. Figure 2-4 and Figure 2-4). Stable deposition layers were not produced with suspensions having pH lower than 6.5 due to rapid coagulation and sedimentation during EPD. Since the stability ratio curve showed a steep slope at around pH 7.5, we expected the effect of stability on EPD to be most apparent in this pH region. In addition, pH 8.3 was considered due to its unexpected lower stability value after the maximum stability at pH 8.0 (see Figure 2-4). For EPD, the suspensions were prepared using the same procedure employed for the zeta potential and stability tests. However, the concentration of PDMS coated Si0 2 particles was increased to 1 g/L in order to reduce deposition time. Stability ratios at higher particle concentration (not shown) displayed similar trends to the 0.1 g/L data shown in Figure 2-4. 57 (a) 170!FC -- 160~ -- --- '51 CTS 0 150: - ..pH 7.4, W abs =6 (D) 0D c 140 .-- pH 7.6, Wa =8 a~H7.9, Wabs =1s .D-pH 8.3, W 130 () 30 =10 60 90 120 150 Deposition Time (sec) 170rI I 180 I -C1- Advancing contact angles -0- Receding contact angles 165 1 160 30 c02 - S)155 S150 2 ?10 o 145 rot 0 60 120 180 Deposition Time (sec) 30 60 90 120 150 Deposition Time (sec) 180 Figure 2-5. Contact angles on films deposited via EPD. (a) Static contact angle as a function of deposition time for four suspensions with varying pH. (b) Advancing and receding contact angles on the surfaces created at suspension pH 7.6. The inset in (b) shows roll-off angles calculated from the advancing and receding contact angles. The error bars shown in (a) represent two-standard deviations of twelve measurements at each point. The maximum contact angle was achieved at pH 7.6, corresponding to a stability ratio of Wabs = 8 and deposition time of 60 s. 58 2.3.3. Contact angle Figure 2-5a shows static contact angle on EPD modified surfaces as a function of deposition time and suspension pH. Contact angles on the films produced were calculated by the tangential method with 3 pL water droplets. The maximum contact angles for each pH were 1600 at pH 7.4, 1660 at pH 7.6, 1610 at pH 7.9, and 1630 at pH 8.3. The highest contact angle was produced with the pH 7.6 suspension after 60 s of EPD. Suspensions with higher stability ratios were unable to obtain higher contact angles than the pH 7.6 suspension. It is interesting to note that the pH 8.3 suspension produced higher contact angles than the pH 7.9 suspension. This is further evidence that contact angle is highly dependent upon suspension stability. Dynamic contact angles were measured on the films produced with pH 7.6 suspension as a function of deposition time, as shown in Figure 2-5b. The difference between advancing and receding contact angles is considered a more reliable method to evaluate superhydrophobicity since water-repellant surfaces can be evaluated strictly by the contact angle hysteresis rather than static contact angles [101]. The roll-off angle, aR, of water droplets can be calculated with the equation aR =sin {yLv'm 'g 'w-( cos OR - cos OA)}, where m and w are the mass and width of the droplet, yLvthe surface tension of water, g the gravitational acceleration, and OA and OR are advancing and receding contact angles, respectively. In the surfaces prepared with pH 7.6 suspensions, the calculated roll-off angles shown in the inset of Figure 2-5b were 30, 20, and 70 for 30 s, 60 s, and 90 s deposition times, respectively. Roll off angles less than 100 indicate that the surfaces can be regarded as having superhydrophobicity in terms of both static and dynamic contact angles. Surfaces produced with the pH 7.4 suspension showed the highest standard deviation in measured contact angle over the range of deposition times. This could be explained by the zeta potential and particle size measurements shown in Figure 2-2, which also displays high variation at pH 7.4. In 59 addition, each suspension has a maximum contact angle as a function of deposition time. Interestingly, there is an optimal deposition time to acquire maximum contact angle for a specific suspension pH. The film produced by the pH 7.4 suspension has the slowest increase of contact angle with deposition time due to its low deposition rate compared to the higher pH suspensions. To explain the observed optimum deposition time we consider the influence of microscale and nanoscale features on surface energy. Lee et al. [11] presented the role of microscale and nanoscale features on wettability. Their work showed that contact angle increases with longer nanofibers but there was an optimal microstructure length scale to maximize the contact angle. Their observed results resemble the present study, given that suspension stability and deposition time affect nanoscale and microscale features, respectively. With EPD, nanoscale surface features are controlled by suspension stability since the agglomerate particle size (order 100 nm) increases with decreasing stability, as shown in Figure 2-2b. Meanwhile, the role of the deposition time is shown in Figure 2-6. Here, EPD surfaces produced with pH 7.9 suspensions are observed as a function deposition time (30 s to 30 min). Patterns on the substrate grow with deposition time, indicating that deposition time influences the micro/macro length scales of the surface. A similar phenomenon has been reported in the EPD process to produce thin and porous alumina membranes [102]. They found that the 'porosity of deposited films decreased after a specific deposition time. In their experiments, the pore structures became smaller and denser after a critical deposition time as the deposition mechanism changed from vertically stacked particles to horizontally organized structures, resulting in decreased pore size. Porosity and surface roughness are comparable with one another and we attribute both decreases after a specific deposition time to the increased length scale of deposits. 60 Figure 2-6. Patterns of EPD films produced by the pH 7.9 suspension with different deposition times. All images have the same scale bar in the top and most left picture. Deposition features grow with deposition time, resulting in a decrease in contact angle when deposition time exceeds 60 sec. 2.3.4. Surface roughness To reveal the mechanism underlying the varying contact angles, surface roughness of the deposited films were measured for each pH. Figure 2-7 shows contact angles with respect to surface roughness for the EPD samples. As shown, contact angle tends to increase with increasing surface roughness. The relation between contact angles and surface roughness is given by the Cassie equation [103], cos=ficos9E- fa. The contact angle on a rough surface, 0, is a function of the native contact angle, OE, of the flat surface, the solid area fraction, fs, and the vapor area fraction, fa, on the rough surface, respectively. The Cassie model assumes that the droplet is positioned on the roughened surface without wetting the porous structure. This function can also be expressed as cos 0= -l + f, (cos OE + 1) since fs + fa = 1 [60]. This means that higher surface roughness leads to higher contact angle when the contact angle on the flat surface is exceeds 90'. The native contact angle of PDMS, the material encapsulating the SiO 2 particles, is 100-112' [24], therefore the result of Figure 2-7 is consistent with Cassie's equation. As expected, the deposition layer created with the 61 pH 7.6 suspension had the highest surface roughness and contact angle, while the pH 7.4 suspension showed the lowest values. This behavior is qualitatively explained in Figure 2-8. In the case of pH 7.4, the large coagulated particles prevent the creation of thick films due to the low electrophoretic force compared to sedimentation, limiting surface roughness. In the case of pH 7.6, coagulated particles, which are smaller than pH 7.4 agglomerates, are reasonably well dispersed in the suspension and have sufficient concentration to produce thick layers. Multiple layers of randomly oriented coagulated particles provide high surface roughness. However, with the pH 7.9 suspension, the deposition layer results in lower surface roughness than the pH 7.6 case because smaller coagulates yield a more ordered deposition surface. Though the thickest deposition layers are achieved with the pH 7.9 suspension, the increased stability leads to reduced surface 180 U 170- , . - roughness. pH 7.4, W =6 0 pH 7.6, W =8 A pH 7.9, W=11 1 pH 8.3, W,=101 SV A ~V A B 150C 0 130 1 , 250 . 300 350 . 400 , 140 450 500 Surface Roughness (nm) Figure 2-7. Contact angle as a function of RMS surface roughness. In surface roughness measurements, the measured area was 40x40 pm2 and three point values were averaged in each sample. The contact angle is roughly proportional to the surfaces roughness, as can be expected by the Cassie mode. 62 To verify the mechanism explained in Figure 2-8, SEM images were taken of EPD sample surfaces (Figure 2-9). From the SEM images, we find that the most uniform deposition layer, with full substrate (black area) coverage, is produced at pH 7.9. At pH lower than 7.9, large islands of deposited particles appear in a random orientation on the substrate. For example, the pH 7.4 sample in Figure 2-9a showed the largest coagulated-particles and the least surface coverage. Unstable M Quasistable + Stable 4E0Q *Q _fN_ -,a,<-Q r* *QQ<-0 <-Q <_<-0 0 <-Q* Flocculated particles Irregular thindeposition layer Flocculated particles Irregular thickdeposition layer Dispersed particles Well ordered deposition layer Figure 2-8. Qualitative schematic of deposition behavior with respect to suspension stability. In unstable suspensions, particles flocculate and thick deposition layers cannot be formed due to sedimentation. In contrast, well dispersed (i.e. stable) particles form uniform films with low surface roughness. At moderate stability ('quasistable') EPD of particle agglomerates yields high surface roughness. 2.3.5. Mechanical durability A sample superhydrophobic surface produced by EPD with contact angle exceeding 1600 is shown in Figure 2-10. In most EPD applications, weak adhesion between particles and the substrate must be addressed in order to obtain robust surfaces. Here, conductive epoxy was used to enhance 63 mechanical durability without degrading the low energy surfaces obtained during EPD of PDMS modified SiO 2 particles. In other EPD studies, various polymers have been used as binders in order to enhance adhesion and avoid cracks in the film, resulting in high mechanical stability [79, 104, 105]. However, longer deposition time were required due to the high suspension conductivity obtained after adding the conductive epoxy [106]. of (b) (C) 10pm10n 1~,,p 10S (d)-- Figure 2-9. SEM images at two different magnifications of deposited films yielding maximum contact angles at pH 7.4 (a), 7.6 (b), 7.9 (c), and 8.3 (d). As pH increases the film become increasingly uniform, consistent with the stability measurements of Figure 2-4. 64 Figure 2-1 Oa shows a representative image of a water droplet on the surface of a substrate modified with EPD, augmented with conductive epoxy. In this case, the average contact angle on the surface is 1690 and the average RMS surface roughness obtained via AFM is 640 nm (Figure 2-10b). In the SEM images of Figure 2-10c-e, irregular porous structures composed of coagulated SiO 2 particles were observed, from hundreds of nanometers to tens of micrometers, providing high surface roughness. (a) U1 (b) Figure 2-10. Characteristics of EPD films with suspensions including conductive epoxy. (a) Image of liquid water droplet on EPD modified surface with contact angle of 1680. (b) 40x40 Pm2 AFM image of the deposition surface and SEM images [(c)-(e)] at three magnifications. In (c) Several ten micron scale features composed of flocculated SiO 2 particles were observed under the SEM. In image (d), micron scale porous structures are visible on the surface, and (e) shows ten nanometer scale SiO 2 particles mechanically connected by the epoxy additive. 65 The mechanical stability of two different superhydrophobic surfaces, one with epoxy and one without, were compared using the peel test [27]. Briefly, in the peel test, the contact angle on a surface is measured before and after an adhesive tape (in this case 1 x 1 cm 2, 1 N/cm 2, PTFE adhesive tape, Cole-Parmer) is attached and subsequently removed from the surface. Changes in contact angle on the two surfaces were measured as a function of the number of peel tests, as shown Figure 2-11. While the surface produced without epoxy lost its superhydrophobicity after the fourth peel test, the surface produced with the epoxy maintained its superhydrophobicity after ten iterations. Clearly, the mechanical stability of the deposition layer was dramatically enhanced with the addition of conductive epoxy. - 170 160 150 4,140 <130120 S110100 0-"- EPD without Epoxy -O- EPD with Epoxy 0 2 4 10 Peel Test Iteration Figure 2-11. Contact angle on EPD surfaces obtained with epoxy (circles) and without epoxy (squares), after successive 'peel' tests. The static contact angle produced without epoxy decreases far faster due to the destruction of the deposition layer after each test. However, the surface obtained with epoxy maintains its superhydrophobicity after successive peel tests. 66 2.4. Conclusions In this study superhydrophobic surfaces were produced by EPD. PDMS modified Si0 2 particles were used to acquire low surface energy and high surface roughness and increase the apparent contact angle. Suspension stability was identified as a key factor to control surface roughness and stability was varied using suspension pH. Two independent techniques were employed to quantitatively compare suspension stability. For the PDMS modified SiO 2 particles, stability ratio increases with higher pH, showing steepest increase at around pH 7.5. Four different pH values, 7.4, 7.6, 7.9, and 8.3 were chosen for EPD to compare surface wettability as a function of suspension stability. The deposited films created with pH 7.6 suspensions and a deposition time of 60 s showed the highest average surface roughness (500 nm) and the highest average contact angle (1660). Surface roughness measurements demonstrate that higher surface roughness leads to higher contact angles, as expected. To explain the results, we suggest that coagulated particle size and mobility are the main factors affecting wettability. EPD of small coagulated particles in stable suspensions leads to low surface roughness and low contact angle. On the other hand, the low surface contact angles produced by unstable suspensions are attributed to sedimentation and low particle mobility, prohibiting thick deposition layers. We conclude that unstable suspensions with sufficient mobility to achieve multi-layer coatings are required to produce superhydrophobic surfaces with EPD. In addition, irregular microscale and nanoscale features on the deposited film increase in size as a function of deposition time, ultimately decreasing contact angle. With regards to durability, conductive epoxy was employed to enhance the adhesion between particles and the surface. Deposition layers with suspensions including conductive epoxy exhibit contact angles over 160* with significantly improved mechanical stability. As shown, conductive epoxy enhances adhesion without degrading the surface properties of the PDMS modified Si0 2 particles. However, more 67 investigation is required to clearly elucidate the function of the conductive epoxy. In this work, we show that by optimizing suspension stability and deposition time, superhydrophobic surfaces can be produced in a one-step process. Ultimately EPD may be an attractive option for manufacturing superhydrophobic surfaces due to its low cost and scalability compared to other fabrication techniques. 68 Chapter 3. Anti-wetting Fabric Coatings Produced by Electrophoretic Deposition of Polymer Stabilized Hydrophobic Nanoparticles 69 3.1. Introduction Cost-effective, scalable fabrication methods for anti-wetting fabric are in high demand because not only can it be used for clothing but also it can replace expensive membranes and filters for water purification and oil-water separation [107-110]. Furthermore, promising fabrication methods can be employed for developing other types of functional fabric including anti-microbial, UVabsorbing, fire retardant and conductive [111-118]. Given the numerous potential applications, many coating techniques have been reported for functionalizing fabric [119, 120]. Among these methods, the use of nanoparticles has generally been accepted as the most promising because various types of functionality can be achieved with different nanoparticles [115-118, 121-126]. Furthermore, the use of nanoparticles is effective to maintain the original fabric properties (e.g. tactility, flexibility, color, and permeability) except the desired properties after coating. Various coating methods have been investigated to implement nanoparticle-based-coatings for anti-wetting fabric. Spray coating, bar-coating, and deep-coating are considered simple methods, but it is challenging to obtain durable and uniform coatings and to control wettability [117, 118, 127-133]. Surface functionalization utilizing chemical vapor deposition, or sol-gel processes have shown good hydrophobicity but the intrinsic characteristics of the fabric are more likely to be distorted during processing [134-141]. Recently, electrostatic assembly techniques have been investigated, demonstrating uniform superhydrophobic fabrics [142-144]. Advantages of this technique include coating uniformity and precise control of coating thickness. However, electrostatic assembly is limited by its considerable processing times, increasing manufacturing costs. Another challenge in coating nanoparticles is to produce patterned functionalized fabrics. For example, in the past decade, patterned superhydrophobic surfaces have been widely investigated to 70 enhance the efficiency of harvesting water from fog [145-147]. To maximize the efficiency, the patterns and wetting properties must be optimized. Conventional micro-fabrication techniques such as photolithography [146, 148], dewetting polymers [149], selective deposition of chemicals and polymers [150, 151], spatial photocrosslinking [147, 152], and patterned imprinting [153, 154] have been employed to produce the patterned superhydrophobic surfaces. However, there have been few attempts to produce patterned anti-wetting fabrics because these conventional microfabrication techniques are difficult to apply to soft and permeable materials. Therefore, we need a new fabrication technique to effectively design the patterns of wetting property on fabric. Furthermore, the technology for patterned anti-wetting fabric can be utilized for other functional fabrics such as conductive fabrics with customized electric circuits [116, 155]. In our previous work, superhydrophobic surfaces were produced by electrophoretic deposition (EPD) with polydimethylsiloxane (PDMS) modified SiO 2 nanoparticles on steel plates [73]. This method provides fast and customizable deposition of superhydrophobic surface coatings. The coating thickness can be controlled by the electric field intensity and deposition time. Furthermore, the morphology on the modified surface can be altered by changing the suspension stability during EPD [73]. Additionally, the durability was dramatically enhanced by co-deposition of nanoparticles with polymeric adhesives. EPD has tremendous potential to overcome the drawbacks of existing methods for coating fabric with nanoparticles. However, to date most EPD processes have been employed on electrically conductive substrates due to the necessity of electric field lines perpendicular to the deposition substrate in order to achieve sufficient adhesion [68, 79, 106]. Some studies have explored the possibility for EPD on electrically non-conductive substrates [96, 97]. Among these investigations, ionic conductive films and porous-substrates show deposits after EPD [156-158]. Fabrics are a promising candidate for EPD because they are permeable to solvents and can be penetrated by electric fields after swelling. However, weak adhesion, poor deposition 71 quality, and limited control of deposition thickness have been plagued previous attempts due to the high electric fields employed [159-161]. The high electric fields result in significant irregularity because concentration polarization near the substrate results in large, weakly adhered nanoparticle agglomerates on the non-conducting substrate. In this work, we suggest a hybrid method employing electrostatic deposition and EPD for coating nanoparticles on fabric. First, using electrostatic deposition, thin polymer-layers are deposited on fabric and then covered by a uniform dispersion of nanoparticles. Next, EPD is conducted with polymer stabilized nanoparticle suspensions. The first polymer layer enhances the adhesion between nanoparticles and the substrate after heat treatment. The initial layer of nanoparticles provides deposition nucleation sites, preventing aggregation on the substrate during EPD. The suspension is stable enough to yield uniform deposition on the substrate due to the steric repulsion between nanoparticles. Our hybrid technique employing electrostatic assembly and EPD results in durable, uniform nanoparticle deposits on fabric. Furthermore we demonstrate anti-wetting fabrics with properties (e.g. color, flexibility, permeability) very similar to the initial fabric. In addition, hydrophobicity can be spatially altered on fabrics modified by EPD with customized electric fields. To verify the patterned wetting property, we demonstrate that water droplets can be coalesced in regular patterns on the fabrics. 3.2. 3.2.1. Experimental Preparation of materials and suspensions In this study we used two types of polyester fabric (Patagonia, Fabric A: brown, 100% polyester, 20 denier in warp and 20 denier in fill direction, and Fabric B: red, 100% polyester texturized fiber 72 without any coating/treatment, 10 CFM in air porous, and plain weave face and twill weave back). Two kinds of polymers, poly(dimethyl-diammonium chloride) (PDDA) (Aldrich, 30 wt.% solution in water) and poly(sodium 4-styrenesulfonate) (PSS) (Aldrich, 35 wt.% solution in water), were used as positively and negatively charged polymers, respectively, for the electrostatic coating process. Both polymer concentrations were 8 g/L in deionized (DI) water. These two polymers have been widely used for layer-by-layer techniques because they can be electrically charged in dielectric solvents, resulting in high electrostatic attractive and repulsive forces.[67, 162] Two kinds of nanoparticle suspensions were prepared; in the first suspension: PDMS modified SiO 2 nanoparticles (PlasmaChem, 14 nm) and polyvinylidene fluoride (PVDF, Sigma-Aldrich) are dispersed into a mixture of methanol (90 vol.%) and water (10 vol.%) with concentrations of lg/L and 0.5 g/L, respectively; and in the second suspension: PDDA is added to the SiO 2 and PVDF suspension described above until the PDDA concentration reaches 3.75x 10- g/L. Finally, the pH of both suspensions is adjusted to 8.5 with a basic solution (Potassium hydroxide, 45 % for HPLC, SigmaAldrich). 73 (a) Step 1: Electrostatic deposition of polymers (b) Cathode Fabric Anode Suspension Power supply Step 2: Electrostatic deposition of particles Step 3: Electrophoretic deposition of particle-polymer assemblies 1000 Fabric PDDA PSS Step 4: Heat-treatment of the fabric Particle PDDA + Particle Figure 3-1. Schematic illustrations of the fabrication process suggested for anti-wetting fabric coating. (a) Step 1: multi-layered polymers are electrostatically deposited on the fabric. Step 2: nanoparticle are electrostatically deposited on the polymer layers of the fabric. Step 3: the polymer and nanoparticle assemblies are deposited by electrophoretic deposition (EPD). Step 4: heat treatment is conducted on the fabric for enhancing mechanical durability. (b) The EPD cell for Step 3 in (a). The cell consists of two stainless steel electrodes and a fabric sheet is wrapped around the cathode electrode. 3.2.2. Electrophoretic deposition for fabric coating The fabrication process for anti-wetting fabric consists of four steps, as shown in Figure 3-1. First, a steel plate (type 316 Stainless steel foil, full hard temper, .004" Thick, 10 cm x 30 cm, Trinity Brand Industries, Inc.) is wrapped with a sheet of fabric (20 cm x 30 cm). In this step, slightly curved steel plates make good contact with the fabric by providing a small amount of tension. Next, 74 the steel plate and fabric assembly is immersed into the PSS solution (8 g/L concentration in DI water) for 1 min and then the fabric is washed with DI water for 10 sec. Afterwards, the assembly is immersed into the PDDA solution (8 g/L concentration in DI water) for 1 min and then washed with DI water for 10 sec. These two immersion-washing steps are conducted twice (Step 1). Next, the fabric-plate assembly is immersed into the nanoparticle suspension (the first suspension) for 5 min and then washed with DI water for 20 sec (Step 2). For EPD in the second suspension (Step 3), the fabric-electrode assembly and a similarly sized stainless steel plate are used as the cathode and the anode, respectively. During EPD, an electric potential difference of 120 V is applied to the electrodes, which are separated by 20 mm. After EPD, the fabric is dried in the laboratory atmosphere, and then heat-treated at 150 - 220 'C for 2 min with a heat-press apparatus (Clamshell, Model PRO-3804X) (Step 4). Zeta potential of polymers and nanoparticles dispersed in solution were measured by a zeta potential meter (Zetasizer Nano ZS, Malvem Instruments, Inc.). Two scanning electron microscopes (SEM, JEOL 6320FV Field-Emission High-resolution SEM and He-Ion microscope, Zeiss) were used to observe the morphologies of modified fabric surfaces. Static and dynamic contact angles were measured with a goniometer (Kyowa, DM-CE1) with a 3 pL drop of DI water. Water droplet behaviors on the fabric were recorded using a digital camera (J2, Nikon). Color changes before/after coating were evaluated by RGB comparison with an image processing program (Adobe photoshop CS4 extended). A customized skin friction generator was used for durability tests. 3.3. 3.3.1. Result and discussion Suspension Zeta potential 75 Surface charge is necessary for electrostatic self-assembly and electrophoresis, and the amount of charge is estimated by zeta potential measurements. Figure 3-2 shows the zeta potential of the suspensions of SiO 2 nanoparticles and polymers used in this work. (b) (a) 30 20 10 0 -10 N (c) -20 -30 Polymer @ pH 8.5 Polyester Zeta potential -45 PSS D -40 2 3 4 O PDDA PSS 5 6 7 pH 8 <SO 2 9 10 -25 PDDA SiO, SiO,+PDDA PVDF PVDF+PDDA +20 [my] 11 -PDDA: Particle suspension solution -25 10 -19 24 poly(dimethyl-diammonium chloride), PSS: poly(sodium 4-styrenesulfonate), SIO2: PDMS modified SiO 2 nanoparticles, PVDF: poly(vinylidene fluoride) Figure 3-2. Zeta potentials of the polymers and the SiO 2 nanoparticles. (a) Zeta potentials of poly(sodium 4-styrenesulfonate) (PSS), poly(dimethyl-diammonium chloride) (PDDA), SiO 2 nanoparticles with respect to pH. (b) Digital image of the suspensions of SiO 2 , polyvinylidene fluoride (PVDF), PDDA, and PSS. (c) The table summarizes the zeta potentials at pH 8.5. The zeta potential of non-treated polyester fiber was obtained from a reference.[163] The solvent for the SiO 2 nanoparticle and PVDF suspensions is composed of methanol 90 vol. % and water 10 vol. %. The others are deionized water. The concentrations are 8 g/L PSS and PDDA and 1 g/L SiO 2 and 0.5 g/L PVDF with and without 3.75x 10~3, g/L PDDA polymers. The error bars indicate one standard deviation from three measurements. The zeta potentials of poly(sodium 4-styrenesulfonate) (PSS) and poly(dimethyl-diammonium chloride) (PDDA) solutions (8 g/L for each) are -25 mV and +20 mV at pH 8.5, respectively. The zeta potentials of SiO 2 nanoparticles, dispersed in the water (10 vol. %) and methanol (90 vol. %) mixtures, were -25 mV without PDDA polymers and +10 mV with 3.75x 10-3 g/L concentration of PDDA polymers. The sign change of the zeta potential of SiO 2 nanoparticles indicates the formation 76 of SiO2-PDDA assemblies, as shown in Figure 3-1. As a reference, the polyester fibers have a zetapotential of -45 mV at pH 8.5 in water, and the isoelectric point (IEP) is pH 2.3 [163]. Based on the zeta potentials measured, the fabrication process, as shown in Figure 3-1, is suggested. The PDDA polymers naturally deposit on the polyester fabric due to electrostatic attraction because the polyester fiber and PDDA have opposite surface charge. Then, PDDA and PSS layers can be produced by alternating immersions of each polymer solution due to their opposite surface charge. Next, immersing the polyester fabric pre-treated by PDDA and PSS into a suspension with 1.0 g/L SiO 2 and 0.5 g/L polyvinylidene fluoride (PVDF) leads to self-assembly of the nanoparticles on the PDDA. Finally, on the SiO 2 -PVDF layer, PVDF-PDDA-SiO 2 assemblies are uniformly deposited by EPD with the suspension composed of 1.0 g/L SiO 2 , 0.5 g/L PVDF, and 3.75x10 3 g/L. The use of EPD leads to rapid fabrication, much faster than the multiple immersions required for thick electrostatic assemblies. To enhance mechanical durability of the modified fabrics, annealing processes are conducted at temperatures slightly lower than the melting points of the polymers after EPD. 3.3.2. Wettability of coated fabric Through the fabrication process shown in Figure 3-1, we produced anti-wetting fabric coated by hydrophobic SiO 2 nanoparticles, as shown in Figure 3-3a. Even though the original fabric instantly absorbed water droplets (Figure 3-3b), droplets on the modified fabric show static contact angles higher than 1500 and roll-off angles less than 20 when EPD was conducted for 90 sec. The modified fabric is still flexible with minimal color change (Figure 3-3a and c). This method results in uniform anti-wetting performance on the entire area modified. The modified surface area is entirely 77 dependent on the electrode size and the electrolyte volume during EPD. Therefore, the fabrication method is easily scalable to large areas of fabric. t=0 t=l10me t=340ms t=8l0ms Figure 3-3. Images of water droplets on polyester fabrics modified by the fabrication process shown in Figure 3-1. Two different fabrics, which color red and yellow, were used to produce antiwetting fabrics. Electrophoretic deposition was conducted for 90 sec in Step 3. (a) The modified fabric does not lose its original color but the droplets show high contact angles and low roll-off angles. Flexibility of the original fabric is maintained after modification while wettability changes dramatically from hydrophilic to superhydrophobic. (b) A water droplet is rapidly absorbed into the original polyester fabric. The scale bars indicate 1 mm. (c) The brown fabrics do not show significant difference in their colors before and after the modification. Water droplets show contact angles higher than 1500 and roll-off angles lower than 5*. The fabric wettability can be altered by the duration of EPD. Static contact angle and roll-off angle were measured on the fabric produced as a function of deposition time during EPD (Figure 3-4). The contact angle of water droplets on the fabric before EPD (time is zero in Figure 3-4) was approximately 1300. This shows that the electrostatic assembly was not sufficient to achieve a superhydrophobic surface due to inadequate surface coverage (the inset of Figure 3-5a). After EPD, the static contact angles were significantly enhanced and low contact angle hysteresis was achieved. With increasing EPD time, the static contact angle increased and the roll-off angle decreases. The maximum static contact angle and minimum roll-off angle were achieved at 90 sec deposition time. 78 A similar relationship between the contact angle and deposition time was found in our previous work on steel plates [73]. 170 140 120 - 160 100 4)0 150 80-O8 08 OContact angle oRoll-off angle ts140 04 130 60 740 5 20 120 0 ,0 30 60 90 120 150 180 210 240 Time (sec) Figure 3-4. Static contact angle and roll-off angle of fabric produced with different deposition times for EPD. Contact angles were measured after heat treatment. Water droplets instantly absorbed into the original fabric resulting in and effective static contact angle of zero degrees. The static contact angle is 1360 on the fabric coated with polymer and thin layers of hydrophobic SiO 2 nanoparticles (after Step 2). Static contact angle is dramatically enhanced after EPD process (after Steps 4). The maximum contact angle of 1590 and the minimum roll-off angle of 2' are achieved at 90 sec deposition time. The error bars indicate 3.3.3. one standard deviation from five measurements. Scanning Electron Microscopy The morphology of the modified fabric was investigated using SEM images, as shown in Figure 3-5. The original fabric has a twill wave pattern composed of 15 micron polyester fibers (Figure 3-5a). After Step 2 in Figure 3-1, Si0 2 nanoparticles are uniformly deposited on the fabric (the inset of Figure 3-5a). The coatings on the fabric become thicker with increasing duration of EPD (Figure 79 3-5b-c) after Step 3. The SiO 2 particles are partially deposited on the fibers after 30 sec of EPD (Figure 3-5b) but fully covered after 60 sec of EPD (Figure 3-5c-). Figure 3-5. SEM images of fabrics produced by electrophoretic deposition (EPD) with different deposition times. (a) Original polyester fabric has twill weave patterns. The inset shows a fabric fiber with SiO 2 nanoparticles deposited on the surface (after Step 2 in Figure 3-1). (b) After 30 sec of EPD, SiO 2 nanoparticle layers are observed on the fabric but do not fully cover the surface. (c) After 60 sec of EPD, the fabric fibers are uniformly and densely coated with SiO 2 nanoparticles. This surface exhibits the maximum contact angle of 1570 and minimum roll-off angle less than 5*. 3.3.4. Color change test Colorfast tests were conducted by comparing the RGB values of the coated fabric with respect to the duration of EPD, as shown in Figure 3-6. The relative RGB values (R*, G*, and B*) were the ratios of the RGB values at a specific EPD time to the original fabric. It is known that thin SiO 2 nanoparticle layers, uniformly deposited, are highly transparent [14, 27, 55]. For instance, Cebeci et al. showed that transparent superhydrophilic glass can be achieved using thin layers of SiO 2 nanoparticles [55]. The colors do not change significantly even after the static contact angle with 80 water reaches 1500 (Figure 3-6a-b). It is worth to noting that dense SiO 2 nanoparticle coatings deposit on polyester fibers, which are not electrically conductive, with the help of the Si0 2 nanoparticles and polymer film coated prior to EPD. Without the pre-treatment processes large, irregularly spaced particle aggregates are deposited rather than uniform thin films, as shown in Figure 3-6d. When the polymerically stabilized SiO 2 suspension was not used, the fabric color changed significantly after EPD (Figure 3-6e). (a) aOR* +G* 4 +B* (b) 0 Q3 33 EPD time (sec) 30 60 90 120 150 180 210 00 30 60 90 120 150 180 210 EPD time (sec) Figure 3-6. Color difference tests using RGB comparison and real images of the modified fabrics. (a) R*, G*, and B*, which are the relative RGB values to the original fabric, with respect to deposition time in electrophoretic deposition (EPD). The error bars indicate one standard deviation from five measurements. (b) RGB values are obtained by using image software (Adobe Photoshop CS4) with captured images of fabric produced with varying EPD time. (c)-(e) Captured images of fabrics produced with three different fabrication processes and with the same EPD time of 90 sec: (c) Fabric produced with Steps 1-4 in Figure 3-1, (d) Fabric produced without Steps 1-2. (e) Fabric without polymeric stabilization of the particles in Step 3. 3.3.5. Air permeability testing 81 To evaluate the breathability of the anti-wetting fabric, water vapor transmission (WVT) rates were measured according to the standard method for water vapor transmission of materials (ASTM E 96, Procedure B, 1999) [164]. The WVT rate can be calculated as WVT = AM/(A -t), where AM is the weight change of liquid water, A is the fabric surface area, and t is the time elapsed to obtain AM. We filled 80 ml of liquid water into 100 ml media bottles and capped the bottles with vented caps sealed with fabric having 9.1 x 10- m 2 test-area. The WVT rates were measured every 5 hours and the average WVT rates were calculated from five measurements at a specific water temperature. The water temperature was controlled from 35 'C to 65 "C at intervals of 10 'C. The test environmental conditions were 24 1 'C and 30 5 % relative humidity with negligible air flow on the fabric during testing. Interestingly, slightly higher WVT rates were obtained from the antiwetting fabric than the original fabric across the entire temperature range. The difference becomes greater at higher temperatures, as shown in Figure 3-7. At the least we can say that the coating does not reduce breathability but the data suggests it may be enhanced. We hypothesize that the hydrophobic nanoparticles can prevent condensation, enhancing direct vapor transport through voids of the fabric. 82 1.6 1.4 - E OOriginal fabric OAnti-wetting fabric 1.2 1.0 0.8 0.6 0.4 0.2 0.0*............................. 30 40 50 60 Water temperature (*C) 70 Figure 3-7. Water vapor transmission (WVT) rate tests for the original fabric (box symbols) and the anti-wetting fabric (circle symbols). The anti-wetting fabric was produced with 90 sec of EPD at 120 V. The WVT rate experiments were conducted with water temperatures ranging from 35 *C to 65 'C. The error bars indicate 3.3.6. one standard deviation from four measurements. Coating durability Durability of the fabric coatings was evaluated by a skin friction resistance test using boundary layer flow. A circulating pump continuously generated water flows at 10 m/s on the anti-wetting fabric attached to a steel plate. The flow speed was chosen to imitate the environment on the surface of a ship, which is stressed from water friction during operation. We measured the static contact angle with respect to test time on the fabric. The fabric was fully dried on a hot plate at 50 'C and cooled down to the environment temperature before the measurement. Figure 3-8 shows the change of the contact angle on the anti-wetting fabric during the test. The fabric maintained contact angles higher than 1100 over 100 hours. Even though the static contact angle decreased, the coatings were 83 not completely eroded. Future work will involve further enhancements to the fabrication process in order to maintain superhydrophobicity in harsh environments. 180 160 140 0)M ~~O'-- 120 100 80 0 60 40 20 t =0 t=35hr t=70hr t=100hr 0 0 20 40 60 80 100 120 Test time (hr) Figure 3-8. Contact angles on the anti-wetting fabric with respect to test time. The durability test was conducted with a customized skin friction generator. The skin friction was induced by boundary layers on the fabric with flows circulated by a water pump. The flow speed was 10 m/s. The contact angles were measured after the fabric was fully dried on a hot plate. Each data point is the average contact angle. The error bars indicate 3.3.7. one standard deviation from five measurements. Patterned anti-wetting fabric One advantage of utilizing EPD for fabric coatings is the ability to produce patterned surfaces. We demonstrate this by producing patterned anti-wetting surfaces using honeycomb-shaped electrodes (Figure 3-9a). As shown in Figure 3-3, the fabric wettability can vary with the duration of EPD. In addition, the electric field during EPD can be utilized to control wettability [73]. When 84 shaped electrodes are used, the electric field on the fabric varies spatially. The weakest electric fields are induced at the center of the holes while the highest electric field occurs at the steel features. The sample fabric coated using the shaped electrode is shown in Figure 3-6a. Even though, the fabric has non-uniform deposits on its surface, there is no discernable color difference on the surface. To test the locally varying wettability, water was sprayed on the fabric at a speed of 0.6 ml/cm 2 -min. The water spray results in droplets growing with time as they merge with the spray droplets .(Figure 3-6b-c). Interestingly, the growing droplets have regular patterns similar to the patterns of the electrode holes (Figure 3-9c). This result shows that designing the electric field using customized electrodes can be used to alter the fabric wettability. This allows us to control fabric characteristics with respect to location as well as deposition time. In nature, one can observe regular distribution of dew drops on leaves, as shown in the inset of Figure 3-9e. The uniform patterns prevent the mass concentration caused by large drops and the subsequent deflection of the surface. This same feature can be obtained on the anti-wetting surfaces with regularly patterned wettability. 85 Figure 3-9. Patterned anti-wetting fabric using a patterned electrode. (a) An electrode with honeycomb shaped holes (left) was used to coat hydrophobic nanoparticles on polyester fabric (right). The fabric coated by the patterned electrode does not show significant color difference relative to the original fabric. (b)-(e) Water was continuously sprayed on two different fabrics, which are uniform anti-wetting fabric and patterned anti-wetting fabric, and the resulting droplets on the surfaces were imaged with respect to time. (a) Droplets grow irregularly on the uniform antiwetting fabric. (c) Droplets grow with regular patterns and ultimately show similar patterns as the electrode. (d) The pattern of coalescent droplets on the patterned anti-wetting fabric is similar to the dew drops on green Oregon grape leaves shown in the inset of (e) - Copyright and photography by Brent Vanfossen. 3.4. Conclusions We have presented a novel method employing electrophoretic deposition to create anti-wetting polyester fabric. Advantages of this method include of scalability, durability, and control of wettability, showing great potential for commercial use. These features have been achieved by a hybrid method employing electrostatic assembly and electrophoretic deposition for coating three 86 different constituents (polymers, nanoparticles, and polymeric nanoparticles) on fabric surfaces. The first polymer layer and the second nanoparticle layer are electrostatically deposited on the fabric surface. The first polymer layer provides strong mechanical networks between the fabric and the polymeric nanoparticles after heat treatment. The second nanoparticle layer provides nucleation sites for the subsequent EPD. The polymeric nanoparticles show low zeta potential but they were highly stable in the suspension because of steric repulsion between particles. As a result, this method simultaneously provides both advantages of EPD and electrostatic deposition: it is fast, cost-effective, and scalable as well as precisely controllable. Water droplets on the modified polyester fabrics show static contact angles exceeding 1500 and contact angle hysteresis less than 20. SEM images of the fabric show uniformly and densely deposited nanoparticles on the polyester fibers. An optimal EPD time of 90 sec was shown to produce highly anti-wetting fabric without significant color distortion. The anti-wetting fabric maintained its hydrophobicity over 100 hours during an aggressive skin friction resistance test. Furthermore, fabric wettability of can be controlled by EPD time and electrode geometry. We expect that this method can be directly employed to develop commercial functionalized fabric. Large areas can be modified by our method in a time and cost effective manner. In addition, the resultant fabric has sufficient durability for practical applications, but more work must be conducted for more aggressive applications. Perhaps the most unique advantage of this method is that patterns of wettability can be implemented using varying electrode geometries. Considering these advantages, we believe that this method can be widely utilized to functionalize fabric with nanoparticles for various purposes. 87 88 Chapter 4. A Hybrid Method Employing Breakdown Anodization and Electrophoretic Deposition for Superhydrophilic Surfaces Reproduced in part with permission from Young Soo Joung, Cullen R. Buie, "A Hybrid Method Employing Breakdown Anodization and Electrophoretic Deposition for Superhydrophilic Surfaces," Journalof Physical Chemistry B, 2013, 117 (6), pp. 1714-1723. Copyright 2013 American Chemical Society. 89 4.1. Introduction In this work, we suggest a novel fabrication method to produce superhydrophilic surfaces with high capillary pressure and fast spreading speed. We show that the method can be utilized to control capillary pressure and spreading speed independently. EPD is utilized to obtain nano-structures composed of TiO 2 nanoparticles, while breakdown anodization method (BDA) was employed to produce micro-porous structures. During anodization, the oxidization film thickness linearly increases with respect to electric potential [75]. However, when the electric potential is higher than a critical value, the film thickness abruptly increases, resulting in a number of micro-scale irregularities. The critical electric potential is called the breakdown point. At electric potentials exceeding the breakdown point, micro-scale porous structures are created on the electrode. This phenomenon was reported a few decades ago but has not been exploited to produce superhydrophilic surfaces [76, 77]. Similar breakdown anodization methods have been used to produce nano-scale titanium structures such as nanotubes and nanowires on titanium substrates [165-171]. These methods have typically used strong etchants in the electrolytes to locally etch the titanium substrate at the breakdown locations. The final nano-structures result from the balance between the etching rate and the oxidation rate. However, in our method, only mildly acidic electrolytes were used to make micro-scale titanium porous structures. This forces the need for higher electric potentials to initiate breakdown behavior. In addition, a balance between the dissolution rate of the oxide layer and the oxidation rate control the resulting structures because weak electrolytes do not locally etch the titanium substrate. EPD and BDA offer independent control of nano-scale and micro-scale features, respectively, resulting in a wide variety of superhydrophilic surfaces. We characterize these surfaces in terms of capillary pressure and spreading speed using CRM. The roles of nanopores (nano-scale spaces 90 created by the nanoparticles and micropores (micro-scale spaces existing between micro structures) are investigated to understand the mechanisms governing capillary transport. Further, we demonstrate specially tailored superhydrophilic surfaces with spatially varying pore radii which result in unconventional capillary flows. 4.2. Theory Capillary flows can be characterized by Washburn's equation. Following this equation, the square of capillary height is linear with rising time. From this relation, two characteristic parameters, capillary pressure and spreading speed constant, can be semi-empirically obtained. These parameters can be utilized to characterize superhydrophilic surfaces. EPD and BDA can be used to produce superhydrophilic surfaces with various capillary pressures and spreading speed constants. In EPD, suspension stability is important to control nano-scale structures [73]. In BDA, micro-scale porous structures are created on anode electrodes when the anodization electric potential is higher than a critical value. The following sections outline the theoretical considerations necessary to control transport in surfaces produced with BDA and EPD. 4.2.1. Capillary flow If the capillary forces exceed the gravitational force, the wetting line can rise. Washburn's equation has been widely used to predict the speed of capillary rise and can be expressed by [172] 91 -= r-2 F2YCOS9pgh1, dt8iyh r (4-1) where h is the height of the wetting line; t is the time after contact between with liquid; r is the pore radius; qy, y, and p are the dynamic viscosity, surface tension, and density of liquid, respectively; 9 is the native contact angle of surface material; and g is the gravitational constant. If the effect of gravity is negligible and the pore radius is constant, the relation between the capillary rise height and time can be expressed by h2rcos 2t7 I 2 (4-2) where Dcap is hydraulic diffusivity. From CRM, the change of capillary height with respect to time can be obtained and hydraulic diffusivity can be calculated by curve fitting. Capillary pressure can be calculated from the maximum capillary height since at this condition the capillary force balances the gravitational force, assuming liquid evaporation is negligible. Thus, the capillary pressure can be expressed by P r H (4-3) where H,,x is the maximum capillary height. From equations (4-2) and (4-3), we can semiempirically calculate the capillary pore size, r, and the contact angle, 0, as follows 92 r 21C. = and Pca, jD 2y 2 Pca f(4 (4-4) If the pore radius is not constant but a function of h, we can obtain various profiles of contact line propagation. 4.2.2. Electrophoretic deposition The electrophoretic mobility, p, is linearly proportional to the zeta-potential, C, and the permittivity, c, of the medium, and is inversely proportional to the viscosity of the medium, 17, as expressed by [78] 2c4- (4-5) 317 assuming spherical particles where the Debye length is small relative to the particle radius. During EPD, the deposition rate, A, is a function of the mobility, the particle concentration, C, and the electric field, E, subjected to the electrodes as follows M=p-C-E (4-6) Thus, if the concentration and the electric field are fixed during the EPD process, the deposited mass can be altered by the deposition time. 93 4.3. 4.3.1. Experimental Suspension and electrolyte preparation EPD and BDA were used to produce superhydrophilic surfaces. By combining both methods, we were able to make three different types of superhydrophilic surfaces; EPD only, BDA only, and a hybrid BDA/EPD method. Schematic illustrations of BDA and EPD are shown in Figure 4-la and Figure 4-lb, respectively. For EPD, TiO 2 nanoparticles (Sigma-Aldrich, titanium oxide, anatase, nanopowder, < 25 nm particle size, ;> 99.7% purity) were used. Acetic acid was used as solvent to prepare 1 g/L concentrations of TiO 2 suspensions. Titanium plates (Ultra-Corrosion-Resistant Titanium Grade 2, 0.020" thick) were used as anode and cathode electrodes. An electric potential of - 30 V was subjected to the electrodes (the gap distance between the electrodes was 10 mm) for 30 120 sec to deposit thin films on the substrate. During BDA, the DI water was adjusted to pH 3 by adding acid (Nitric acid, 70 % ACS reagent, Sigma-Aldrich). Two titanium plates (same material used for EPD) were used as cathode and anode electrodes (the gap distance between electrodes was 10 mm), and electric potentials were supplied in the range of 30 - 120 V for 30 min. During BDA, the electrolyte temperatures were maintained at 10 - 75 'C because breakdown initiation is affected by electrolyte temperature, as shown in equation (1-22). Furthermore, it has been reported that electrolyte temperature affects the resulting electrode morphology [76, 173]. Therefore, the temperatures were adjusted using a circulating bath (polystate, Cole-Parmer) in order to investigate the effect of electrolyte temperature on wetting properties. Throughout BDA experiments the electrolyte was rigorously stirred with a magnetic bar. 94 To produce nano- and micro- scale hierarchical structures on the titanium substrates, we used a hybrid BDA/EPD method. In this method, BDA was conducted first and then the anodized electrode was re-used as the substrate for EPD. The EPD process in the hybrid method was conducted with the same operating conditions as the surfaces modified solely by EPD. (a) DCPower Supply SDC Power Supply Electrodes Nanoparticle Magnetic bars Solvent Water bath Acidic electrolyte [00 Stirrer 1 Stirrer Figure 4-1. Schematic illustration of the experimental setups for breakdown anodization (BDA) (a), and electrophoretic deposition (EPD) (b). The water temperature is controlled to maintain constant electrolyte conditions during BDA. TiO 2 nanoparticles are dispersed in acetic acid solvents with 1 wt% concentration for EPD. The solvents of both methods are stirred with magnetic bars. 4.3.2. Capillary rise measurement (CRM) CRMs were performed to evaluate the capillary pressure and the spreading speed of the superhydrophilic surfaces. Figure 4-2 shows a schematic illustration of the CRM setup. During CRM the sample is enclosed in a glass column to prevent evaporation from the surface during the test. The sample is fixed vertically and the height of water bath is gradually increased to make contact between the bottom of the sample and the water surface. Capillary rise through the substrate 95 is recorded with a digital camera (Canon, PowerShot S90). A ruler fixed adjacent to the substrate is used to estimate the capillary rise height with respect to time. Sample Ruler Watr bath Stage Figure 4-2. Schematic illustration of the capillary rise measurement (CRM) system. Capillary height is recorded by a digital camera and estimated with customized image processing software (Labview). Evaporation from the sample surface is prevented by encapsulating the sample in a glass column. 4.3.3. Characterization Morphologies of the deposited films were characterized with a scanning electron microscope (SEM, JEOL 6320FV Field-Emission High-resolution SEM). Static contact angles (CA) were calculated using the tangential curvefitting method. A goniometer (Kyowa, DM-CE1) was used to dispense and image a 3 uL drop of DI water on each sample. X-ray diffraction (XRD) data were collected from the PANalytical X'Pert Pro Multipurpose Diffractometer with Cu anode material at 96 45 kW and 40 mA to investigate the material composition and atomic structure of the resulting surfaces. The step size and the scan step time are 29= 0.0 17 and 42.55 sec, respectively. 4.4. 4.4.1. Results and discussion Surface morphology Figure 4-3 shows SEM images of the fabricated surface structures under different magnifications. Highly irregular and entangled microstructures appear on the titanium electrode after BDA in Figure 4-3a-c. In the samples prepared by EPD with a 30 sec deposition time, well ordered TiO 2 nanoparticles were observed, as shown in Figure 4-3d-f. Hierarchical structures, as shown in Figure 4-3g-i, were observed in the case of the hybrid BDA/EPD method. Here the micro structures were highly curved and entangled while TiO 2 nanoparticles were well dispersed on the surface. This suggests that the anodized surfaces do not completely lose their electrical conductivity. Static contact angles were measured on each of the modified substrates. Each surface exhibited contact angles near zero degrees. Therefore, following the classical definition of superhydrophilic surfaces [60], the three fabrication processes resulted in superhydrophilicity. However, the time required to achieve zero contact angle differed amongst the samples. This reveals that the samples in fact have differing wettability, which is not evident from static contact angle measurements. 97 Figure 4-3. SEM images of superhydrophilic surfaces fabricated by breakdown anodization (BDA) (a)-(c), electrophoretic deposition (EPD) (d)-(f), and the hybrid BDA/EPD method (g)-(i). BDA was conducted at the electric potential 90 V for 30 min with an acidic electrolyte (pH 3) at 10 'C. EPD was conducted with the electric potential 30 V for 90 sec with I wt.% TiO 2 in an acetic acid solvent at ambient temperature. The surface produced by BDA consists of several entangled micro-porous structures while the surface deposited with TiO 2 nanoparticles by EPD shows uniformly distributed nano-porous structures. The hybrid BDA/EPD method yields hierarchical micro- and nano- porous structures. 98 4.4.2. Capillary pressure and hydraulic diffusivity To further evaluate the superhydrophilic surfaces, spreading speed and capillary pressure were measured with CRM, as shown in Figure 4-2. As can be expected from equation (4-2), the square of capillary rise height is linearly proportional to time (Figure 4-4a). The slope of each linear fit line represents hydraulic diffusivity, %2 Dcap. Capillary pressures were calculated from equation (4-3) using the maximum capillary rise height of each surface. The surfaces produced by BDA show higher capillary pressure and spreading speed constant than the surfaces produced by EPD. This is attributed to the pore sizes of each surface. The BDA surfaces have micro-scale pores as shown in Figure 4-3a-c, while the EPD surfaces are composed of nano-scale pores as shown in Figure 4-3d-f. From equation (4-2), hydraulic diffusivity is linearly proportional to the effective pore radius. Therefore, it is reasonable to expect the BDA surfaces to show higher spreading speed constants than the EPD surfaces. However, unlike hydraulic diffusivity, capillary pressure is inversely proportional to the pore radius. Viscous drag through smaller pores reduces capillary speed at low heights. For time scales on the order of days, the capillary rise height of the EPD surfaces can be considered as quasi-static. Therefore, the EPD surfaces have lower effective capillary pressures than the BDA surfaces even though the pore size is significantly smaller. Interestingly, the surfaces produced by the hybrid BDA/EPD method maintained both high capillary pressure and spreading speed constant. From this result, we conclude that the overall transport characteristics are governed by micro-scale structures and only locally influenced by the nano-scale structures. 99 40 0 O A 0 BDA: 120 V BDA: 90 V BDA: 60 V BDA: 30 V *BDA: 120 V + EPD 35 30 25 E20 E20 - (a) 00 %%0 15 --- 10 -A -- 1--- -5 0 5 10 15 20 25 30 35 40 Time (sec) (b) BDA: 120 V BDA: 90 V BDA: 60 V BDA: 30 V BDA: 120 V+ EPD EPD H,,, (cm) P,8 p (kPa) D,.p (mm 2 /s) 16.1 14.2 9.2 7.2 13.4 1.2 1.6 1.4 0.9 0.7 1.3 0.12 145 118 47 38 79 6.6 Figure 4-4. Square of experimentally derived capillary rise height is linearly proportional to time, as predicted by Washburn's equation (a). The samples produced by BDA and EPD show different capillary pressures and spreading speeds (b). The sample produced by the hybrid BDA/EPD method shows slightly lower spreading speed than the sample produced by BDA at the same conditions. In BDA, electrolyte temperatures were maintained at 10 'C and electric potentials were supplied for 30 min. EPD was conducted at the electric potential 30 V for 90 sec. Hydraulic diffusivity, Dcap, of the surface can be obtained from the slope of each curve as shown in equation (4-2). The capillary pressure is calculated from equation (4-3) using the maximum rise height. Different electric potentials and electrolyte temperatures during BDA were investigated to control wettability and capillary transport properties. Capillary pressure and spreading speed constant with respect to electric potentials and electrolyte temperatures are shown in Figure 4-5a-b. Interestingly, for a given temperature the electric potential to achieve the maximum capillary pressure was not always consistent with the value to obtain the maximum spreading speed. This highlights why both 100 the capillary pressure and the spreading speed are necessary to evaluate and optimize superhydrophilic surfaces. For electrolyte temperatures less than 50 'C, the capillary pressure and hydraulic diffusivity increased with increasing of electric potential. However, all surfaces showed similar capillary pressure and spreading speed at electrolyte temperatures greater than 50 'C (Figure 4-5). This behavior can be explained from the mechanisms of breakdown initiation and oxide layer dissolution. The electric potential to initiate breakdown is inversely proportional to the electrolyte temperature, as shown in equation (1-22). Therefore, high electrolyte temperatures are favorable for BDA at low electric potential. In contrast, because the dissolution rate of the oxide layer is low electrolyte proportional to the electrolyte temperature as shown in equation (1-23), temperatures promote thick and rough surfaces at high electric potential. At electrolyte temperatures exceeding 50 *C, both capillary pressure and spreading speed constant decrease due to significant dissolution of the oxide film into the electrolyte. (a) (b) 2.0 1.8 1.6 -*-30 V -- 1.4 1.2 rk oV 120V 160 140 -0-30 V 120 90V - A -120V a) 1.0 EA 0.8 0.6 0.4 0.2 E 0.0 10 20 I 30 I 40 I 50 60 60Q 40 20 0 I0 70 Temperature (C) . -* 10 20 30 40 * 50 I .... 60 70 Temperature (*C) Figure 4-5. Changes in capillary pressure (a) and spreading speed constant (b) with respect to electric potential and electrolyte temperature during BDA. Four different electric potentials (30 V, 60 V, 90 V, and 120 V) and four different electrolyte temperatures (10 'C, 25 *C, 50 'C, and 75 'C) were used to obtain the surfaces. The error bars indicate four measurements. 101 one standard deviation resulting from Figure 4-6. Sample SEM images of surfaces produced by BDA. At low electrolyte temperature and low electric potential (a), initiation of BDA is not facile; therefore, partially burst structures are observed on the surface produced at 30 V and 10 'C. When the electrolyte temperature was maintained at 75 'C during BDA, the highly interconnected porous structures shown in Figure 4-3ac dissolve in the electrolyte, leaving smooth surfaces with large pores (b). This theory is supported by SEM images of the surfaces at different electrolyte temperatures. The surfaces produced at an electrolyte temperature of 10 *C and electric potential of 30 V do not have porous structures, as shown in Figure 4-6a. Instead they feature small cracks that could not develop into pores. This suggests that high electric potentials are necessary to initiate BDA at low electrolyte temperatures. Conversely, the surfaces produced at high electrolyte temperatures show large porous structures, as shown in Figure 4-6b. This SEM image further suggests that dissolution at high electrolyte temperatures facilitates pore formation. The final weight of each substrate after BDA is further evidence of the influence of dissolutions. When the electrolyte temperatures were lower than 50 *C the weight of the substrate increased due to oxidation. However, for temperatures exceeding 50 'C the substrate weight decreased due to the influence of dissolution. The relative amount of dissolution can be qualitatively measured by the amount of sediment in the electrolyte after the BDA processes. The sediments result from oxidized titanium through the dissolution process (equation (1-16)) and are the same color as the porous oxide titanium structures shown in Figure 4-3a-c. As the electrolyte temperature increases, more sediment is observed. 102 4.4.3. Relationship between wetting properties and surface structure To understand the role of nano-porous structures on capillary transport, TiO 2 nanoparticles were deposited on both bare titanium substrates and micro-porous surfaces by EPD. The capillary pressure and spreading speed were significantly affected by the deposition time (Figure 4-7). On the bare titanium substrate, the capillary pressure was linearly proportional to the deposition time while the spreading speed decreased for deposition times longer than 90 sec. To explain this result, the porosities were obtained from the deposition weights and the deposition heights. The porosity of the EPD surface can be estimated by. Porosity =1 Deposition weight Deposition volume x TiO 2 density The density of TiO 2 nanoparticles is 3.9 g/cm 3 (Sigma-Aldrich, titanium oxide, anatase, nanopowder, < 25 nm particle size, ;> 99.7% purity). The deposition weight increases with EPD time, as shown in Figure 4-7c (red-circles). The deposition volumes were calculated with the average deposition-heights measured using a profilometer (Measurement system: Tencor P-16 Surface Profilometer (TM)). The average deposition-height increases with the EPD time (blackboxes in Figure 4-7c). The porosity calculated by equation (4-7) shows a minimum at the deposition time of 90 sec (blue-diamonds in Figure 4-7c). It is clear that hydraulic diffusivity is inversely proportional to the porosity and the capillary pressure is proportional to the deposition height. A similar result, namely increasing spreading speed with deposition thickness, was also observed in Cebeci et al. [55]. 103 (a) 0.150.14- -10 -*-PCW -8 0.13' -6 0.12- -4 E S0.11CL0.10- -2 0 0.09- P 1% - 0.08 - - - - - - - - - - - - - - - - - - - 2 340 100 60 80 Time (sec) (b) 0.920.88- 120 PCap/F Cap D ID COP Cap 0.84 od 0.80 %0.76 0.72 ---------- 20 35 30 25 60 80 Time (sec) 160 120 00 r%^ +0-Weight -85 Height -0- - .80 SPoroty 20 -75 15 -70 10 ES . (c) 40 -65& 25 -60 40 60 80T16m Time (sec) 120 Figure 4-7. Capillary rise measurements of surfaces modified by EPD. EPD processes was conducted on bare titanium substrates (a) and on surfaces augmented by BDA (b). PCaIP*cap and Dcap/D*cap are the ratios of capillary pressure and spreading speed constant before and after EPD, respectively. EPD was conducted at 30 V and the BDA conditions were 120 V, 30 min, and 25 *C. Porosity of the deposits by EPD was calculated using the deposition weight per unit area and the deposition height (c). The error bars indicate measurements. 104 one standard deviation resulting from three For EPD on micro-porous surfaces produced by BDA, the capillary pressure shows opposite tendency as the bare substrate, decreasing with increasing deposition time. Hydraulic diffusivity behaves similarly to the case of EPD on flat substrates. The nano-porous structures produced by EPD increase the viscous drag resistance; thus, hydraulic diffusivity displays a maximum irrespective of the initial substrate condition. However, micro-porous surfaces with thicker nanoporous layers (longer deposition time) show lower capillary pressures because the capillary pressure is affected by the additional viscous drag through the nano-porous structures. 4.4.4. Surface chemical composition and atomic structure The material compositions and atomic structures of the surfaces were investigated with X-ray diffraction (XRD) (Figure 4-8). The XRD patterns of a bare titanium substrate and TiO 2 deposits were measured (Figure 4-8a-b) to serve as reference surfaces. Notably, the surface produced by the hybrid BDA/EPD method show a combination of titanium and TiO 2 patterns (Figure 4-8c). Further, the amount of oxidized titanium produced by BDA affects the intensity of the peaks shown in Figure 4-8e-i. It is interesting to note that the surface produced at 120 V and 10 C shows nearly no peak (Figure 4-8i), identical to the spectra of the sediment obtained after BDA (Figure 4-8j). According to other studies, the sediment can be assumed to be amorphous titanium oxide which lacks crystal structure [174, 175]. To confirm this hypothesis, we heat treated the BDA surfaces and the sedimentation at 450 *C for 2 hours to initiate crystallization. After heat treatment, the spectra of both samples (Figure 4-8k and 1) are similar to anatase TiO 2 (Figure 4-8b). This result reveals that the porous structures created by BDA are composed of amorphous titanium oxide. At electrolyte temperatures higher than 50 *C, oxidized titanium quickly dissolves into the electrolyte through the reactions in equation (1-16). As a result, titanium peak patterns are observed 105 on these surfaces, as shown in Figure 4-8e and f. It is worth noting that the BDA surfaces produced with high electrolyte temperature show different spectra after prolonged exposure to air (Figure 4-8d). This indicates that the surface titanium slowly oxidizes to titanium oxide. In addition, we found that the wettability is degraded by these transitions. EPD of high surface energy nanoparticles such as TiO 2 and SiO 2 alleviates the effect of surface oxidation in air. We believe that the nanoporous TiO 2 structures contribute to the higher durability of the BDA/EPD surfaces relative to the BDA surfaces. As explained in Cebeci et al. [55] and Drelich et al.[61], superhydrophilic surfaces can be achieved even with materials that have native contact angles higher than 65 - 700 if the surface roughness is extremely high. The nanoporous TiO 2 structures on the BDA surfaces greatly enhance the surface roughness. Therefore, the nano/micro porous structures can maintain their superhydrophilicity longer than the BDA surfaces alone. In addition, we hypothesize that the enhanced durability is due to reduced oxidation of Ti relative to the BDA fabricated structures. To investigate this, we compared XRD data of the BDA/EPD and EPD surfaces immediately after they were produced and again 30 days later. The XRD patterns of the EPD surface did not change after 30 days. This suggests that the chemical composition and atomic structure of TiO 2 nanoparticles are relatively fixed. Therefore, the change in wetting properties of the BDA/EPD surface originated from property changes of the BDA surface underneath the EPD layer. Since the BDA/EPD surface shows higher durability than the BDA surface, we believe that the EPD layer on the BDA surface helped to retard the degradation of wetting properties. We compare the degradation rates of five superhydrophilic surfaces that were produced by BDA with 120 V electric potential at 10 'C, 25 *C, 50 'C, and 75 'C electrolyte temperature. We also examined the BDA/EPD method with BDA at 120V and 10 'C and EPD at 30 V for 90 sec. The five samples were maintained in laboratory air under ambient conditions for 30 days. The degradation rates were calculated by 106 P Deraaiate=1- 4, where Pap.i and Pcap.2 are the initial and the final capillary pressures, respectively, of the surface. Table 4-1 shows the degradation rates of the prepared surfaces. The BDA surfaces produced with different electrolyte temperatures show similar degradation rates and the average degradation rate is 20 %. As expected, the lowest degradation rate was observed from the BDA/EPD surface due to the nanoporous structures, as mentioned above. Table 4-1. Changes in capillary pressure for five superhydrophilic surfaces 30 days after fabrication. 10*C Degradation rate 23% I BDA BDA 25*C 50*C 75*C 17% 21% 21% aIII 107 BDA/EPD 7% Ti Tro Ti Ti TrI T lii t4 I Titanium plate T42 T2 T10 2 a) Ti (a) ~Ti Ti0 2 TiO_ EPD with T2 .1 ( C) I ODA + EPD (d) (d) II BDA 90V SI at 75GC in 30 days I (b) I i 1- (e) BDA 90V at 75 OC MDA 9WV at 50 OC BOA 10V at 10OC Amorphous Titanium (k) Heat treated BDA 120V at 10 GC A(1) I Heat treated amophoua Titanium 10 20 30 40 50 60 70 80 90 Position (20) (Copper (Cu)) Figure 4-8. XRD patterns of the surfaces produced by EPD, BDA, and the hybrid BDA/EPD method. A bare titanium plate and the TiO 2 deposit surface were used as control XRD substrates showing the spectra of titanium (a) and titanium dioxide (b). Surfaces produced by variations of EPD and BDA shows aspects of both Ti and TiO 2 spectra. Sediments obtained after BDA shows very weak peaks (j), which are indicative of amorphous titanium oxide [174, 175]. After heat treatment, the surface used in (i) and the sediments used in (j) show the spectra of anatase TiO 2 as shown in (k) and (1). 108 4.4.5. Relationship between the effective pore radius and surface roughness From equations (4-2) and (4-3) we know that the pore size strongly influences capillary pressure and spreading speed constant. To determine the role of electric potential in BDA, surface roughness was measured using the profilometer. Surface roughness was measured for the substrates used in Figure 4-5 with the exception of the 50 'C and 75 *C electrolytes. At these high electrolyte temperatures BDA resulted in highly interconnected pores as shown in Figure 4-6b; therefore, it is not appropriate to compare with other surfaces. To show the correlations between the wetting properties of the BDA surfaces and the surface structure, first we compare the effective pore radius with the surface roughness, as shown in Figure 4-9a. The effective pore radius can be considered a key parameter to characterize the surface structure. The pore radius can be calculated by equation (4-4) with the values of Pcap and Deap obtained from capillary rise measurements (CRM). In Figure 4-9a, we confirm that the pore radius is roughly linearly proportional to the surface roughness. Capillary pressure asymptotes with surface roughness (and therefore pore radius), as shown in Figure 4-9b. However, hydraulic diffusivity is linear with surface roughness and pore radius (Figure 4-9c). This result reveals that capillary pressure and spreading speed can be tuned with both electric potential and electrolyte temperature because the surface roughness and pore radii are changed. 109 14 - (a) 12 0 30 V. 10 *C * 30 V, 25 -C 060V, 10*c 0 60 V, 25 *C a 90 V, 10 *C A 90 V, 25 *C 0 120 V, 10 *C * 120 V, 25 *C 10 :. 0 8 13 6 0 10 20 3 2 (b) 1.5 030 V, 10 *C /30 V, 25 *C 060 V, 10 *C l60 V, 25 *C A90 V, 10 *C A90 V, 25 *C 0*120 V, 10 *C 0 120 V, 25 *C ................................ .... I 1 0.5 0 0 10 0 30 V, (C)140 20 3C 10 -C * 30 V, 25 *C 0 60V, 10 *C @ 60 V, 25 @C 0 100 a E 80 E E60 90 V, 10 25 A 90 V, :c * 120 + 0~ * 120 V, 10 -C *120 V, 25 *C 40 20 n 5 1~p0) r(Atm) 15 Figure 4-9. Plots of the pore radius (a), the capillary pressure (Pcap) (b), and hydraulic diffusivity (Dcap) (c) with respect to surface roughness. The pore radius obtained by equation (4-4) with Pcap and Dcap shown in (b) and (c) is linearly proportional to the surface roughness (a). The box, circle, triangle, and diamond symbols indicate the electric potentials of 30 V, 60 V, 90 V, and 120 V, respectively, used in the BDA processes. The fill colors indicate the electrolyte temperature, 10 'C (white) and 25 'C (red). Morphological data was obtained with a profilometer (Measurement system: Tencor P-16 Surface Profilometer (TM)). The error bars indicate E one standard deviation resulting from four measurements. 110 4.4.6. Modification of capillary flows From the relation between electric potentials and spreading speed constants (Figure 4-9), we developed superhydrophilic surfaces that have unconventional capillary flows. To vary the electric potentials continuously through an electrode used in BDA, various profiles of electrodes were used as shown in Figure 4-10 from Case 1 to Case 3. In addition, triangle substrates were used to demonstrate unconventional flows can be achieved with different shapes of electrodes (Case 4 and Case 5 in Figure 4-10). Same size (10 mm x 70 mm) of rectangular electrodes was used in Case 1-3. Constant electrode gap distance (10 mm) was used in Case 1 and 4-5. To show the effect of nanoporous layers on capillary flows, TiO 2 nanoparticles were partially deposited by EPD with constant electric potential (30 V) for 5 min on the surface produce by BDA with the electrode profile given in Case 3. With the electrodes designed in Figure 4-10, various unconventional capillary flows were obtained (Figure 4-11). The results of CRM show superhydrophilic surfaces where square of capillary height can be non-linear in time. Case 2 shows faster spreading speeds than Case 1 for short times but becomes slower over time. We attribute this result to pore radius changes resulting from the varying electrode gap distance. Notably in Case 3, the surface with partial TiO 2 deposit via EPD, there is a discontinuous change of spreading speed. This shows that the nano-porous structures produced by EPD can locally change the spreading speed. Cases 4 and 5 show that capillary flows can depend on the electrode shapes and orientation. For Case 4, hydraulic diffusivity increases with time, while Case 5 is nearly identical to Case 1. The trend lines in Figure 4-11 are linear and quadratic fitting lines of the experimental data. However, theoretical predictions for the capillary flows are possible but beyond the scope of this paper. We are now preparing a follow up paper investigating the effects of electrode configurations and shapes on capillary flows. The 111 approach to predict the capillary flows is as follows. The capillary pressure and hydraulic diffusivity are functions of the rise height (h shown in Figure 4-10) depending on the configuration and shape of the electrode in the BDA method. As a result, we can formulate the general capillary flow equations in terms of the capillary pressure and spreading speed constant. Case I Case 2 Case 3 Case 4 Case 5 10 mm Triangle electrode Triangle electrode 55 mm 50mm quadratic E E E E0 a. E E E E E so CO linear h 10m M 10mm 55mm 50mm Figure 4-10. Schematic illustrations of electrode gap-profiles and electrode shapes used to demonstrate capillary transport modification with BDA and EPD. Case 1 was used as a reference with a constant gap-distance (10 mm) and a rectangular electrode shape (10 mm x 70 mm). In Cases 2 and 3 the electric potentials vary with gap distance between anode and cathode electrodes, with identical electrode cross sectional area as Case 1. In Case 3, TiO 2 nanoparticles were partially deposited on the surface produced by BDA using EPD. In Cases 4 and Case 5, triangular electrodes were used with the same electrode gap-distance as Case 1 (10 mm). 112 25 p foPP 15 0' - 20 10 Dca 1: eMA (sOW, 25-C) 5 dV OCMs 1: BDA (9w., 10*C) 2: BOA (sOW, 26*C) *Case 3: BDA (OWV, 250C) #Case 3: BOA (OW., 250C) + EPD 80we *Case 4: 4M (90V, 10*C) *Case 5: BOA (qDv, 10-C) 0 0 20 40 so so 100 Time (sec) Figure 4-11. Square of measured capillary rise height with respect to time for superhydrophilic surfaces produced by the BDA and EPD methods presented in Figure 4-10. In the BDA processes, two different electrolyte temperatures (10 C and 25 C) and one electric potential (90 V) were used. In the EPD process, 30 V was supplied for 5 min to deposit Ti02 nanoparticles. In Cases 2-4 the square of capillary height is nonlinear in time, unlike conventional capillary flows. With augmentation via EPD, the spreading speed can be discontinuously changed, as shown in Case 3. 4.5. Conclusions Superhydrophilic surfaces with hierarchical micro- and nano-scales pores were produced a hybrid BDA/EPD method. BDA creates micro-scale porous structures on titanium substrates. BDA morphology is a function of electric potential and electrolyte temperature in constant pH electrolytes. We found that there were optimal values of electric potential and electrolyte temperature to obtain high capillary pressure and spreading speed. The electrolyte temperature affects both the breakdown potential and the thickness of the resulting oxide layer. High electrolyte 113 temperatures decrease the breakdown potential but also decrease the oxide layer thickness due to dissolution. These opposing contributions result in an optimal electrolyte temperature for a given electric potential. Amorphous titanium oxide layers provide high surface energy to the micro-porous structures, resulting in highly wetting surfaces. Nano-porous titanium dioxide layers were deposited on the micro-porous structures produced by BDA using EPD. With the EPD process, capillary pressure and spreading speed were precisely controlled by the deposited mass. The nano-porous films offer stability to the BDA surfaces. The surfaces produced by the hybrid method showed significantly higher capillary pressure and spreading speed than the surfaces produced by EPD alone. Ultimately the characteristics of the superhydrophilic surfaces are mainly controlled by BDA but the EPD films can offer various surface energies and nano-structures to micro-porous surfaces produced by BDA. We have also presented various methods to achieve unconventional capillary flows. These capillary flows showed varying spreading speed constants at different locations on the substrate. The methods presented can be used to create low-cost superhydrophilic surfaces with specially tailored transport properties. Furthermore, the hybrid BDA/EPD method has great potential to be used for various applications requiring high surface area or a large number of reacting sites. Numerous material compositions and nano-structures can be effectively altered by EPD with different kinds of nanoparticles, depending upon the desired application. 114 Chapter 5. Hybrid Electrophoretic Deposition with Anodization Process for Superhydrophilic Surfaces to Enhance Critical Heat Flux Reproduced in part with permission from Young Soo Joung, Cullen R. Buie, "Hybrid Electrophoretic Deposition with Anodization Process for Superhydrophilic Surfaces to Enhance Critical Heat Flux," Key EngineeringMaterials,Vol. 507 (2012) pp. 9-13. Copyright 0 2012 Scientific.net. All rights reserved. 115 5.1 . Introduction Pool boiling is a heat transfer method with high heat transfer rates attributed to phase change and high mass transfer through bubble generation. In pool boiling, enhancement of critical heat flux (CHF) has been considered the most important issue to not only enhance the heat transfer efficiency but also escape from burn out incidents [176]. At high superheat temperatures, liquid vapor covers the entire heater surface and abruptly increases the thermal resistance, resulting in system failure. Another challenge is to reduce the boiling inception temperature (BIT) in order to achieve high heat transfer rates at temperatures closer to the liquid saturation temperature. In heat exchanger systems operating near liquid saturation temperature, BIT has a stronger influence on heat transfer efficiency than CHF enhancement. Methods to enhance CHF can be classified in two categories. The first is modification of the working fluids [177]. For instance, CHF can be enhanced with nanofluids, which are mixtures of liquids and nanoparticles. Different CHF values are shown during pool boiling depending upon the specific nanoparticles utilized. The mechanism of CHF enhancement is the deposition of nanoparticles on the heater surface, resulting in changes in the wettability and porosity of the heater surface [57]. This approach can provide an easy way to enhance CHF but it exhibits limited control. The second method is the direct modification of the heater surface. Theoretically, CHF is the function of roughness and surface energy of the heat transfer surface. Porous structures and various wetting surfaces have been investigated to improve CHF [178]. Both approaches rely upon altered wettability to enhance CHF and it has been shown that hydrophilic surfaces typically show higher CHF values than hydrophobic surfaces. However, to date porous superhydrophilic surfaces with high capillary pressure have not been investigated and BIT has rarely been considered in previous work related to CHF. 116 In this work, electrophoretic deposition (EPD) and break down anodization (BDA) were utilized to produce surfaces with varying wettability from superhydrophilic to superhydrophobic surfaces in order to enhance CHF and reduce BIT. Recently, we have developed novel methods to produce superhydrophobic surfaces with EPD and superhydrophilic surfaces with BDA [73, 179]. Superhydrophilic surfaces produced by BDA show high CHF with high BIT values. To reduce the BIT of the superhydrophilic surfaces, hydrophobic nanoparticles were deposited on the superhydrophilic surfaces in order to provide nucleation sites. With the hybrid method, unconventional surfaces with mixed wettability are obtained, resulting in higher CHF and lower BIT. 5.2. 5.2.1. Experimental Hybrid method of electrophoretic deposition of TiO 2 nanoparticles and breakdown anodization of titanium plates BDA and EPD were used to prepare heat transfer surfaces. In the BDA process, the pH of DI water was adjusted to pH 3 with acid (Nitric acid, 70 % ACS reagent, Sigma-Aldrich). Two titanium plates (Titanium foil (99.7%), 0.05 mm Thickness) were used as cathode and anode electrodes and electric potentials up to 90 V were applied for 10 min. For the EPD method, PDMS modified Si0 2 nanoparticles (14 nm, PlasmaChem) in a mixture of 90% methanol and 10% DI water by volume were used to make 1 g/L concentration Si0 2 suspensions. Titanium plates were again used as anode and cathode electrodes. An electric field of 30 V/cm was subjected to the electrodes for 30 seconds to deposit nanoparticles on the substrate. Ultimately three kinds of heat 117 transfer surfaces were prepared, superhydrophobic, superhydrophilic, and mixed wettability surfaces. Superhydrophobic surfaces and superhydrophilic surfaces were produced by the EPD and BDA processes, respectively. Both BDA and EPD were employed to create the mixed wettability surface. 5.2.2. Capillary rise measurement Capillary rise experiments were used to evaluate the superhydrophilicity of the prepared surfaces in terms of capillary pressure and spreading speed [179, 180]. In summary, the capillary pressure, Pap, can be calculated from the maximum capillary height using the equation of Pcap = [2ycosOIR]= H,,xgp, where H.,x is the maximum capillary rise height, y is liquid surface tension, 0 is a native contact angle, and p is the liquid density. Hydraulic diffusivity, Dap, can be obtained from the equation h 2 = (Rycos/27)t = Dcapt, where h is the rise height, I is the liquid viscosity, and t is time. The morphologies of prepared surfaces were characterized with a scanning electron microscope (SEM, JEOL 6320FV Field-Emission High-resolution SEM). A goniometer (Kyowa, DM CEl) was used to dispense and image 3 pL drops of DI water on each sample. Static contact angles (CA) were calculated using the tangential curve-fitting method. A digital camera was used to record bubble dynamics on the heat transfer surfaces. 5.2.3. Critical heat flux in pool-boiling CHF tests were conducted with a custom built pool-boiling experimental system composed of a heater assembly, a water bath, and a temperature control unit. The heater assembly includes thermocouples positioned on three different distances from the top in order to calculate heat flux 118 and uncertainty [181]. The heat flux, q ", can be calculated with the equation q" = kA T/Ax, where k is the thermal conductivity of heat transfer surface, AT is the temperature difference between thermocouples with a separation distance Ax. The uncertainty in heat flux can be estimated by the equation Eq" = q"[(Ex/Ax) where ex is the distance uncertainty between the thermocouples and ET is the temperature uncertainty. The water bath was maintained at the saturation temperature, 100 'C, during CHF tests and the water level was kept constant with a condenser to recycle water vapor. 5.3. Results and discussion Table 5-1 shows the wetting characteristics of the superhydrophobic, superhydrophilic, and mixed wettability samples including untreated titanium as a reference. Sample 1, produced by EPD of PDMS modified Si0 2 particles showed a very high contact angle of 168*, confiming it's superhydrophobicity. Sample 2 was prepared by the BDA process and it showed a contact angle near zero with a high capillary pressure and spreading speed constant. As expected, sample 3 displayed mixed wettability. The average contact angle on the surface was 280 but the capillary pressure and hydraulic diffusivity were 538 Pa and 9.36 mm2/s, respectively. This suggests that the layer of hydrophobic SiO 2 particles increased the contact angle while still allowing liquid transport along the surface. 119 Table 5-1. Evaluation of prepared surfaces with BDA and/or EPD methods. Reference Method Bare Ti plate Sample 1 EPD Sample 2 BDA Sample 3 BDA and EPD Contact angle 550 1680 00 280 Capillary pressure (Pa) Spreading speed constant (mm2/s) n/a n/a n/a n/a 851 31 538 9.36 SEM images in Figure 5-1 illuminate the mixed behavior of sample 3. Nanoporous layers in Figure 5-la were observed with the samples produced by EPD while micro porous structures were observed in the samples prepared by the BDA process, as shown Figure 5-lb. The dual scale micro and nano porous structures in Figure 5-1c were produced by the hybrid method. Figure 5-1. SEM images of prepared surfaces; (a) superhydrophobic surface by EPD, (b) superhydrophilic surface by BDA, and (c) mixed wetting surface by the combined BDA/EPD process. 120 Table 5-2 shows the CHF test results. The untreated titanium surface showed CHF and BIT of 441 kW/m 2 and 114*C, respectively. Conversely, BIT was 140 'C on the superhydrophilic surface produced by the BDA process. The higher BIT could be explained with well-known boiling theory. BIT can be estimated by the equation Psat(T) = Po+2a/R, with the assumption that the contact angle is 90* on the surface and the gas pressure is ignored. Psat(T) is the saturation pressure inside a bubble at the wall temperature Ts, P. is the ambient pressure around the vapor bubble, a is the liquid surface tension, and R, is the critical bubble departure radius [176]. Table 5-2. CHF results for modified surfaces. Reference: an untreated titanium surface. Sample 1: a superhydrophobic surface produced with EPD. Sample 2: a superhydrophilic surface produced by BDA. Sample 3: a hybrid surface produced by the coupled BDA and EPD method. Boiling Inception Superheat (AT) Critical Heat Flux (kW/m2 ) Critical Superheat (AT) Reference 14 441 81 Sample 1 5 598 78 Sample 2 40 536 148 Sample 3 10 507 176 On superhydrophilic surfaces (Sample 2), the contact angle is nearly zero; therefore the assumption of 900 contact angle must be reconsidered. In this case the nucleation site becomes smaller, resulting in a higher BIT. The higher CHF on the superhydrophilic surface can be explained with the maximum heat flux equation, q,,x ~ hfgpVmax, where hfg is the liquid latent heat, p, is the liquid density and Vmax is the maximum vapor velocity. V,,, can be approximated by V,,, ~ (u/pvL)'12, where Lc= [u/(pt-pv)g]112 is the characteristic length of heater assuming a 90* contact angle on the surface [176]. Considering superhydrophilic surfaces, the characteristic length of Lc become smaller, increasing the maximum velocity and resulting in increased maximum heat flux. 121 Conversely, the BIT of the superhydrophobic surface (Sample 1) was close to the water saturation temperature at 105 *C. We believe that during boiling vapor bubbles trapped in the superhydrophobic surface merge to find local energy minima, resulting in macro scale bubbles. Therefore R, becomes on the order of a few millimeters so that the term 2a/Re can be neglected, resulting in Psa,(T) ~ P. Interestingly, the CHF on the superhydrophobic surface was larger than the superhydrophilic surface at the low superheat. This phenomenon is an ongoing area of research in our group and we speculate that bubble generation on the superhydrophobic surface dramatically enhanced the heat transfer coefficient at low superheat regions [182]. Sample 3, which had nano/micro porous structures with different wettabilities, showed a low BIT of 110 *C but the film boiling was effectively delayed up to the high superheat of 176 *C, exhibiting the high CHF value of 507 kW/m 2. Fast nucleate boiling was attributed to the hydrophobic nanoparticles providing additional nucleation sites. 122 Figure 5-2. Boiling behavior on the prepared surfaces. Images of the superhydrophobic surface of sample 1 at the saturation temperature (a), the boiling inception temperature (b), and the nucleate boiling region (c). Images of the superhydrophilic surface of sample 2 at the saturation temperature (d), the boiling inception temperature (e), and the nucleate boiling region (f). Images of the hybrid wetting surface of sample 3 at the saturation temperature (g), the boiling inception temperature (h), and in the nucleate boiling region (i). The former descriptions can be verified by observing bubble generation during boiling. Figure 5-2 shows the bubble generation on the modified surfaces. On the superhydrophobic surface vapor bubbles were generated at the water saturation temperature of 100 *C as shown in Figure 5-2a. Boiling inception at 105 'C was observed on Sample 1 with vapor bubbles larger than other surfaces, as shown in Figure 5-2b. On the superhydrophilic surface, Sample 2, bubbles were not be until 140 'C and boiling began abruptly as shown in Figure 5-2e. On the hybrid wetting surface, Sample 3, small bubbles were observed at the water saturation temperature as shown in Figure 5-2g. This 123 confirms that the hydrophobic layer provides additional nucleation sites. Aided by hydrophobic nanoporous structures, boiling inception occurred at the low temperature of 110 *C. We conclude that on the hybrid wetting surface the nanoporous hydrophobic layers provide numerous nucleation sites, reducing BIT, while the microporous hydrophilic layers enhance CHF by preventing film boiling. 5.4. Summary A novel surface with low BIT and high CHF was produced by a hybrid EPD and BDA method. The superhydrophobic surface displayed low BIT, which is attributed to numerous nucleation sites on the nanoporous structures. The superhydrophilic surface fabricated by BDA did not generate vapors even at superheats up to 40 *C due to limited nucleation sites on the surface. Low BIT and high superheat at CHF were observed on the hybrid wetting surface produced by the hybrid BDA and EPD process. The resulting mixed wettability surface was composed of a hydrophobic nanoparticle layer on top of a superhydrophilic layer. The hydrophobic nanoparticle layer provides nucleation sites while the superhydrophilic layer prevents film boiling, resulting in lower BIT and higher CHF than untreated titanium surfaces. This work demonstrates the potential to use processes based on EPD to enhance heat transfer surfaces. 124 Chapter 6. Design of Capillary Flows with Functionally Graded Porous Films Controlled by Anodization Instability Reproduced in part with permission from Young Soo Joung, Bruno Michel Figliuzzi, Cullen R. Buie, "Design of Capillary Flows with Functionally Graded Porous Titanium Oxide Films Controlled by Anodization Instability," Journal of Colloid and Interface Science, 2014. Copyright C 2014 Elsevier B.V. All rights reserved. 125 6.1. Introduction Capillary flows through thin porous media have been widely utilized for small liquid transportation systems such as chromatographic analyzers, biosensors, micro chemical reactors, and paper-based microfluidic devices [183-190]. These devices are generally cheap and simple because additional pumping systems are not necessary to make liquid transportations. Furthermore, the capillary flows are predictable and effectively scaled down; therefore, they are appropriate to develop small devices with precise liquid deliveries. To take these advantages of capillary flows, many different porous media have been applied to the devices because capillary flows show different characteristics depending on the media; examples include capillary tubes and wedges, micro-fabricated structures, porous media composed of small particles or fibers, and sponges [61, 191]. However, in most media it is difficult to tailor the capillary flow for a given application because it is challenging to realize arbitrary shapes and spatially functionalized micro-scale and nano-scale structures. Therefore, the media mentioned above can only be used for conventional capillary flows obeying Washburn's equation and the modifications thereof [180, 192, 193]. Given background, new fabrication techniques and corresponding capillary flow equations of motion must be developed for more demanding applications. In the literature, equations of motion for capillary flows have been derived from the NavierStokes equations, or modifications thereof [172, 180, 192-194]. These approaches result in Washburn's classical equation for one-dimensional capillary flows assuming uniform capillary pressure, no inertial-effects, and constant channel cross section [172]. This equation can be used to estimate capillary flows through wetting porous media with uniform porous structures and surface energy. However, Washburn's equation cannot be used to estimate the capillary flows for porous media with spatial variations in surface energy, pore radius, and cross sectional area. Eeyssat et al. 126 recently developed an imbibition model aimed at studying capillary flows for porous media with varying circular cross-section, demonstrating that shape variations affect the dynamics of the capillary rise [195]. Recently, we developed a method to produce highly wetting porous surfaces using anodization instabilities [196]. During conventional anodization, oxide film thickness increases linearly with time [75]. However, when the electric potential of anodization is higher than a critical value, the anodized films burst due to ionic shear stress that exceeds the surface tension of the films, resulting in irregular structures on the substrate [90]. This fabrication method is known as breakdown anodization (BDA). Surfaces produced by BDA on titanium plates show extremely low effective contact angles (nearly zero) and fast water spreading. These properties result from the highly porous surface composed of amorphous titanium dioxide, which provides high surface energy. In addition, we've shown that capillary flows on surfaces produced by BDA can be expressed by Washburn's equation [196]. However, if porous structures of the surface are non-uniform, the capillary pressure changes locally, resulting in varying propagation speeds. In this case, the capillary flows do not follow Washburn's equation. Therefore, we modify the equations of motion to account for local variations in capillary properties. Capillary pressure and spreading speed are well-known parameters for characterizing porous materials such as packed beads [180]. Both parameters can be obtained from capillary rise measurements (CRM). Capillary pressure and spreading speed depend on the surface energy and bead size and are not generally proportional to each other. Thus, we consider them separately for evaluating superhydrophilic surfaces. In this study we utilize CRM to characterize superhydrophilic surfaces. The surfaces can be considered as bundles of capillary tubes with various tube diameters [172]. 127 In this work, for the first time, an electrochemical fabrication method using anodization instability is presented for the design of capillary flows with functionally graded porous films controlled by electric fields. Additionally, we derive capillary flow equations of motion to predict the transport in these systems. First, we suggest a simplified conceptual model of highly wetting porous surfaces. Then, we derive a general capillary flow equation of motion from the conceptual model for predicting capillary speeds on heterogeneous surfaces. Finally, the capillary flows are expressed as functions of the local capillary properties of capillary pressure and capillary diffusivity, which are functions of capillary rise height. With the fabrication method and the theoretical model, we show that various capillary flows can be designed by BDA with spatially variable electric fields. 6.2. Theory From the Navier-Stokes equations, if inertial effects are negligible and the flow is laminar, Poiseuille's law gives the average flow velocity in a single cylindrical tube: r2 V,= ap- -+pg 8p o'y (6-1) where Vavg is the average flow velocity over the entire cross sectional area of the tube, r is the tube radius, aP/Sy is the pressure gradient in the flow direction y, g is the gravitational acceleration constant, and p and p are the liquid viscosity and density, respectively. From equation (6-1), one-dimensional capillary flow in a uniform cylinder of radius r, can be described by Washburn's equation [172], 128 -= dt -27cs pgh h 8p r (6-2) where h denotes the height reached by the liquid in the capillary, t the time, 0 the surface contact angle and y the air-liquid surface tension. From equation (6-2), when the height h of the liquid is small enough, the hydrostatic pressure can be neglected, and we can obtain a simple expression of the capillary rise height as a function of time, = 2p Dt (6-3) 2 Here, we introduce the capillary diffusivity Dcap = rcycos9/p, which has units of m2/s. When the properties of the material are uniform in the capillary, the surface contact angle remains constant and Dcap remains constant. Similarly, the capillary pressure, Pcap, can be calculated from the maximum capillary rise height, hmax. At equilibrium, the gravitational force on the liquid column balances the capillary force. Thus, the capillary pressure can be calculated as 2ycos9 P= r hrgp (6-4) From equations (6-3) and (6-4), the capillary tube radius (re) and the surface contact angle (0) can be obtained as follows: rc = (2puDcap/Pcap) and cosO= (2DcapPcap/yL). We model highly wetting porous films as multi-layered capillary tubes oriented in the flow direction, as shown in Figure 6-1. Similar capillary tube models have been employed to simplify 129 N - flows in porous media [197] but here we newly suggest a capillary tube model for wettable porous films, which have varying surface width and capillary properties. The imaginary capillary tube has the effective radius r. The number of capillary tubes (N,.y) present at the capillary height, y, can be obtained using, N 2R, HcRH = 2r 2"2,2 (6-5) where Ry and He are the half width and the thickness of the film, respectively, at the capillary height of y. Therefore, the total cross-sectional area of capillary tubes (Ay) can be expressed as A =r 2 N. The volumetric flow rate at a given cross-section of the porous film can be calculated as (6-6) 2 -V, Combining both equations (6-1) and (6-6), the relation between the pressure gradient and the volumetric flow rate can be found to be: 8 Aiy +p ) = 2(6-7) 7rRHc From mass conservation, we know that the volumetric flow rate is not a function of capillary height. In addition, the local capillary radius can be expressed as a function of the capillary pressure and capillary diffusivity as re., = (2pDcap./Pcap.y)"', where the subscript y indicates a function of capillary 130 height. Then, we can integrate both sides of equation (6-7) and derive the relation between capillary pressure and volumetric flow rate: 16-V IP~aP dy + pgh f0h ca.=h RH DaY (6-8) At y = h, the volumetric flow rate is -= 7rRH dh 2 dt (6-9) Therefore, we can obtain the general capillary flow equation of motion in wettable porous films as the following: dh dt Pa,. - pgh R (6-10) h ,, R, Dca, If the porous material has uniform wetting properties and constant cross sectional area, equation (10) becomes identical with Washburn's equation, as shown in equation (6-2). 131 77 T ube radius: r\ Y II \ ' I I \ Flow direction Figure 6-1. Schematic illustration of the conceptual model for wettable porous films with varying cross sectional area. The porous film has the constant height, He, and varying half width, R. The porous film can be modeled as multi-layered capillary tubes vertically oriented in the capillary height, y. The number of the capillary tubes in x-direction and y-direction is varied according to the radius of a single capillary tube, re, and the width of R. Equation (6-10) can be further modified when the surfaces have either a constant width or uniform wetting properties throughout the volume. First, if we assume constant capillary pressure 2 , and capillary diffusivity, but the surface width changes linearly in the flow direction: Ry=Ciy+C where C1 and C2 are constants, equation (6-10) becomes 2cos 0pg r 2Y dh dt rC 8p C1 (6-11) 1+ Eh In C2 )(C2 I+ C'h 132 When C h << C2 , equation (6-11) can be simplified into V = dh = r2P -P C - pg). (I- C'h (6-12) . As a result, different capillary flow speeds can be obtained depending on the values of C1 and C2 When the surface width has an exponential form such as Ry= C2 ecrY, where C2 is the surface width at y = 0 and C1 is a constant value, the capillary speed can be expressed as follows: V = ,P-pgh- q tc8Ai 8 Vh eq'h-1 (6-13) Therefore, the capillary flow can be altered by the constant C1 . When Clh << 1, equation (6-13) simplifies to V dh dt r\ 8 up =-=-(P,-pgh)- h(I +Ch) (6-14) Second, in the case of constant surface width, and varying capillary pressure and capillary diffusivity, equation (6-10) becomes dh _Pp,,hdt 4 Pgh hPcapy dy 0 D 133 (6-15) Therefore, the flow velocity can be expressed as a function of both Pcap.y and Dcap.y. When the capillary pressure and the capillary diffusivity vary linearly with rise height and have the forms of Pcap.y=Pjy+P 2 and Dcap.y=Diy+D2 , respectively, equation (6-15) simplifies to dh dt PJh+P2-pgh + (P 2 D 12 4f:' SD2 D. YD2)y +1 - (6-16) dy If Pjh <<P 2 and Djh << D2, equation (6-16) becomes dit = 1P P+-pg= 4 P2 P+-pg h (6-17)(2f 8p h here, D2 /P2 = rc2 /2p. As a result, we can control capillary flows by altering the capillary pressure . based on the constants P, and P2 In the conceptual model of porous wetting surfaces (Figure 6-1), it is assumed that the porous surfaces behave like bundles of capillary tubes (Figure 6-2). Therefore, the control volume for equation (6-10) can be determined depending on the surface shapes, as shown in Figure 6-2. The rule of determining the control volume is that the conceptual capillary tubes that have been fully filled with water are excluded from the control volume. As a result, wettable porous surfaces that have decreasing width show the same capillary rising behaviors as constant width surfaces if we can ignore evaporation on the surfaces. 134 Evaporation must be considered when the surfaces have varying cross sectional area and the rise height is high. If a surface has an evaporation rate of e [kg/s-m 2 ], mass conservation can be expressed as V pA, -f y=h 4 -2Rdy=pKAh (6-18) Therefore, the capillary flow speed V can be expressed as follows: V =V Rh y ,, 4Qf-IR,,dy + _yy prRH, (6-19) Equation (6-19) can be used instead of V.g in equation (6-6) to derive the capillary flow equation (equations (6-10)) from equation (6-7) when accounting for evaporation. However, the use of equation (6-19) complicates calculation of the rise height; therefore, commercial mathematics software (Maple 16) was used to solve equation (6-10) when equation (6-19) was applied. 135 (a) FTF Partially filled capillary tubes (b) Partially filled capillary tube V- Wicking surfaces Fully filled capillary tube Control volume Capillary rise height rWater Capillary rise height Figure 6-2. Schematic illustrations of control volumes depending on the shapes of wicking surfaces. The wicking surfaces can be conceptually modeled as vertically packed capillary tubes. (a) The control volume covers the whole wetted area when the capillary width increases with respect to the capillary height. (b) The wet area that has the same column width as the top wetting line is considered as the control volume when the wicking surface has decreasing width. 6.3. 6.3.1. Experimental Fabrication method Breakdown anodization (BDA) was used to produce highly wetting porous surfaces [196, 198]. For the BDA electrolyte, DI water was adjusted to pH = 3.0 with nitric acid (70 % ACS reagent, Sigma-Aldrich). Electric potentials up to 120 V were applied for 30 min during the anodization process. The electrolyte temperature was maintained at a constant value during the BDA process using a water-circulating bath (Polystate, Cole-Parmer). Titanium plates (Ultra-Corrosion-Resistant Titanium Grade 2, 0.020" thick, McMaster) were used as anode and cathode. 136 6.3.2. Sample characterization Capillary rise measurements (CRM) have been used to determine wetting properties of BDA surfaces including capillary diffusivity and capillary pressure. Static contact angles (CA) were calculated using the tangential curve-fitting method. A goniometer (Kyowa, DM-CE1) was used to dispense and image a 3 pL drop of DI water on each sample. Droplet spreading on the surfaces was recorded with a high speed camera (FASTCAM SA5, Photron USA, Inc.). The surface roughness (Ra) was obtained with a profilometer (Tencor P-16 Surface Profilometer (TM)). Morphologies of BDA surfaces were characterized with a scanning electron microscope (SEM, JEOL 6320FV FieldEmission High-resolution SEM). 6.4. Results and discussion Contact angles were measured on surfaces produced by BDA under the following conditions; anodization potential of 90 V, 30 min duration, and electrolyte temperature of 25 *C. When a droplet contacts the surface, the bottom of the droplet rapidly expands in the radial direction the droplet is separated into two distinct volumes (Figure 6-3). The liquid in contact with the surface spreads rapidly, illustrating the near zero effective contact angle. 137 Figure 6-3. Droplet spreading on the surface produced by breakdown anodization (BDA). BDA was conducted with 90 V for 30 min in pH 3.0 electrolytes at 25 'C. The droplet suspended from the needle is immediately separated into two parts after contacting to the surface, finally showing almost zero contact angles. The scale bars in the images indicate 1 mm length. Surfaces produced by BDA showed highly irregular and porous structures, as shown in Figure 6-4a. Interestingly, the micro-porous structures have fish-scale like sub-micron features (Figure 6-4b). Notably, inside the amorphous titanium oxide layers, we can observe multiple layers of sponge-like capillary channels in the cross-sectional SEM images (Figure 6-4c-e). We found that the porous structures are composed of amorphous titanium oxide, [198] which has high surface energy [199], so the micro- and nano-structured surface can greatly enhance the wetting properties. 138 Figure 6-4. SEM images of surfaces fabricated by breakdown anodization (BDA). BDA was conducted at the electric potential 120 V for 30 min with an acidic electrolyte (pH 3.0) at 10 'C. The surface shows highly irregular micro porous structures (a)-(b) that have submicron scale-like structures on the surface. The cross sectional area of the surface is composed of multi-layers of amorphous titanium oxide, which have micro-scale wicking channels (c)-(e). From the results of CRMs (Figure 6-5), we can confirm that the square of capillary height is linearly proportional to time for the BDA surfaces, as expected from Washburn's equation. Therefore, we can obtain the capillary pressure (Pcap) and the capillary diffusivity (Dcap) from the CRM data of each BDA surface using equations (6-3) and (6-4). It is worth noting that the BDA surfaces show approximately ten times higher capillary diffusivity and three times higher capillary 139 pressures than the commercial TLC plate, which is composed of 25 ptm silica particles with 200 ptm thickness. The relatively large micro-scale pores and the channel-like nanostructures (Figure 6-4) contribute to the high spreading speed and capillary pressure. (b) 120 V at 10 *C 40 35 O 120Vat 25 30 25 8 60 V at 25 *C C o 15 0o 10 111" a' "0. U 0 0 5 15 20 25 Time (sec) 30 35 025 C +506C 475 *C -+75*C 1 100 0.5 50 0 10 010 *C ..- 15 &500C d cv. (C) 200 *C 1.5 * TLC plate 20 *10 OC 2 -25 30 60 90 Poenil0 Potential 120 150 0 30 60 90 Potential 120 150 15 ( (a) Figure 6-5. Results of the capillary rise measurements with highly wettable porous films produced by breakdown anodization (BDA). As expected from Washburn's equation, the square of capillary height is linear to time at initial region of rising time. The maximum capillary diffusivity is 155 mm2/s and the maximum capillary pressure is 1.75 kPa. These values are ten times and three times higher than the commercial wicking sheet composed of micron SiO 2 particles (200 tm thickness of 25 ptm silica particles). Samples were produced by BDA with titanium plates (10 mm x 250 mm) in pH 3.0 electrolytes. The gap distance between anode and cathode electrodes was 10 mm. The error bars indicate one standard deviation resulting from four measurements. These high capillary pressure and fast capillary diffusivity are obtained when the anodization potential is high and the electrolyte temperature is low, as shown in Figure 6-5b and c. This is due to the correlation between the capillary properties (capillary pressure and capillary diffusivity) and the surface roughness. The surface roughness decreases at high electrolyte temperature due to an increased oxide dissolution rate. Therefore, the capillary pressure and capillary diffusivity are relatively constant when the electrolyte temperature exceeds 50 'C (Figure 6-5b and c). However, the capillary pressure is increases with anodization potential when the electrolyte temperature is less 140 than 25 *C. The capillary diffusivity also increases with anodization potential at low electrolyte temperatures, except for 120 V at 10 *C. This deviation is due to the decrease in effective pore radius resulting from the low oxide dissolution rate. This mechanism has been verified in our previous work [198]. We explore two ways to alter capillary flows; surface width (e.g. cross sectional area) and the electrode gap profiles. BDA surfaces produced by parallel electrodes have uniform capillary pressure and capillary diffusivity on their surfaces due to the uniform electric field. However, to alter capillary flows, the capillary pressure and capillary diffusivity must vary with rise height, as shown in equation (6-10). We also see that the capillary flow can be manipulated by changing the surface width. Under given options, first, we demonstrate capillary flows through surfaces produced by BDA with various electrode shapes but uniform electrode spacing. In this case, the capillary properties are constant but the surface width can be expressed as a function of the rise height. Next, we use varying gap distance between electrodes instead of parallel electrodes to vary the local electric field, resulting in spatially dependent capillary properties. The distance profile between anode and cathode electrodes is expressed as a function of the capillary rise height. 6.4.1. Case 1: Capillary flows with constant capillary properties and varying surface width Capillary flows were investigated on BDA surfaces with varying surface width. In addition, chromatography papers (Gel-Blotting paper, 340 tm in thickness, Genesee Scientific) were compared with BDA surfaces to show the general use of the conceptual model suggested. When the electrode shape is a linear function of the rise height (R(h) = Clh + C 2 ), as shown in Figure 6-6a-b, the capillary flows can be expressed with equation (6-12). If the electrode shapes vary exponentially 141 with rise height, e.g. R(h) = C2 eC (Figure 6-6c-d), the capillary speed can be found from equation h (6-14). In both cases the capillary speed can be controlled by the constants C1, and C 2 . As shown in Table 6-1, various electrode shapes with different values for Ci and C2 were used for surface modification via BDA. Certain anodization conditions were held constant: electrolyte pH 3.0 and temperature of 25 'C, 10 mm electrode spacing, and 90 V electric potential applied for 30 min. Therefore, all BDA surfaces had the same capillary pressure and capillary diffusivity of 1.2 kPa and 133 mm 2 /s, respectively. For comparison, the chromatography papers were cut to the same shapes as the electrodes shown in Figure 6-6 and Table 6-1. The capillary pressure and the capillary diffusivity of the chromatography paper are 1.4 kPa and 18.7 mm 2 /s, respectively. Refere nce Type A Type B Type C TY pe D R(h) R(h) Th R(h) R(h) _ - R(h)=Clh+C R(h)=Cc," 2 Figure 6-6. Shapes of the BDA surfaces and the chromatography papers (Genesee Scientific) used for the capillary rise measurements. Type A and B have linearly changed shape functions, and Type C and D have exponentially changed shape functions. All BDA processes had the same condition of electrolyte (pH 3.0 and 25 *C), electrode gap distance (10 mm), and electric potential (90 V for 30 min). 142 Table 6-1. Shape functions of the BDA surfaces and the chromatography papers used to verify the alternation of capillary flows. As a reference, a BDA surface was produced with two parallel electrodes, which have a constant width of 10 mm and an electrode gap distance of 10 mm. Parameter Function [mm] Reference Type A-a Type A-b Type B-a Type B-b Type C-a Type C-b Type D-a Type D-b R(h)=Clh+C 2 R(h)=C 2 eCh I C, C2 0 10.0 12.5 25.0 5.0 5.0 10.0 2.5 66.4 44.8 -0.075 -0.2 +0.075 +0.2 +20 +40 -20 -40 Equation (6-10) was compared with the corresponding experimental data for the electrode shapes shown in Table 6-1. In the theoretical calculations, a water evaporation rate of 8.5 x 10-6 kg/m 2s % was employed (experimentally obtained in a typical laboratory atmosphere at 25 'C and 32 humidity) in equation (6-19). However, in the chromatography papers, water evaporation is negligible because the papers are relatively thick to the BDA surfaces; therefore, at a given height the evaporation rate is small relative to the flow rate. As shown in Figure 6-7, the theoretical calculations show good agreement with the experimental data for the BDA surfaces and the chromatography papers. As a result, the suggested equations can predict the capillary flow speeds on the highly wettable surfaces with varying surface width, thus they can be used to design capillary flows. The results also validate conceptual models shown in Figure 6-1 and Figure 6-2. 143 e9 Reference Type A-a -- 0-0 Type B-a Type A-b -- 0-- Type B-b Type C-a S 60 50 POO - 9--Type D-a - 40 4' S Type C-b - 6--Type D-b 00 1 304r4 BDA surfaces $ 9- 20 Papers 10.0 0 50 100 150 2(0 Time (sec) Figure 6-7. Capillary flows on the BDA surfaces and the chromatography papers with the surface shapes shown in Figure 6-6 and Table 6-1. The symbols and the lines indicate the experimental data and the theoretical expectations calculated by equation (6-10). The gray small dots and solid-line show the reference capillary rise on a constant width surface. 6.4.2. Case 2: Capillary flows with constant surface width and varying capillary properties As shown in Figure 6-5, the capillary pressure and the capillary diffusivity are dependent upon the anodization potential, particularly when the electrolyte temperature does not exceed 25 *C. It means that capillary pressure and capillary diffusivity can be modified locally using the electric field, which can vary spatially depending upon the electrode spacing. Therefore, tailoring capillary flows is possible if we know how capillary pressure and capillary diffusivity vary with electrode spacing. First, we fixed the electrolyte temperature at 25 144 0 C to find simple linear relationships between the electric field and the resulting surface properties because the capillary properties linearly increase with anodization potential at 25 'C, as shown in Figure 6-8. 2 *1 1.8 1.6 1.4 Ca. U, M Pcap: 30 V, 25 0C * Pcap: 60 V, 25 *C A Pcap: 90 V, 25 *C * Pcap: 120 V, 25 *C 200 180 160 140 1.2 120 1 100 0.8 - SDcap: * Dcap: * Dcap: * Dcap: - 0.6 80 0.4 0.2 0 1 0 0 I 10 a 20 M 2-- 0 E 60 30 V, 25 *C 60 V, 25 *C 90 V, 25 *C 120 V, 25 *C a E 40 20 0 0 30 Figure 6-8. Capillary pressure (Pcap) and capillary diffusivity (Dap) with respect to surface roughness (Ra) and electric potential (V) of breakdown anodization (BDA) at the electrolyte temperature of 25 'C. The box, circle, triangle, and diamond symbols indicate the electric potentials of 30 V, 60 V, 90 V, and 120 V, respectively. The red and the white colors inside the symbols correspond to the capillary pressure and the capillary diffusivity, respectively. When the electrolyte temperature is 25 'C, the capillary pressure and the capillary diffusivity are linearly proportional to the surface roughness, which is linearly enhanced by the electric potential of BDA. The error bars indicate + one standard deviation. Varying electrode gap distance was employed during BDA, as shown in Figure 6-9a, to find the relationship between the gap distance and the capillary pressure. The gap distance between the two parallel electrodes was varied from 10 mm to 85 mm while BDA was conducted at 120 V for 30 145 min at 25 *C. At the specified BDA conditions, the capillary pressure linearly decreases with increasing electrode spacing, as shown in Figure 6-9b. Therefore, we can find an empirical relationship between electrode spacing and capillary pressure. As a result, when the electrode spacing varies linearly with the height, the capillary pressure and capillary diffusivity also vary linearly with respect to height. This linear variation can be expressed by Pcap(x) = PI-x + P2 [kPa] and Dcap(x) = DIx + D2 [m2/s], where PI, P2, D, and D2 are constants obtained from the experimental data. For the case of BDA at 120 V and 25 0C, PI, P2 , D, and D2 are -8.2x103, 1.76, 1.04x1 06 and 2.6x1 0 -4, respectively, when x is in millimeters. In addition, the semi-empirical relationship between the surface roughness and electrode spacing can be obtained from Figure 6-9b. At the specified conditions, the surface roughness is a linear function of electrode spacing, Ra(x)= Rjx + R2 [pm], where R, and R 2 are constants: R1 = -0.21 and R 2 = 27.57 when x is in millimeters. 146 (b) (a) 10 2 40 1.8 35 1.6 30 z 1.4 0. 1.2 2520 20 1 0 0.8 't 0.6 o Capillary 0.4 o Surface 0.2 h 15 0 0 0 pressure roughness 5 . 0 x C 0 10 20 30 40 50 60 70 80 90 Electrode gap distance (mm) Figure 6-9. Capillary pressure and surface roughness produced by breakdown anodization (BDA) with varying electrode gap distance, x. (a) Two parallel electrodes were used with varying the electrode gap distance (x) in BDA processes. (b) The capillary pressure and the surface roughness are inversely proportional to the electrode gap distance. The electric potential of 120 V and an electrolyte temperature of 25 C were used to produce the BDA surfaces. The error bars indicate one standard deviation. To produce functionally varied capillary properties, we employed height dependent electrode spacing for (a)-(b) (linear) and (c)-(d) (quadratic), as shown in Figure 6-10. Expressions for the height dependent electrode spacing are shown in Table 6-2. Because the electrode spacing is a function of height: x = J(h), as shown in Table 6-2, the capillary pressure can be expressed as a function of the height, i.e. Pcap(x) = P1 -j(h)+ P2 . The BDA conditions were 120 V for 30 min at pH 3.0 and 25 'C, in each case. 147 Reference Type A Type B Type C Type D T E E E E 0 E E 65 mm E E thi 10 mm E E 0 0 80mm -80 mm E x 6 mm x 120 mm (a) 10 MM (b) + H 10 mm 90 mm (c) (d) Figure 6-10. Electrode distance profiles used in breakdown anodization (BDA) processes. The constant gap distance profile was used to produce a reference surface. Four different gap profiles, which linearly change (a)-(b) and quadratically change (c)-(d), were used in BDA processes. All BDA processes had the same condition of electrolyte (pH 3.0 and 25 C), electrode width (10 mm), and electric potential (120 V and 30 min). The left side electrodes (the red lines) in each image indicate the anode electrodes converted to highly wetting porous surfaces. The anode electrodes have different surface heights (H). 148 Table 6-2. Profile functions of electrode gap distance shown in Figure 6-10. In Type A and B, linearly changed gap distance was employed. Circular shape electrodes were used in Tape C and D. The reference BDA surface was produced with a constant electrode gap distance of 10 mm. Function Reference Type A Parameter [mm] C, C2 xCihC 2 0 0.5 0.9 -0.9 Type C x=Ch+C 2, 0 h < 60 x=Ch+C 2,60 h (x-C,)2+(h-C2)2=C 22 10 120 10 65 10 80 Type D (x-(C,+C 2)) 2+(h-C 2)2=C 22 10 80 Type B Capillary rise measurements were conducted on the surfaces produced using the electrode configurations shown in Figure 6-10. The capillary flow speed can be found from equation (6-10) because the capillary pressure and capillary diffusivity are known from the results of Figure 6-9 and the electrode gap profiles from Table 6-2. Equation (6-10) is compared with experimental data, and the theoretical calculations show good agreement with the experiments, as shown in Figure 6-11. 149 250 200 150 Ee 00 -: 100 SReference 0 Type A M Type B 50 50 0 Type C 0 0 200 400 600 Type D 800 1000 Time (sec) Figure 6-11. Capillary flows on surfaces produced by breakdown anodization (BDA) with the electrode gap profiles shown in Figure 6-10 and Table 6-2. The solid lines and the symbols indicate the theoretical data and the experimental data, respectively. The solid-circle, solid-box, circle, and box symbols are obtained with the electrode configurations shown in Figure 6-1 Oa, (b), (c) and (d), respectively. BDA surfaces were produced with different electrode gap profiles but the same BDA conditions: 120 V for 30 min in pH 3.0 electrolyte at 25 'C. The surface roughness (Ra) was measured on the surfaces used in Figure 6-11. In our previous work, we showed that the surface roughness is linearly proportional to the effective pore radius of BDA surfaces [198]. Surface roughness is predictable using the relationship between surface roughness and electrode spacing found from Figure 6-9. With the profile functions shown in Table 6-2, the surface roughness can be expressed as a function of the capillary height, i.e. Ra(h) = Rrf(h) + R 2. From 150 20 Figure 6-12, we can see that the surface roughness varies with capillary height and there is good agreement between the experimental data (symbols) and the predictions (dashed-lines). This result indicates that BDA can effectively produce functionally graded porous structures with designed electric fields. From Figure 6-9b, we found that capillary pressure is a linear function of electrode spacing. This relationship is also evident in Figure 6-12. Type A Type B Type C Type D 1 0.8 - 0.9 0.7 -a - 0.6 CD 75 0.5 cc 0.4 Of a 4- 0.3 0 0.2 2 I 0.1 0 0 10 20 30 0 10 20 30 0 10 20 30 Surface roughness (pmn) Figure 6-12. Surface roughness changes with respect to the non-dimensional height of surfaces produced by the electrode profiles shown in Figure 6-10 and Table 6-2. The surface height (H) shown in Figure 6-10 was used to obtain the non-dimensional capillary height in the x-axis. The surface roughness (Ra) is functionally varied according to the profiles of electrode gap distance. The symbols and the dashed-lines indicate the measured data and the predictive data, respectively. The predictive data were obtained from the relationship between the surface roughness and the electrode gap distance. 151 6.5. Conclusions We've shown that breakdown anodization (BDA) can produce highly wettable porous films with high capillary pressure and capillary diffusivity. These properties are attributed to the micro/nano scale amorphous titanium dioxide structures. High anodization potentials and low temperatures are necessary to yield hierarchical surface features. In BDA, we used various electrode configurations to control capillary flows on the surfaces. We successfully varied capillary pressure and capillary diffusivity on BDA surfaces by varying electrode spacing. The alteration of wetting properties is attributed to changes in surface morphology. Surface roughness of the BDA surfaces showed good correlation with electrode spacing. To predict the capillary flows, we suggested a conceptual model of highly wettable porous films. From this model, we obtained simple equations of capillary flows dependent upon surface width and capillary properties. Capillary flows on BDA surfaces and chromatography papers showed good agreement with the theoretical predictions. This work shows that capillary flows on BDA surfaces can be effectively manipulated with the applied electric field and electrode configurations. The suggested formulas and the corresponding BDA processes can be used to tailor capillary flows for applications including paper based microfluidic devices and chromatographic analysis. This promotes new opportunities to develop novel devices employing the control of capillary flows. 152 Chapter 7. Scaling Laws for Drop Impingement on Thin Porous Media Reproduced in part with permission from Young Soo Joung, Cullen R. Buie, "Scaling Laws for Drop Impingement on Porous Films and Papers,"PhysicalReview E, 89, 013015 (2014). Copyright 2014 American Physical Society. 153 7.1. Introduction Interesting phenomena can be observed when a droplet impacts liquid or solid surfaces. The crown formation of milk droplets upon impact with water represents a well-known example of this phenomena [200]. Recently, however, drop impingement on solid surfaces has gained significant attention. This is largely due to state of the art micro and nano fabrication methods which have enabled novel functional surfaces. Among the functionalized surfaces, superhydrophobic surfaces have been a major thrust [201, 202] in part due to their unique impingement properties. Conversely, drop impingement on highly wetting porous films has been largely neglected. Though there have been investigations on the dynamic behaviors of droplets on porous media [203-207], most studies have explored thick surfaces where the droplet radius is negligible compared to the substrate thickness. In this study we investigate droplet dynamics on highly wetting thin porous surfaces. First, we present a modified energy equation accounting for capillary effects that successfully captures droplet spreading upon impact on highly wetting porous surfaces. The modified energy equation reveals a dimensionless parameter denoted the capillary-Weber number, which expresses the relative influence of the matrix potential and the kinetic energy of the impinging drop. Additionally, dimensional analysis leads to a new non-dimensional parameter, which we will call the WashburnReynolds number (Rew), which strongly correlates with impingement modes on highly wetting porous surfaces. The Washburn-Reynolds number balances inertial effects of the impinging droplet and capillary transport in the porous film. 154 7.2. 7.2.1. Experimental Preparation of micro and nanoporous films Electrophoretic deposition (EPD) and breakdown anodization (BDA) were used to produce highly wetting micro/nano porous surfaces [73, 196]. For EPD, TiO 2 nanoparticles (20 nm, anatase, Sigma-Aldrich) and acetic acid were used to make 1 g/L TiO 2 suspensions. Electric potentials up to 120 V were supplied for 10-30 min during the anodization process to make thin films on the substrate. DI water pH was adjusted to pH = 3 using nitric acid (70 % ACS reagent, Sigma-Aldrich) for the electrolyte during BDA. The electrolyte was maintained at a specified temperature using a water reservoir surrounding the BDA cell. Titanium plates (Ultra-Corrosion-Resistant Titanium Grade 2, 0.020 inches thick, McMaster) were used as anode and cathode for both EPD and BDA. BDA was conducted first to make micro-porous layers followed by EPD of TiO 2 nanoparticles on the micro porous layers to realize hierarchical structures. In BDA, the electric potential and the anodization time were varied to obtain different surface properties. In EPD, however, the same conditions were used on each surface to provide consistent nanoscale features. BDA results in micro-scale porous titania surfaces on titanium substrates (Figure 7-la). Nanoscale porous layers were produced by EPD on the micro porous surfaces with titanium dioxide nanoparticles (Figure 7-1b). 155 Figure 7-1. SEM images of as-prepared surfaces show micro-scale and nano-scale porous structures produced by breakdown anodization (BDA) (a) and electrophoretic deposition (EPD) (b), respectively [198]. The combined BDA/EPD process yields highly wetting micro/nano scale hierarchical porous structures. The resulting surfaces display effective contact angles near zero degrees with high capillary pressures and spreading speeds. The porous surfaces showed near perfect wetting for several liquids including water, ethanol, ethylene glycol, and glycerol. The surfaces can be characterized with respect to capillary pressure, Pcap, and spreading speed constant, Dcap, through capillary rise measurements [180]. The capillary rise height, h, can be expressed as h2 = 'Dcapt (Washburn's equation) [172], with Dcap = rcycos0/p, and rc is the capillary radius, y is the liquid surface tension, 0 is the native contact angle of capillary surface, and p is dynamic viscosity of the liquid. The value of Dcap can be determined by the gradient of the time dependent experimental capillary rise height. Capillary pressure can be obtained from the maximum rise height, hmax, and is expressed by Pcap = pghmax= 2ycos0 Irc. 156 7.2.2. Four impingement modes on wettable porous films When liquid droplets impact the surfaces, we observed four impingement modes (Figure 7-2). We find that impingement modes are determined by the value of Rew = Uoprecos9/p. Mode-Al consists of oscillating and spreading of the droplet. In Mode-A2 the droplets show necking followed by fast spreading. Mode-B consists of smooth spreading without oscillations or necking. Finally, Mode-C is characterized by radial jetting and spreading. Mode-Al Rew=0.01 Mode-A2 Ref=fO.I Mode-B Rew=0.5 wm 12m 60m Mode-C Rew=2.0 Figure 7-2. Droplets show four different impingement modes depending on the WashburnReynolds number after impacting highly wetting porous surfaces. The Washburn-Reynolds number can be expressed as Rew = Uoprecos9 /t, where U is the impact velocity, p is the liquid density, r, is the effective capillary radius, 0 is the surface contact angle, and pt is the liquid viscosity. ModeAl: compressing-oscillating, Mode-A2: necking and spreading, Mode-B: spreading, and Mode-C: radial jetting and spreading. A high-speed camera (Photron) was used to record the behavior of liquid droplets on the surfaces. The scale bar in each image is 1 mm. 157 Table 7-1. Liquid and surface properties used in the drop impingement experiments and numerical simulations. The properties of the glycerol and water mixture were obtained from the literature [208]. Highly wetting porous titania (TiO 2 ) surfaces were produced by an electrochemical fabrication method [198]. The porous titania surfaces showed wide variations in capillary pressure and spreading speed. The thin layer chromatography (TLC) plates were obtained from SigmaAldrich. The Gel-Blotting paper is a chromatography paper (grade 230) obtained from Genesee Scientific. The coffee filter is a commercial product made by Melitta USA, Inc. The symbols correspond to the symbols in the plot of drop impingement modes (Figure 7-8). Effective Surface Symbol A B C D E F G H I J K L M N 0 P Liquid Water Porous Titania surface I Water Porous Titania surface I Water Porous Titania surface I Water Porous Titania surfae 2 Water Porous Titania surface 3 Water Porous Titania surface4 Ethanol Porous Ttia surface 5 Ethyleneglycol Porous Titania surface 8 Porous Tilanlasurface 8 Glycerol60 %+ water 40 % Porous Titania surface 8 Glycerol90 %+ water10 % Glycerol Porous Titania surface 7 SJoaTLC Water Water Ceuose TLC Water Ahuminum TLC Water Ge-loftng piper Water Coffoeflter 7.3. 7.3.1. pore radius re ( 10.6 10.6 10.6 13.4 13.1 13 8.85 28 25 21 13.6 4.2 5.8 5.1 5.6 7.9 Layer Droplet Surface thicitess diameter tension H Win) (urn) 8.6 8.6 8.6 9.2 9.2 9.2 9.5 10.5 10.5 10.5 9.4 250 100 250 340 120 Do (mm) 2.8 2.2 5 2.8 2.8 2.8 2.2 2.8 3 2.9 2.8 2.8 2.8 2.8 2.8 2.8 y (mN/m) 72.8 72.8 72.8 72.8 72.8 72.8 22.3 47.3 67 63.6 63 72.8 72.8 72.8 72.8 72.8 Viacoelty i (cP) 0.9 0.9 0.9 0.9 0.9 0.9 1.1 16 9 164 1258 0.9 0.9 0.9 0.9 0.9 y p (kgWrm) 998 998 998 998 998 998 789 1113 1154 1232 1261 996 998 998 998 998 Capilary pressure PW (Pa) 783 783 783 1048 930 773 1084 863 733 665 495 959 1076 1546 1395 1420 Hydraulic diffusivity Dow (mfs) 49 49 49 104.4 88.6 72.2 38.6 21.0 19.8 0.85 0.37 9.5 20.4 22.4 24.0 60.4 Theory Energy conservation of a cylindrical model of droplets Energy conservation is employed to predict droplet spreading upon impact. The energy conservation approach has shown good agreement with experiments, and it has helped to find useful scaling parameters for droplet dynamics [209-211 ]. In this study we modify the energy equation with two additional terms accounting for viscous dissipation inside porous media and the matrix 158 potential, which we denote as (D, and Em,,p, respectively. The modified energy equation can be expressed as =0 .,+ t,+P,+k+D,+9,+9 (7.) where Ek is the droplet kinetic energy, Eg is the gravitational potential energy, E, is the surface energy of the droplet, D1 is line dissipation, and (D, is viscous dissipation inside the droplet. For simplicity, we model water droplets impacting solid surfaces as cylinders with a time dependent radius, R(t), and height, H(t) (Figure 7-3) [209, 210]. The model droplet has the same volume as the actual droplet. Assuming mass conservation (neglecting evaporation) and fluid incompressibility, the height and radius have the following relationship: H(t)= where K, is the volume ,V JrR (t)2 of the actual droplet with an initial diameter, D. The modeled droplet has the same impact velocity as the actual droplet and thus the same kinetic energy. 159 UO: droplet velocity Figure 7-3. Schematic diagram of the cylindrical droplet impact model with time dependent radius, R(t), and height, H(t). The model cylindrical droplet has the same volume and impact velocity as the real droplet. 7.3.2. Kinetic energy The kinetic energy of a droplet after impact can be calculated from the internal velocity field in the droplet. In the cylindrical model, we assume flow in the droplet is irrotational [212]. The vertical flow velocity, v, and the radial flow velocity, vr, can be defined as, v =-2-- z dR Rdt r dR R dt and vr=-. These velocities are internal flow velocities at a specific instant such that R and dR/dt are spatially independent. The kinetic energy of the droplet can be obtained by integrating the internal velocities over the droplet volume, -V 0, as follows: 160 E,=-y(+g#=--02 p C 4iR6 2 (2 3 7?2R) 1 -+- == 2 1 r . 2 ,6 i827R* (7-2) with the initial droplet kinetic energy, Ek,, = %pV 0 U0 2 , the dimensionless drop radius, R* = R(t)I(Dd/2), and the dimensionless time, t* = t-Uo/DO. 7.3.3. Gravitational energy Utilizing the location of the time dependent center of mass, H(t)12, gravitational energy can be determined: 2 1 H~t) _pgV2 Eg=pg4-o = =Eg 32 2 2 2,rR 3R (7-3) where the initial droplet gravitational energy is defined as, Eg,o = %/2pgV0 D. 7.3.4. Droplet surface energy The surface energy of a droplet on a solid surface can be calculated by integrating the surface tension of the entire surface, A. In the cylindrical impact model, the surface tension of the liquidsolid interface can be obtained with Young's equation, and the surface energy is found to be E, = Ld =r(A,, - -4,At, cos , ), 161 where Aair is the droplet surface area in contact with air, A,,,t is the wetted area (the projected area of the solid-liquid interface), and 0, is the equilibrium contact angle on the surface. Since the surfaces used in this work exhibit complete wetting (i.e. contact angle near zero), the equation for a cylindrical droplet can be reduced as follows: 2ry o =-2 ayg,, =_,2 E, E= 27iRH= R 3 k 3 (74) . with the initial droplet surface energy, Eso = nyDO 2 7.3.5. Line dissipation Dissipation occurs in the process of moving the contact line and the amount is proportional to the contact line speed, VR, and an effective viscosity, PL. According to the de Gennes approach [213], the line dissipation can be expressed by 12 (DI =-pL 2 1 8 S VRdS=PL s YR d- R 2 L where, S is the moving contact line, dt 2( R2 dR*2=D dt* VR is the line velocity, obtained from 162 (7-5) R* (dR 4 dt* 1,o = Y2pLTDO U02 , and pL can be 31* where 1* = ln(lmax/min) with maximum length scale, 1m. (the droplet diameter), minimum length scale, min (the microscopic length scale, 2r, in this work), and 0, the sliding contact angle, which is the contact angle at the interface of the droplet, the top surface of the porous layer, and air when the droplet is spreading, as shown in Figure 7-4. In other words, the sliding contact angle is the advancing contact angle of the spreading droplet. Sliding contact angles were experimentally obtained from high speed images at 2R/D0 =2.5 when the actual droplets were spreading after impact, e.g. to obtain Figure 7-6, sliding contact angles were determined to be 2.60, 2.90, 7.4', 5.7*, and 19.60 for water, ethanol, ethylene glycol, a 60/40 (v/v) glycerol and water mixture, and a 90/10 (v/v) glycerol and water mixture, respectively. Viscous dissipation inside a droplet The total viscous dissipation in the boundary layer can be found by integrating the shear stress, , 7.3.6. multiplied by slip velocity, vs, for the non-slip area, As. To calculate the shear stress generated in the boundary layer inside the droplet, the boundary layer thickness must be estimated. It is clear that the boundary layer thickness is time-dependent from lubrication theory, and the time-dependent boundary layer thickness has shown close agreement with experimental results [214, 215]. However, to simplify the analysis, we employ a time-independent boundary layer thickness assuming irrotational flow inside the droplet. In the literatures [210-212, 216], others have successfully formulated the kinetic energy and the viscous dissipation assuming irrotational flow and timeindependent boundary layer thickness to predict the droplet behaviors. To calculate the shear stress, 163 we assumed that the effective boundary layer thickness is D/(2fb), where fb is a correction factor, which was proposed by Bechtel et al. [211]. The slip velocity, v,, was assumed to be the radial velocity, vr, and the non-slip area is equal to the projected area of the cylinder. Thus, we obtain the following expression for viscous dissipation, J4 ' -rsd4-A ~, S 4 Ir APR 2( dR 2 D dt fbU D0 2 vrdA R I '"64 A d d2R* 2 (7-6) ) D, = dt* with Dvr= 2 andf4=j, where the Ohnesorge number is Oh = p/(pyD 0 )". 7.3.7. Matrix potential Matrix potential is the surface energy inside the capillary network that drives fluid transport [217]. The matrix potential is proportional to the wetted volume of the porous region and the capillary pressure [218]. Taking the droplet as the system of interest, work is done on the droplet so the matrix potential can be expressed as E =,HR 2 164 =-f (7-7) where 4 ,,t is the wetted volume of the porous region, H, is the height of the porous region, f, is the ratio of the height of the porous region to the pore diameter, 2re, ( = He/2r,) (Figure 7-4), and E,,,,, is the reference matrix potential, Emp, .> = 2cr,PpKr Here, the thickness, He, is the average height of the porous layer considering its void volume measured by a profilometer (Measurement system: Tencor P-16 Surface Profilometer (TM)). The negative sign indicates that energy is put into the droplet. 7.3.8. Viscous dissipation within the porous media The shear stress, xc, in the porous region can be approximated in the same manner used to estimate the viscous dissipation in the droplet (equation (7-6)) as follows: c . CP Vr where v,. is the flow velocity in the porous region at a distance R from the center of the droplet (Figure 7-4). If we assume that the contact line of the droplet coincides with the wetted boundary of the porous region, the total viscous dissipation inside the porous region can be expressed as 165 ,4 ~-7R cv= =o16,V 4 r2 - (Dc =f zd 2r (7-8) ,) where A, is the non-slip area of the porous region and the characteristic viscous dissipation can be expressed as, = pV-wet,o , <De r 7 and the reference volume of the wetted porous region is -J ,o=rHDo2/4. 2r4 R I I2rc Hc=fc-2rc Vr I Droplet Porous layer HcI Solid substrate I R Figure 7-4. Schematic diagram of capillary flow for the assumed cylindrical droplet geometry. The porous region can be considered as several cylindrical capillaries in parallel, each having a height, 2rc. The porous region has height, He, and the flow velocity, Vr, at a distance, R(t), from the center, assuming the ratio of the height of the porous region to the initial droplet diameter, H/D0 , is small. 6, is the sliding contact angle, the contact angle of the droplet on the top surface of the porous layer during spreading. 166 7.3.9. Dimensionless energy equation with the capillary-Weber number When equations (7-2) to (7-8) are substituted into equation (7-1), the final energy conservation equation can be obtained as a function of the time dependent droplet radius, UO d 1 8 E 16 1 ) (dR* dt* 27 R*6 2 2 1 2E +E'"o3 - 2 +3 R 3 R. )2 1 R(dR. 2 R2 t '64 .t0' 64 "'"16 *2 -EP,oR =0 We can divide this equation by Eko-(Uo/Do) to obtain a dimensionless energy equation: 1 TdI 8 + f3 1 dt 8Re2dt* 8 ReC dt = 2 Re 16 1 27R* dR* 2 1 d R**2 1 S2(7-9) *2 IdR* 3 1 . dR* . 8 1 12 R*2 e R*Wee +3 + R*2 ( dR* ( d 0 Here, the initial drop diameter, Do, and impact velocity, UO, are used to formulate a dimensionless time, t* = t/(D/Uo), andfL = pL/p is the dimensionless contact line viscosity. The first term is the kinetic energy of droplet. The gravitational energy and the surface energy are a function of the Froude number, Fr = U0 IgDo, and the Weber number, We = pDOU 0 2/y, respectively. The matrix potential is a function of the capillary-Weber number, Wec, where 167 We pD0 U fyrcos0 Eko E, p U0 F,,oP, (7-10) The capillary-Weber number is the ratio of the initial kinetic energy (Eko) of the droplet to the = Hx7r(Do/2)2 ) reference matrix potential (Emp,o) corresponding to the initial wetted volume (,,et,o of the surface. In the case of either Hc=O (no porous region) or 9=90' (non-wetting), the capillary energy term is negligible. In the dissipation terms, we can see the dimensionless contact line viscosity,fL = pLp, the Reynolds number, Re = pDOUO/p, and the capillary-Reynolds number, Rec= . prcUO/P To solve equation (7-9) numerically, the initial conditions were specified in order to yield the same surface energy and kinetic energy as the real droplet [210]. Therefore, the radius and the impact velocity of the cylindrical model droplet are slightly different from those of the real droplet used in the drop impingement experiment. First, we set the initial radius of the cylindrical model droplet as R(0) = D,/2. The initial height of the cylindrical model droplet, H(0), can be calculated using mass conservation as H(O) = Vd/(nR(0) 2), where V = 1/6nD"3 . The cylindrical model droplet has the same initial surface area as the real droplet, Amocie = Areai, where Amodel and Areal are the surface areas of the cylindrical model droplet with radius R(0) and the real droplet with radius Rreal, respectively. This information is used to calculate the initial radius of the cylindrical model droplet. 2 The cylindrical model droplet has the same initial kinetic energy as the real droplet; %p V U 00 2p Vrea Ureai 2, where Vrai and Ureai are the volume and the impact velocity of the real droplet used in the experiment. The initial spreading speed of the droplet radius, dR/dt,=o, can be calculated using equation (7-2). As a result, the initial conditions for equation (7-9) are R*(0) = 1 and dR*/dt*t=o = 1.18. We find that each dissipation term in equation (7-9) has different significance depending on Wee, 168 as shown schematically in Figure 7-5. Contact line dissipation and viscous dissipation inside the droplet are important at high Wec (Figure 7-5a). However, for low We, (Wee < 10), viscous dissipation inside the porous region is the main dissipation term in equation (7-1). As depicted in Figure 7-5b, line dissipation and viscous dissipation are negligible at low Wec because droplet expansion can be considered as a free surface flow driven by the capillary pressure in the porous region. (b) Viscous dissipation Contact line inside the droplet Solid substrate dissipation (a) Viscous dissipation inside the porous layer Porous layer Capillary energy driven flow Kinetic energy driven flow Figure 7-5. Schematic illustrations of droplet spreading on highly wetting porous surfaces. At high capillary-Weber numbers (Wee > 103), the kinetic energy determines droplet expansion, while the contact line and viscous effects dissipate mechanical energy (a). At low capillary-Weber numbers (Wec < 10), the capillary energy governs droplet spreading, and viscous dissipation is most prominent inside the porous region (b). 7.4. 7.4.1. Results and discussion Simplified energy equation at high capillary-Weber number In Figure 7-6, we compare the predicted droplet spreading obtained from equation (7-9) with the 169 experimental results when Wee > 103. The liquid and surface properties utilized are given in Table 7-1. As suggested in Figure 7-5, viscous dissipation inside the porous layer was not considered in the energy equation for high Wee. The surfaces showed varying Pcap and Dcap values depending on the liquid utilized. The predicted droplet spreading is in good agreement with the experimental data (Figure 7-6). This suggests that the matrix potential is essential to appropriately predict droplet spreading. In spite of the geometrical simplifications, the energy equation captures the major energy transfer mechanisms during impingement. We derived simplified energy conservation equations for four cases with different We, Fr, and We, regimes to investigate the dependence of R*(t) on each dimensionless parameter. To simplify the expressions, we ignored the constant values of each term and consider the line dissipation dominant because Rexff' is smaller than the other dissipation terms, as shown in Figure 7-6. In addition, the initial conditions of R*(O) = 0 and/or dR*/dt*,=o = 1 are used to derive the simplified expressions as functions of well-known dimensionless parameters. These initial conditions are different from those of the numerical solutions shown in Figure 7-6. Table 7-2 provides a summary of the simplified energy equations and the dimensionless droplet radius change with different ranges of We, Fr, Wec. From Table 7-2, we see that the gravitational energy and the surface energy do not significantly affect droplet spreading. The kinetic energy influences expansion largely in the initial stage of impact, and the radius is asymptotes to a specific value dependent upon the Re. With respect to the matrix potential, the dimensionless droplet radius increases linearly with time, and the expansion rate is proportional to the ratio of Re to Wec. As the matrix potential is dominant in Table 7-2, equation (7-9) can be simplified to dR*/dt*~Re/Wec for large t* when the kinetic energy is dissipated (We0 << 1). This simplified equation means that the droplet expansion rate is determined by the ratio of Re to We0 . Therefore, theoretically, R(t)* does not achieve a constant value if the capillary pressure (Pcap) is non-zero. In Figure 7-6, the curves for highly viscous liquids appear to 170 converge to constant values but in actuality they slightly increase at a rate of Re/We. In Figure 7-6, the initial drastic increase of R* is mainly attributed by the kinetic energy, and the linear increase of R* after the initial stage of impact is due to the matrix potential. As a result, the assumption that the line dissipation is dominant can be justified by comparing the simplified energy equations and the experimental data in Figure 7-6. The kinetic energy and the matrix potential are the main driving forces for droplet spreading when We >> 1 and We0 << 1, respectively. Table 7-2. Simplified energy conservation equation and dimensionless droplet radius at different physical regimes. The line dissipation in equation (7-9) is considered dominant and the other dissipation terms are ignored to obtain simplified energy conservation equations as functions of common dimensionless parameters. The dimensionless droplet radius can be expressed as a function of the Reynolds number and another dimensionless parameter; therefore, we can characterize the droplet radius change with respect to the relative influence of the Reynolds number. Dominant Dimensionless effect Simplified energy equation parameters d rrdR'f -) R ( R* =Kinetic e tanh4 energy t* asymptoticto We>> Re% 1, Fr >>1, Wee>> 1 Gravitational potential energy Surface energy We << 1, Fr >> 1, Wee>> 1 C Matrx potential energy e droplet radius Characteristic smpoi oR .) ~ d We >> 1, Fr << 1, We >> 1 1 Dimensionless d* 2 1 (1(2Re dt FrR eFr) - dt* kWe R* d I dt We, R* =(R-t*rt R* - Re R* Re 171 2 RR (R*2 R* = dt * dt* (~we *d= We. t) weak due to the powerofl1/5 rdfIto the power of 1/4 linearly proportional to Re/Wee. 7.4.2. Simplified energy equation at low capillary-Weber number In the case of We. < 10, viscous dissipation in the porous region is the dominant dissipation mechanism and the other dissipation terms are ignored, as shown in Figure 7-5. To justify this assumption, note that Re, is small relative to Re in the energy equation. When Re, and Re are compared in the dissipation terms, however, we must carefully consider the no-slip area (A,). In the viscous dissipation inside the droplet and the line dissipation, the contact surface between the droplet and the surface is assumed as the no-slip surface. This assumption is valid when We0 is sufficiently high (We0 > 103). When We. is small, however, the no-slip surface becomes a slip boundary because the flows inside the porous layer induce spreading via capillary pressure. Therefore, the viscous dissipation inside the droplet and the line dissipation are negligible when We0 is small (We, < 10). Furthermore, gravitational energy and droplet surface energy are much weaker than the matrix potential when We0 < 10 and R* > 1. Therefore, the energy equation becomes a function of the viscous dissipation and matrix potential. In this case equation (7-9) for the dimensionless droplet radius becomes d dt 12 1 R 3 R*2 +---fR We' Rec 8 . dR* 2-, dt*) =0. From equation (7-11), the dimensionless droplet expansion rate is found to be dR* dt 64 Re --,- --64 c.= - 1 f We. R D* (7-12) R* By integrating equation (7-12), the droplet radius can be expressed as 172 (7-11) R 12= t*=128D t* 2 8 IR fc Wec (7-13) As a result, R(t)* is only dependent to t* and Dcap* and the initial condition of equation (7-13) is not needed. Equation (7-13) effectively predicts the droplet radius change on wetting surfaces at low We,, as shown in Figure 7-7. Here the independent axis is Dcap*xt* and the dependent axis is R*2 in Figure 7-7 to display the linearity of equation (7-13) and the corresponding experimental data. We used We, less than 100 for the drop impingement experiments in Figure 7-7, but the theoretical data obtained from equation (7-13) assumes that We, is less than 10. Our intent was to show that the experimental data becomes closer to the theoretical data when their We, is close to 10. As shown in Figure 7-7, the experimental data obtained with We0 higher than 10 show larger deviations from the theoretical data. However, the nearly linear experimental data obtained when We, is higher than 10 is recovered after initial impact because the kinetic energy is dissipated through the initial moments of droplet expansion. We0 only reflects the initial impact speed of the droplet. Therefore, after the droplet loses it's kinetic energy the expansion more closely follows the theoretical prediction. This is why droplets with We, higher than 10 show nonlinearity in the initial impact region. It reveals that the droplet expansion rate at low We is governed by hydraulic diffusivity, Dcap. In highly viscous liquids, the initial kinetic energy is dissipated through droplet deformation resulting from viscous effects inside the droplet after impact. The impingement modes of highly viscous liquids are confined to Mode-A and mainly governed by the matrix potential. 173 6 0 Experimental date 5 00 4O R* 3 2 100 Kinetic energy Matric potential dominant dominant 0 Data Surface 15 10 5 0 Wec RexfC1 Rexfb 1 R Fr We 355 212 69 374 224 73 14700 27.9 7100 21.6 4100 12.3 258.0 199.5 114.1 Rew Mode 83.4 64.5 38.8 2.1 1.6 0.9 B B B 'xf 1 2 3 A A A 4 G 89 149 1400 7.0 91.8 16.6 1.9 B 5 6 7 1 H J 265 427 205 402 15200 772 17800 327 17800 8.0 6.8 1.5 82.9 76.7 146.0 47.4 38.8 1.7 1.9 1.7 0.1 B B Al Figure 7-6. Theoretical and experimental droplet spreading for multiple liquids after impact on highly wetting porous surfaces. Solid lines represent spreading radii obtained from equation (7-9) while circles are experimental data. The impact velocities are selected such that Wec exceeds 103. The table provides the surface and liquid properties and the dimensionless parameters of the energy equation for each data set. Viscous dissipation inside the porous layer, which is the column highlighted in gray in the table, is not considered when Wec > 103. Here we only considered impingement Mode-Al and Mode-B because the droplet mass is not conserved in Mode-A2 and Mode-C due to necking and jetting behavior, respectively. 174 10 9 8 7 6 R 2 5 4 0 1. Water 3 0 2. Ethanol 2 A 3. Ethylene glycol * 4. Glycerol 60% 5. Glycerol 90% 6. Glycerol 100% 0 11 0.02 Data Surface Fr We 1 A 0.9 0.9 2 G 0.7 1.2 3 H 0.9 1.6 4 1 1.3 2.0 5 J 15.3 24.4 6 K 0.1 0.2 0.04 C*Cap We 15 7 37 81 86 43 0.06 . --- 0.08 * 0 Recxfc' Rew Mode 4.14 0.1 A2 1.47 0.2 A2 1.82 0.2 Al 3.32 0.1 Al 0.45 0.003 Al 0.01 0.002 Al Figure 7-7. Experimental results for droplet spreading at low capillary Weber number for the liquids shown in Table 7-1. The symbols and the black solid line indicate the experimental data when We, < 100 and the theoretical data obtained from equation (7-13), respectively. Viscous dissipation inside the droplet and line dissipation, which are the columns highlighted in gray in the table, are not considered in the energy equation because Wec is low. The square of the dimensionless radius change is linearly proportional to the dimensionless time. The table provides the surface and liquid properties and the dimensionless parameters of the energy equation for each data set. The impingement modes are confined to Mode-Al and Mode-A2 because Rew < 0.2 when We, < 100. 175 7.4.3. The Washburn-Reynolds number We can see Mode-Al and Mode-B in Figure 7-6 (high We0 ), and Mode-Al and Mode-A2 in Figure 7-7 (low We,). In Figure 7-6 and Figure 7-7, we used the experimental data obtained from Mode-Al and Mode-B because the mass conservation isn't valid in Mode-A2 and Mode-C due to necking and jetting behavior, respectively. From the investigation of We0 and Rew, we can say that very low and high We, numbers correspond to Mode-A and Mode-C, respectively, because these We0 regimes correspond to low and high Rew. However, to predict the transition between the impingement modes, viscous dissipation inside the porous region must be considered. From equation (7-13), we see that Dcap includes the viscosity effects in the porous region; therefore, Rew incorporates the effects of kinetic energy, matrix potential, and viscous dissipation. This is why we can predict impingement mode transitions with Rew. Dimensional analysis is a useful tool to extract pertinent dimensionless groups using parameters obtained from the modified energy equations. Dimensionless groups are formed from the parameters, Dcap, Pcap, and He, which relate to the highly wetting surfaces in addition to the parameters, Do, Uo, p, p, and y. We find that a dimensionless parameter obtained from a combination of the dimensionless hydraulic diffusivity, D*cap = Dcap1(U 0D0), and the Weber number governs the drop impingement modes. We denote this nondimensional number the Washburn-Reynolds due to its similarity to the traditional Reynolds number and its relationship with Washburn's equation [219]. The Washburn-Reynolds number, Rew, is expressed by, Re,=D* -We -U,prp(7-14) cos6 07 CWU 176 4 Hydraulic diffusivity, Dcap, is obtained from capillary rise measurements and combined with the Weber number to obtain Rew. Using the liquids and surfaces in Table 7-1, drop impingement experiments were conducted with varying impact velocities. It is worth noting that the WashburnReynolds number Rew is close to the capillary-Reynolds number Rec when the contact angle is small (cos9 ~ 1). For the case of contact angles near zero, Rew approaches Rec, the Reynolds number inside a single capillary tube with radius r, and flow speed U. 7.4.4. Impingement mode transitions correlate with the Washburn-Reynolds number When the impingement modes were plotted with respect to We or Re, there were several orders of magnitude differences between mode transitions for different liquids. However, Re w correlates with mode transitions for a wide range of liquid and surface properties, impact velocities, and for various highly wetting porous surfaces (Figure 7-8). As shown in Figure 7-8, increasing Rew governs transitions between impingement modes. Interestingly, in Mode-A (Rew < 0.5), observation of Mode-Al or Mode-A2 appeared to vary with cos9/Oh 2, where Oh is the Ohnesorge number expressed as Oh = p/(rep) and 9 is the Young's contact angle of the surface. This parameter has the same form of another dimensionless group of Dcap/u, where v is the kinematic viscosity of liquids used. We found that these two dimensionless groups can effectively characterize the wetting ability of surfaces for various liquids. In Mode-Al we believe that kinetic energy is transferred to viscous dissipation through the observed oscillations in the low cos9/Oh 2 region (cos9/Oh 2 < 30). Conversely, for the high cos9/Oh2 region (cos9O/h2 > 30) the matrix potential is compensated by 177 the necking behavior observed in Mode-A2. Mode-Al and Mode-A2 are combined as Mode-A since both are observed in the same Re w range. Further investigation is needed to illuminate the role of D,,p for the transition between Mode-Al and Mode-A2. The biggest advantage of the WashburnReynolds number is that we can predict the impingement modes based upon three factors: surface properties (r, and 6), liquid properties (p and p), and impact velocity (U). Therefore, once the surface and liquid are determined, we can change the impingement modes using the impact velocity. Alternatively, if the impact velocity is limited to a specific range, the impingement modes can be altered by the surface and liquid properties. However, in this case, the modes available are limited because Dap and p have narrower ranges than U,. The Washburn-Reynolds number (Rew) can be interpreted as the ratio between the inertia of the impinging droplet and capillary driven transport in the porous thin film. At high Rew, the droplet inertia is dominant, resulting in Mode-C (jetting). In Mode-B, the inertial and capillary effects are balanced and smooth spreading is observed. Conversely, Mode-A (low Rew) results in capillary driven flows. When the capillary radius is large and the contact angle is small, the surfaces generally show high spreading speed. This can result in oscillating motions (Mode-Al) at low Dap and necking (Mode-A2) at high Dcap. In Mode-A, droplets spread largely due to the matrix potential since the initial droplet kinetic energy is negligible. 178 10 'Mode C I0 0.1 *A Mode Al C Mode A2 H MN m 0.01 L 10 - 1 0.001 0.1 G M N C; K 0.01 - 0.0001 a -""IM a am''""E I - 0.001 0.01 100 10 1 0 .0 0 0 1 IN ' men - - '"I 0.1 1 2 cosO/Oh -' ""0 onoe M I -- I So@ 10 100 1000 Figure 7-8. Impingement mode transitions correlate with the Washburn-Reynolds number, as shown in the log-log plot of Rew vs. cos0/Oh2 , where Oh is the Ohnesorge number and 0 is the Young's contact angle of the surface. Dcp/u has the same form of cos9/Oh 2, where u is the kinematic viscosity of liquids used. Both dimensionless parameters can be used to distinguish wetting properties of the surfaces. Droplets fall from heights ranging from 0.003 m to 1 m prior to impact. Symbols: 0, El, A, and 0 indicate Mode-Al, Mode-A2, Mode-B, and Mode-C, respectively. The capital letter under each vertical line indicates the surface name shown in Table 7-1. The inset shows the mode transitions in a narrow Dcap/u region of 1 to 100. 179 7.5. Conclusions In this work, we propose a modified energy equation to predict droplet spreading on highly wetting porous thin films. Notably, we include the matrix potential to accurately simulate droplet spreading. This effect can be evaluated with the capillary-Weber number, Wec, which is the ratio of the initial kinetic energy to the initial matrix potential. Furthermore, we discovered a dimensionless parameter, the Washburn-Reynolds number (Rew), which correlates with impingement modes on highly wetting porous surfaces. Rew can be derived from dimensional analysis or the simplified energy equations at low and high Wec. At high Wec, the Weber number governs the droplet expansion rate but at low We0 numbers, the dimensionless hydraulic diffusivity, D*CaP, governs droplet spreading. Rew can be obtained from We and D*cap. Rew has the same form as the Reynolds number but possesses an effective length scale, recos9, which reflects the wetting properties of the surface. Rew effectively correlates with impingement modes across a wide range of fluid and surface properties. Rew provides useful insight into droplet dynamics and can be used to guide the design of highly wetting surfaces for applications ranging from spray cooling to paper microfluidics and inkjet printing. 180 Chapter 8. Sparkling Droplets: Evidence of Aerosol Generation from Drop Impingement on Porous Media 181 8.1. Introduction Aerosols, which disperse chemicals and microorganisms into the atmosphere [220, 221], have been intensively investigated because of their significant impact on the environment and human health [222-226]. To date, bubbles breaking on the sea and fossil fuel combustion have been considered the main origins of atmospheric aerosol dispersion [224, 227-230]. However, aerosol generated from rainfall on soil has not been considered to date. Furthermore, the origins of atmospheric bioaerosols consisting soil elements and environmental microorganisms still remain illusive [220, 226]. Here we report that rainfall on soil can also generate aerosols. High-speed imaging provides visual evidence of aerosol generation when liquid water droplets hit soil at velocities consistent with rainfall. During drop impingement, tiny gas bubbles form inside the droplet and are fed by air escaping from the porous soil. When the tiny bubbles break, they release jets that quickly break into droplets tens of microns in diameter. Within a specified range of impact velocities, we observed frenetic bubble generation and ejection of tiny droplets, producing aerosol above the surface. We can predict when the frenetic aerosol generation will occur from the surface properties and impact conditions. This work demonstrates that aerosols can be generated on porous surfaces like soil when impinged by a liquid droplet. Hence, our work has widespread implications for future remediation of air pollution, global warming, and the migration of disease causing microbes from aerosol. We have found evidence of aerosol generation from raindrops hitting soil (Figure 8-la-c). Using different impact velocities, we observed the drop impingements on the soils with a high-speed camera. When small water-droplets imitating raindrops were dropped on three representative types of soils: sandy, clay and loam-clay [231], aerosol generation was successfully reproduced with both of the clay and loam-clay soils but not with the sandy soil. When a droplet, which was the similar 182 size to a rain drop (~ 2.6 mm), hit the clay soil surface at an light rainfall velocity of 2 m/s (Figure 8-la), trapped bubbles were observed inside the droplet (Figure 8-1b). Then, multiple tiny jets, which diameters were approximately few tens of micro meters, were ejected from the droplet at a top speed of around 10 m/s during the droplet was flattened and spreading (Figure 8-ic). It can be speculated that the bubbles pinned on the soil surface break when the upper surface of the droplet compresses the bubble tops, resulting in simultaneous water jets, as it is known that water jets occur when gas bubbles break at the interface of air and water [232-235]. This phenomenon was first observed during drop impingement on thin layer chromatography (TLC) plates. TLC plates can be considered ideal soil-like surfaces, because they have a similar range of water absorption speeds to soils while they are topologically and chemically homogenous and mono-colored (Table 8-1). When a water droplet hit a TLC plate, which has the similar waterabsorption speed to clay soil, at an impact speed of 0.4 m/s, a large bubble was trapped inside the droplet (Figure 8-1d). When the bubble met the water-air interface and broke (Figure 8-le), a thin water pillar spurted (Figure 8-1f). Then, the water pillar was separated into small droplets, which can float in the air (Figure 8-1g). Depending on impact speeds and surfaces, we observed different bubble generations and breaking. Interestingly, frenzied water-jets (aerosol dispersion) were investigated in a specific range of impact velocity depending on surface property, as shown in Figure 8-ih. In this mode, hundreds water-jets, which diameters were in the range of 10 - 50 micro meters, were ejected from the droplet on an area of 30 mm 2 for 20 micro-seconds. At the moment, hundreds of bubbles inside the droplet were breaking in a short time. This frenzied aerosol dispersion is very attractive because quite a number of aerosols can be produced within a few microseconds to trigger aerosol generation, transportation, and reaction [236]. Aerosol generation after drop impingement on porous media is a three step process consisting of bubble formation, bubble expansion, and bubble bursting, as shown in Figure 8-2. To account for 183 the mechanism of aerosol generation, we observed bubble's life inside droplets with different impact velocities and surfaces. From the high speed images of Figure 8-2a-c, we can simply express the process of aerosol generation with the schematic illustrations of Figure 8-2d-g. When a droplet hits the surface, the droplet compresses the solid surface due to the kinetic energy of the droplet, and tiny bubbles are trapped at the interface of the droplet and the surface because the expansion speed of the droplet is higher than the water absorption speed of the surface (Figure 8-2d). After the droplet reaches the maximum radius, the droplet oscillates or trembles, relieving its compressive pressure. At this time, bubbles are growing by receiving air escaping from the pores of the surface due to water soaking into the pores (Figure 8-2e) while the height of droplets is thinning because water is being absorbed into the surface (Figure 8-2f). Finally, the bubble tops meet the air-water interface, resulting in breaking of the bubbles (Figure 8-2g) and consequently ejecting small droplets [223, 228, 237]. From the observation, we know that bubble's behavior inside droplets is strongly affected by impact condition and surface property, resulting in different characteristics of aerosol generation. 184 Figure 8-1. Aerosol generation from droplets hitting soils and porous surfaces. a, The clay soil has irregularly rough surfaces and a droplet falls with an impact speed of 2.0 m/s. b, Tiny bubbles are formed under the droplet. The bubbles are inside the white circles in the image. c, Tiny water-jets are ejected from the droplet during impact. The white circles and arrows in the image highlight aerosols and jets ejected from the droplet. d-g, High-speed images of the process of generating aerosol when a single bubble breaks inside the droplet. d, A bubble is trapped inside the droplet. e, The bubble breaks at the interface of water and air. f, A water pillar is jetted from the droplet. g, The water pillar breaks up into tiny droplets. h, Many simultaneous water-jets and aerosols are generated on a thin layer chromatography plate within a specific range of drop impact speeds. 185 Figure 8-2. Bubble generation, growth, and breaking inside droplets hitting porous surfaces. a-c, High-speed images of bubble formation inside water-droplets on a thin layer chromatography plate with respect to impact velocity. Bubble size and distribution become uniform with impact velocity, resulting in simultaneous water-jets on the droplet surface. d-g, The schematic illustration of the process of water-jet generation. d, A droplet impacts a porous layer and the droplet radius is expanded. e, After it reaches the maximum radius, tiny bubbles are inflated at the interface of the droplet and the porous surface. f, The droplet height thins due to the absorption of water into the porous layer and the bubble size grows due to the inflation with air squeezed from the porous layer. g, Finally, the tops of the bubbles meet the interface of air and water, and they are ruptured, generating tiny water jets. The maximum number of bubbles inside the droplet increases with impact velocity. The scale bars in a-c indicate 1 mm. 8.2. 8.2.1. Theory Measurements of hydraulic diffusivity 186 The capillary rise measurement (CRM) has been widely used to characterize liquid transports in porous media [180, 238, 239]. CRMs were performed to evaluate hydraulic diffusivity on the prepared surfaces and soils (Table 8-1). For the capillary rise measurements of the soils, we used a common experimental setup according to other literatures [180, 239, 240]. A glass tube, which inner-diameter is 10 mm, is filled with soil up to 100 mm in height and capped with a glass membrane, which pore size is in the range of 70-100 micron, at the bottom (the glassware and membranes purchased from ACE glass). The glass tube is fixed vertically and the height of the liquid bath is slowly increased to initiate contact between the sample and the liquid surface. The capillary rise is recorded using a digital camera. For the capillary rise measurements of the other surfaces, the samples, which measured 2 cm and 10 cm in width and length, respectively, were used instead of the glass tubes. Hydraulic diffusivity was originally derived from Darcy's law; therefore, we need to understand how the hydraulic diffusivity can be measured with the capillary rise measurement. As a result, we can say that the hydraulic diffusivity obtained from the capillary rise measurement has the same meaning as one obtained from Darcy's law, under the assumptions of uniform porous structures and unit changes of the moisture content inside the porous media. Following two sections describe the derivations of hydraulic diffusivity from both approaches. 8.2.2. Hydraulic diffusivity from the capillary rise measurement Capillary flows can be considered a subset of Poiseuille flow in a narrow tube with radius, r. The velocity within a capillary can be written as 187 vC = where , (8-1) aP/Oz is the pressure gradient in the direction of z. In the tube, the capillary pressure, Pcap, can be calculated from the Young-Laplace equation, Pa, = 2,vcos 0 r2 (8-2) , where y is the surface tension, 0 is the native contact angle on the capillary surface. If the capillary pressure is higher than the hydrostatic pressure in the capillary tube, the liquid rises. When a vertical capillary tube comes into contact with the free surface of water, the flow velocity can be expressed by modifying equation (8-1). For a vertical tube with the rise height, h, from the liquid free surface to the propagating liquid front, and the average velocity, vc, can be expressed as the change in capillary height with respect to time. Finally, Washburn's equation is obtained as [172] v dh = dt r2 2 cos0 8ph r (8-3) pgh If h is small, the gravitational effects can be neglected and Washburn's equation can be simplified to dh =rycos0 dt (8-4) 4ph 188 Equation (8-4) can be integrated with respect to h and t to obtain a function relating rise height with time, resulting in the expression: h2 = 2pu t =--D2 2 t, (8-5) where Dcap is hydraulic diffusivity with the units of m 2/s. Therefore, hydraulic diffusivity can be obtained from the capillary rise speed. 8.2.3. Hydraulic diffusivity from Darcy's law From Darcy's law, the water discharge, q, through porous media can be expressed as q=-K-, (8-6) az where K is the hydraulic conductivity and ah/az is the hydraulic gradient. When we assume the capillary head, hcap, only the driving force for the water discharge, from equation (8-1) and equation (8-6), we can express the hydraulic conductivity as PcaP h . K=C8/rc- (8-7) Considering of the continuity of water content, the Richards' equation [241, 242] can be derived from equation (8-6) as 189 = a (K at az (8-8) az where 0 is the moisture content of porous media. Here, capillary pressure is also considered only the driving force, and finally we can express equation (8-8) with hydraulic diffusivity, D, as follows: at (8-9) where D=K ah". (8-10) ao If we assume that the porous medium is initially dry (0 = 0) and becomes fully wet (0 = 1) after capillary suction, and the capillary pressure become zero when 0 = 1, the hydraulic diffusivity when the porous medium is dry can be expressed as = D (r =rycol= , (8-11) from equations (8-7) and (8-9). Therefore, the hydraulic diffusivities obtained from Washburn's and Richard's equations have the same form. 8.3. 8.3.1. Experimental Preparations and characterizations of porous media 190 We used twenty eight different porous media (see Table 8-1) to examine aerosol generation from droplets hitting the porous media. Eight kinds of soils (from clay loam soil to peat) were purchased from World's Science (Nasco soil samples, Fort Atkinson, Wisconsin). We used the classification of the soils based on the product information from the company. The following eight kinds of soils collected at Boston area in USA. The detailed locations of sampling the soils are as - follows; Soil-A: Killian court of MIT, Soil-B: Amherst Alley, Soil-C and -D: Briggs field, Soil-E, F, and -G: Charles River, and Soil-H: Nahant beach. The soils were approximately classified with the consideration of the particle sizes and the locations of sampling. The five kinds of commercial thin chromatography (TLC) plates purchased from Sigma-Aldrich, and the seven different titania porous films produced by electrochemical processes [243]. The porous titania plates were produced by an electrochemical fabrication method with different electric-potentials [243]. To prepare dry soil substrates, first we gently broke the soils into fine grains with hands. We did not filter and grind the soils. Then, each soil was placed on a dish with the soil height of 1 cm and then wet with water-spray. The wet soils were dried on a hot plate at the temperature of 50 *C for three days. After cooling the soils down to the ambient temperature, the soils were used for the drop impingement experiments. The thin layer chromatography plates were used without any purification and rinse. The porous titania plates were rinsed with deionized water for 1 minute after the electrochemical processes and dried in a common laboratory environment. From the capillary rise experiments, the hydraulic diffusivities of the media were evaluated with six different liquids: water, ethanol, ethylene glycol, and glycerol. For the soils, the hydraulic diffusivity and the aerosol generation were evaluated with water, and for the TLC and porous titania plates with the every liquids. The particle sizes of the media were approximately evaluated by an image processing and analysis software (ImageJ, U. S. National Institutes of Health, Bethesda, Maryland, USA). Table 191 8-1 shows the summary of the surface properties and the liquids used in this works for the measurements of hydraulic diffusivity and aerosol generation. 192 Table 8-1. Characteristics and wetting properties of the media used to examine aerosol generation. Medium thickness Particle size (ptm) (pm) 104 104 104 104 104 104 104 104 < 500 < 300 300-600 Clay loam soil Loam soil Sandy soil Clay soil Muck Sandy loam soil Silt loam soil Peat Soil A (Clay loam) Soil B (Peat) Soil C (Loam) Soil D (Sandy loam) Soil E (Sandy Clay) Soil F (Sand gravel) Soil G (Silt loam) Soil H (Beach sand) TLC A (Silica) TLC B (Cellulose) TLC C (Aluminum) TLC D (Silica) TLC E (Cellulose) TLC A (Silica) TLC B (Cellulose) TLC C (Aluminum) TLC D (Silica) TLC E (Cellulose) TLC A (Silica) TLC B (Cellulose) TLC C (Aluminum) TLC D (Silica) TLC E (Cellulose) Titania A (120 V, 10 *C) Titania B (150 V, 10 *C) Titania C (120 V, 75 *C) Titania D (120 V, 25 *C) Titania E (90 V, 25 *C) Titania F (150 V, 25 *C) 104 104 250 100 250 200 250 250 100 250 200 250 250 100 250 200 250 190 330 80 150 140 240 Titania G (60 V, 50CC) 80 20-50 Titania G (60 V, 50 *C) Titania G (60 V. 50 *C) 80 80 20-50 20-50 < 100 < 500 200-500 < 400 < 500 < 600 104 104 104 104 < 1000 < 1500 < 1000 < 300 100 - 5000 < 500 100 -400 10 2-20 11-14 25 50 10 2-20 11-14 25 50 10 2-20 11-14 25 50 50-100 20 -200 20- 100 < 400 10-200 < 600 193 Liquid Hydraulic diffusivity (mm2/s) Water Water Water Water Water Water Water Water Water Water Water Water Water Water Water Water Water Water Water Water Water Ethanol Ethanol Ethanol Ethanol Ethanol Ethylene glycol Ethylene glycol Ethylene glycol Ethylene glycol Ethylene glycol Water Water Water Water Water Water Ethanol Ethylene glycol Glycerol 2.9 3.1 91.9 3.4 0.1 28.4 3.7 0.1 3.8 0.02 14.8 33.5 16.8 26.4 7.2 80.0 9.5 20.4 22.4 24.0 37.0 2.3 14.7 4.7 4.7 26.4 0.5 2.2 1.1 1.2 2.9 85.6 50.1 52.4 144.6 117.8 88.3 38.6 21.0 0.4 8.3.2. Characteristics of typical soil and rainfall To display the regimes of rainfalls hitting soils on the characterization map of aerosol generation shown in Figure 8-4, we used the contact angles and effective pore radius of soils and the radius and terminal velocities of rainfalls, which have been reported in other literatures [164, 240, 244-252]. In the literatures, the contact angles and effective pore radius were obtained from the capillary rise merriments same as the method we used in this work. If we assume that glass surfaces have nearly zero contact angles, the effective pore radius of soils ranges from 1 micron to 1 mm corresponding to clay and sand, respectively [244, 245]. The contact angles of various soils have been reported in many literatures [164, 240, 246-251]. Based on the literatures, we know that the contact angles are approximately varied from 300 to 80* depending on the chemical compositions of soils. Therefore, the range of the hydraulic diffusivity of each soil can be estimated by equation (8-5) with the contact angles and the effective pore radius obtained from the literatures. Rain types can be classified with light, moderate, and heavy rains depending on their precipitation rates [252]. The drop size of rainfalls is varied from 0.5 mm to 2.5 mm with the terminal velocities of 2.1 m/sec and 9.1 m/s, respectively [252]. From the relationship between the drop size and the terminal velocity, the Weber numbers can be estimated with respect to the kinds of rainfalls. 8.3.3. Characteristics of droplets and bubbles with respect to drop heights We investigated the behaviors of droplets hitting porous surfaces using high-speed imaging. We used the three different TLC plates (TLC-A, TLC-B, and TLC-C), with wetting properties shown in Table 8-1, and varied the drop height to change the impact velocity. First, we compared the droplet 194 film thickness after spreading and the bubble diameters, as shown in Figure 8-3a. The droplet film thickness is measured when the droplets reach their maximum radius after impact. The bubble diameter is the average diameter of the bubbles inside the droplet when the first bubble breaks. At low velocities, the droplet film thickness is greater than the bubble diameter. However, the film thickness and the bubble diameter become similar at 1 m/s impact velocity. Therefore, impact velocity governs the maximum bubble diameter because bubble diameter cannot exceed the droplet film thickness. The maximum number of bubbles is linearly proportional to the impact velocity, as shown in Figure 8-3b, because the number of bubbles is linearly proportional to the droplet contact area to the surface. As a result, the number of bubbles increases with impact velocity. Jetting initiation time is the time it takes to observe the first aerosol dispersion after the droplet reached the maximum diameter after impact. The jetting initiation time shows the effects of surface wettability and impact velocity on aerosol generation. The jetting initiation time is mainly governed by the wetting properties of the surface at low impact velocity (Figure 8-3c). However, the droplet film thickness significantly affects the jetting initiation time at high impact velocities. TLC-A and TLC-C have the lowest and the highest hydraulic diffusivities, respectively. Therefore, the initial jetting occurs on TLC-C first and on TLC-A last at low impact velocities (less than 1.4 m/s). At higher impact velocities (higher than 2.0 m/s), however, the droplet height is the highest on TLC-B, as shown in Figure 8-3d; therefore, the initial jetting time is the greatest on TLC-B. 195 (a) (b) 1.4 45 0 1.2 Bubble diameter 1 40 0 Liquid film thickness S35 1 30 E E 0.8 Frenetic aerosol 4 dispersion ~0.6 25 20 I0.4 15 BWb Z 10 0.2 03 5 0 0.5 1 1.5 U 2 (m/s) 0 2 I0 (d) dominant I 0 TLC-A: Dc,, = 9.5 mm 2/s 2 0 TLC-B: D, = 20.4 mm /s 0) . d 6 2 TLC-C: D, = 22.4 mm /s E Droplet film "b 7 I thickness dominant 0.01 - - - - -- - - ---- - - 0.5 1 1.5 2 U 2 (m/s) 2 4 3 %b 2 GOI TLC-A:D = 9.5 mm /s2 -0- TLC-B: D. = 20.4 mm /s 2 SI 0m. TLC-C: D, 0.001 -a--!- 0 -- 2 ------- 4 - - 6 0 U02 (m/s) 2 2 4 U 2 (m/s) 2 = 22.4 mm 2/s - - (c) 2 - 0 0 6 Figure 8-3. Characteristics of droplets and bubbles generated after the droplets hit the three TLC plates. The hydraulic diffusivities of TLC-A, TLC-B and TLC-C are 9.5, 20.4, and 22.4 mm 2 /s, respectively. (a) The average bubble diameter does not change significantly by impact velocity U 2 but the average droplet film thickness is inversely proportional to U 2 . (b) The maximum number of bubbles inside the droplet linearly increases with respect to U 2 . (c) The time it takes to observe the first jet is affected by the surface wettability at low impact velocity but by the droplet film thickness at high impact velocity since the droplet film thickness governs the maximum bubble size at high impact velocity. (d) The droplet film thickness h,ni, after hitting the surfaces is the highest on TLCB. 196 8.4. Results and discussion From the relationship between the kinetic energy and the surface energy of the droplet, we can estimate the size and distribution of bubbles after impact. It is known that jet-size and -velocity are strongly correlated with bubble size [227, 234, 253]. Therefore, different aerosol generation behaviors can be interpreted by the relationship between bubble size and impact velocity. By the energy conservation of the kinetic and surface energy of a droplet, the maximum radius of the flatten droplet after impact is linearly proportional to the impact velocity; therefore, the minimum droplet height is inversely proportional to the square of impact velocity as following: U' l/hmin, where U is the impact velocity, d,, is the maximum droplet diameter, and hmin is the minimum height of the droplet (Figure 8-3). The size and the maximum number of bubbles can be measured from snap shots of high speed movies. From the observation, we know that the number of bubbles is proportional to the wet area of the surface. The wet area can be calculated with the maximum radius of the flatten droplet. Therefore, we can find the relationship between the maximum number of bubbles and the impact velocity as following: U0 2 -x Nma, where A.,, is the maximum wet area of the surface, and N,,a is the maximum number of bubbles formed inside the droplet. (Figure 8-3) Therefore, we can employ the Weber number; We = p DU 2/a, where p is the liquid density, D, is the initial droplet diameter, and a is the liquid surface tension, to evaluate the effect of impact velocity on the number of bubbles and the bubble size. When We >> 1, we can expect that aerosol generation cannot be observed because the height of the droplet is too low to reach the sufficient bubble size to generate aerosols after breaking. In contrast, when We << 1, the number of bubbles decreases and the bubble size is much smaller than the droplet height, resulting in the delay of bubble bursting. 197 The droplet permeation and the initial aerosol dispersion are affected by surface properties, mainly the wettability (see Figure 8-3c). The water absorption speed of porous media can be characterized by the hydraulic diffusivity, Dcap [242]. If we assume that dry soil becomes fully wet after capillary suction (e.g. there are no impenetrable pores), the hydraulic diffusivity can be found using the equation for capillary rise in the absence of gravitational effects: h2 ~ Dcapt, where h is the rise height and t is time [172, 198, 254]. The hydraulic diffusivity can be experimentally estimated from capillary rise measurements [198]. We now define a modified Peclet number (Pe = (U-D 0 )/Dea), where D, is the initial droplet diameter and U is the impact velocity of the droplet. The modified PNclet number is the ratio of the advective transport rate (U -D,) to the hydraulic diffusivity (Dap) and used to represent the effects of surface wettability. 198 14 103V - We 102-- Frenetic Jets region 10 1 Commercial soils Local soils Nano-porous plates Smooth absorption Micro-porous plates 1 4 0 1 10 102 103 104 105 106 Pe Figure 8-4. Characterization of aerosol generation when droplets hit soils and porous surfaces. The x-axis indicates a modified Peclet number (Pe (U&-Do)/Dap), where U and D, are the impact velocity and the diameter of the droplet. The y-axis indicates the Weber number (We pDU2 mainly varied by the impact velocity. We and Pe represent the impact condition and the surface property, respectively. The Dcap values of the soils were measured by the capillary rise experiments. The red symbols indicate the observation of aerosol dispersion from the droplets at the corresponding We and Pe. The blue circle named "aerosol generation region" highlights the group of the data points where aerosols are generated. The yellow symbols inside the aerosol generation region show the data points of frenetic aerosol generation. Rainfalls on soils are placed on the upper part of the characterization map, and clay and sandy-clay soils have concentrated regions in the sparling zone. a-c, the reprehensive images of drop impingements in the regions classified with We. 199 Based on the drop impingement results, we found a domain which resulted in aerosol generation. We employed the Weber number (We) and the modified PNclet number (Pe) parameters in a two dimensional map of our experimental conditions. We observed drop impingements on various types of porous media, including soils, with different liquids and impact velocities. In total we conducted roughly 600 experiments. Experiments were performed on sixteen different soils (eight commercial soils purchased from World's Science, and other soils collected from the local area), five different TLC plates (Sigma-Aldrich, micro- and nanoporous surfaces: silica, cellulose, and aluminum oxide with different pore sizes), and seven porous titania films fabricated in-house [198] (Micro-porous surfaces fabricated with different conditions) with four different liquids (water, ethanol, ethylene glycol, and glycerol) (see Table 8-1). Figure 3 summarizes the observations with respect to Pe and We. We and Pe were varied from 1 to 104 and from 1 to 106, respectively. Aerosol generation is observed in a specific region of We and Pe, as shown in Fig. 3. There was no aerosol generation in the low and high We regions (We < 10 or We > 103). When We > 103, we do not observe aerosol generation because splashing in radial direction (Figure 8-4a) leads to air entrainment. By contrast, when We < 10, few bubbles are entrained and they're much smaller than the droplet height, resulting in delayed bubble bursting and reabsorbed bubbles into the surface. In addition, the surfaces did not show aerosol generation at high or low hydraulic diffusivity (Pe < 10 or Pe > 104). When Pe > 104 , bubbles cannot become sufficiently large to generate aerosols during droplet spreading because the droplet does not permeate into the surface. Conversely, if Pe < 10, the droplet quickly absorbs into the porous surface, faster than the droplet expands. Therefore, air is not trapped at the interface of the droplet and the surface, and there is no aerosol generation. Frenetic aerosol generation is observed at the core of the aerosol generation region on the characterization map (c.f. yellow symbols in Figure 8-4). To achieve frenetic aerosol generation, 200 bubbles have to be pinned on the surface for the duration of reaching sufficient size to disperse aerosol when they break. Therefore, moderate Pe (102 < Pe < 103) is necessary for frenzied aerosol generation since this balances bubble size (favoring low Pe) while minimizing the time it takes for the initial droplet to spread and the bubbles to burst (favoring high Pe). At low We (Figure 8-4c), bubble-bursting is delayed because the average bubble height is smaller than the average droplet height after impact. When We is high, bubbles forming inside the droplet are not large enough to generate water-jets because the droplet becomes very thin after impact. According to the study by Lee et al. [234], bubbles must have a minimum size (the minimum radius is about 26 pm in water) to generate liquid jets when they burst. The average bubble height and the average droplet height become similar within a specific range of We (Figure 8-3a). Frenetic aerosol generation occurs when the bubble diameter is very close to the droplet height, leading multiple bubbles to burst at roughly the same time after impact. This frenetic aerosol dispersion is very attractive because quite a number of aerosols can be produced within a few microseconds, which could be useful for applications requiring fast aerosol generation at ambient temperature. From the characterization map we conclude that aerosol generation can occur during light and moderate rainfall on soils with hydraulic diffusivity less than sand. To locate the impingement domains for rainfall on soil (red, yellow, and green regions in Figure 8-4), the hydraulic diffusivity of the soils were obtained from the literature [240, 244, 246] and the impact velocity of raindrops was set from light rain (U < 4.0 m/s) to heavy rain (U 0 > 7.0 m/s) [252]. When the impingement domains are added to the characterization map, the overlapping regions of aerosol generation and rainfall are evident (Figure 8-4). We verified aerosol generation experimentally for conditions consistent with light and moderate rain on soil with similar water-absorption properties as clay and sandy-clay soil. Aerosol generation was not observed on sand or during conditions consistent with heavy rainfall. 201 8.5. Conclusions This work is meaningful in two respects: first, we present visual evidence of aerosol generation from rainfall hitting soil; and second, we provide an effective means to generate aerosol using simple drop impingement on porous media. Until now, the effects of aerosol from soil have been rarely investigated due to the lack of knowledge of its generation. However, from the result of this work, we can understand the strong correlation of aerosol generation with wetting property of soils and impact velocity of droplets. This is the key to open the study of the environmental and geological effects of aerosol generation from soil. Using the correlation, the rate and the duration of aerosol generation can be effectively controlled. In addition, the chemical composition of aerosols can be altered with porous surfaces pre-permeated with the chemicals. Therefore, this aerosol generation method using liquid droplets and porous surfaces will provide crucial opportunities to develop novel chemical synthesis methods for next-generation materials beyond the aerosol investigation itself. Furthermore, we will possibly illuminate the mechanism of the human infection and the migration of viruses and bacteria originated in soils. 202 Chapter 9. Thesis Summary and Future Work 203 9.1. Thesis Summary The use of electric fields is both cost- and time-effective, as well as mass-producible, when creating and customizing nano- and micro-scale structures and properties. In this dissertation, electrophoretic deposition [73] and breakdown anodization [198] were used as electro-fabrication methods to structure nano- and micro-morphologies and to vary their material properties (Figure 9-1). Electrophoretic deposition uses electrophoresis of nanoparticles dispersed in suspensions. In this work, the use of suspension stability to control nanostructures is unique. Depending upon the quality of dispersion, the nanostructures of the deposition layers can be effectively controlled. In addition to electrophoretic deposition, breakdown anodization was used to create microporous structures with nano-scale feather patterns, resulting in highly porous films [196, 255]. The porous structures can be functionally varied with customized electric fields [256-258]. A hybrid method of electrophoretic deposition and breakdown anodization was proposed to control surface roughness and deposit various nanoparticles on surfaces. In this work, diverse functional surfaces and films were produced by using electro-fabrication methods. Superhydrophobic and superhydrophilic surfaces are good applications for demonstrating the effective control of surface structures and properties using electric fields [73, 198]. Wettability, as a function of surface roughness and energy, can be varied from superhydrophilic to superhydrophobic regions using customized electric fields and different nanoparticles [198, 256]. This technology has been successfully applied to produce highly durable anti-wetting fabric that maintains its original breathability and flexibility [259]. The anti-wetting fabric is a strong candidate for cost-effective and large-area membranes for water desalination and oil-water separation (Figure 9-2a). Critical heat flux in pool boiling was greatly enhanced with hierarchically 204 structured superhydrophilic surfaces produced by both of breakdown anodization and electrophoretic deposition (Figure 9-2b) [196, 255]. Electrophoretic Deposition Breakdown Anodization DC PowerSup DC Power SupPlY Electrode Acidic electrolyte 00 St Nano-structure 0 Mc Nano stimng Micro-structure Figure 9-1. Fabrication processes for micro/nano hierarchically structured surfaces and Scanning Electron Microscopy (SEM) images (a)-(f). For micro-porous structures, breakdown anodization (BDA) is used with different electric fields, (a)-(c). For nano porous structures, electrophoretic deposition (EPD) is used with different nanoparticles, (d)-(f). The hierachical micro- and nanoporous structures are produced by a series of BDA and EPD, (g)-(i). The electrolyte temperature and the nanoparticles affect the surface morphology and the surface energy, respectively. 205 Figure 9-2. Functional surfaces produced by electro-fabrication. (a) Anti-wetting fabric with biomimicked coating-layers for water harvesting. Each fiber of the fabric was uniformly coated with hydrophobic silica nanoparticles, resulting in high static contact angle and low roll-off angle. The fabric maintains its original flexibility and breathability after the modification. The water droplets were growing with a regular pattern similar to those on a real leaf during condensation. (b) Hierarchical surfaces for critical heat flux enhancement. The micropillars, created on a silicon wafer, were coated with hydrophilic silica nanoparticles, enhancing hydrophilicity. The coatings prevent vapor-blankets on the heater surface and dramatically enhance critical heat flux and heat transfer efficiency in pool boiling [255]. In drop impingements on porous films, a new dimensionless parameter, named the WashburnReynolds number, is suggested to predict the impingement modes of droplets hitting the porous films [260, 261]. Use of the Washburn-Reynolds number offers a convenient method for controlling the impingement behaviors of liquid droplets for ink-jet printing, spray painting and spray cooling. The energy equation suggested in this work can be generally utilized for understanding liquid propagation mechanisms on wettable porous films. Furthermore, aerosols can be generated from droplets hitting porous surfaces (Figure 9-3b) [262, 263]. The aerosol generation provides clues to explain the migration of bacteria and viruses encapsulated in the aerosol. 206 Figure 9-3. Aerosol generation from droplets hitting porous surfaces. Frenzied aerosol dispersion can be observed in a specific region of the Weber number of droplets and the hydraulic diffusivity of surfaces. 9.2. 9.2.1. Future Research Porous Thin Film Microfluidic Devices Designed by Breakdown Anodization A new methodology is presented to develop microfluidic devices using highly wetting porous titania films. Production of low cost diagnostic tools, allowing for small sample volumes, is in high demand. Thus the use of capillary flows employed in paper-based microfluidic devices has been considered highly attractive. Capillary driven flows through thin porous media have been widely utilized for small-scale liquid transportation systems, including chromatographic analyzers, biosensors, micro-chemical reactors, and paper-based microfluidic devices (Figure 9-4). 207 (a) Reagent Sample Microfluidic I:Pump P device (b) Porous thin film microfluidic device I 00 0 -0 -r Reagents -00 Sample -_O - 0 _ Porous thin film channel Figure 9-4. Schematic comparison between (a) a conventional microfluidic device and (b) the porous thin film microfluidic device. Capillary flows show different characteristics depending on the porous substrate. Examples include glass capillary tubes, porous media composed of densely packed particles or fibers, and sponges. However, for most media it is challenging to realize arbitrary shapes and spatially functionalized micro-structures with variable flow properties. It is also difficult to control the liquid transport properties and to scale-down dimensions under hundreds microns with media such as paper. Recently, we developed a unique anodization technique, called breakdown anodization (BDA), with titanium plates to produce highly wetting porous thin films. The porous layer thickness is a few tens of microns, which is much thinner than nearly all types paper. Furthermore, when we characterize liquid transport speed in terms of hydraulic diffusivity, the porous films showed much higher hydraulic diffusivity than commercial chromatography paper. Hydraulic diffusivity of BDA surfaces linearly increases with electric potential during anodization, and the liquid transport distance can be predicted with a conceptual capillary tube model. 208 In this work, BDA is applied to the development of microfluidic devices using porous titania thin films. Using a conventional soft lithography technique, micro-patterned electrodes are prepared on titanium sheets for anodization (Figure 9-5). After BDA, highly wetting porous films is generated on the micro-patterned surfaces (Figure 9-6). The porous thin film microfluidic (PTFM) channels show fast liquid transport over long distances with minimal sample volume. A B UV illumination 4 I Photo IIIIImask C Developing D Electrolyte Counter electrode Breakdown anodization E BDA surfaces Figure 9-5. Fabrication process. A photoresist Figure 9-6. Porous thin film microfluidic layer channels produced by BDA. (a)-(c) Patterned Ti is patterned using photolithography. Breakdown anodization is conducted with the sheets patterned Ti electrode in an acidic electrolyte. after BDA with different electric potentials. (d)-(f) Thin channels generated by BDA. The PTFM chips will be used to identify various properties of a simulated urine sample including the presence of glucose, phosphate, chloride, and pH. The corresponding reagent, placed on the 209 PTFM chip changes color depending on the concentration of each element. By comparison with commercial porous media, the PTFM chips facilitate precise test results with one sixth smaller sample volumes in m 2 /L and five times faster diagnosis in m 2 /sec than a commercial chromatography paper (Figure 9-7). Thin titania porous films produced by BDA are an attractive alternative substrate for microfluidic diagnostic devices requiring small sample volumes. Advantages of this technique include design flexibility, fast and low sample volume diagnosis, and low-cost fabrication. For our future work we will demonstrate multi-channel electrospray from PTFM devices, which can be used in ambient ionization sources, for high sensitivity sample analysis. 120$ E 1.5 *21 Hydraulic diffusivity 100 E 80 60 - E . Wetted area - 2- 40-a S0.50 -20 1rN 1 AUmmmm 0 Wicking media Figure 9-7. Wettable area and wicking speeds of different porous media with a one microliter sample of 1 g/L Rhodamine B in deionized water. The wetted area of the BDA surface is significantly larger due to its high permeability and low thickness. 210 9.2.2. Conductive Hydrogel Produced by Electrophoretic Deposition at the Interface of Two Immiscible Liquids A novel method is presented to fabricate hydrogel composite nanomaterials using electrophoretic deposition at the interface of two immiscible liquids. Hydrogels have been employed for biomaterials due to their outstanding bio-compatibility, high porosity, and notable swelling capabilities. Thin hydrogel membranes are desired because diffusion and mass transfer effects across the films can be improved. Further, conductive hydrogel films have been investigated because they can provide a critical methodology to sense and diagnose biological systems or used for energy storage. To date, vacuum filtering techniques and electrochemical deposition have been utilized to fabricate conductive polymer films. However, the filter textures and pressure gradients distort the polymer structures; especially when mechanically weak hydrogels are used. Here, we propose a novel process employing electrophoretic deposition (EPD) at the interface of immiscible liquids to create composite hydrogel films (Figure 9-8). During interfacial EPD, nanoparticles such as carbon nanotubes (CNTs) migrate to the oil/water interface, where crosslinking of polymers is induced to form composite hydrogel membranes. The key aspect of this method is that polymerization occurs away from a solid substrate while surrounded by both polar and nonpolar media, allowing for the integration of CNTs or other nanoparticles and hydrogel. Properties of the composite hydrogel films are controlled by the deposition and polymerization time, allowing for mass production without the need for complex machinery. This fabrication method is cost-effective and scalable for composite hydrogels with tunable electrical, mechanical, and biological properties. Potential applications include fabrication of doped hydrogels for drug delivery and conductive hydrogels for biological sensing. 211 Oil phase Cross-linker CNT & hydrogel compositeayr Water phase Electrophoretic deposition CNT& surfactant Hydrogel _- monomer Materials " Sodium alginate " Carbon nanotube " Sodium dodecyl sulfate (SDS) " Calcium chloride Figure 9-8. Fabrication procedure for the carbon nanotube and hydrogel composite layer and the experiment setup for electrophoretic deposition with two immiscible liquids. Both water and oil are added to a container and the electrode assembly is placed into the container. The oil-water interface must be positioned at the middle of both electrodes. During an electric potential is applied to the electrodes, the crosslink solution is added to the top of the oil phase. After the polymerization, the oil phase is extracted and then the composite hydrogel film is gently lifted up. Experiment setup for electrophoretic deposition at the oil-water interface. 9.2.3. Dispersion of Carbon Nanotubes in Aqueous Solutions of Ionic Surfactants A mean-density model of surfactants is suggested to illuminate the mechanism of carbon nanotube (CNT) dispersion in aqueous solutions of ionic surfactants. Ionic surfactants have been used to make well-dispersed aqueous CNT suspensions. However, the mechanism of the dispersion is not clear. To understand the mechanism, molecular dynamics (MD) simulations were recently employed, but they require high computing power to obtain significant results. With the meandensity model, we can effectively estimate the mean force potential between CNTs stabilized with 212 surfactants. Notably, we find that the osmotic pressure between CNTs plays an important role in the suspension stability. The mean-density model can help to determine appropriate surfactants and concentrations necessary for stable CNT dispersions. In the mean-density model, the surfactant heads suspended on a CNT are assumed to constitute a single cylindrical group. When a CNT is dispersed in aqueous solutions with ionic surfactants, the surfactant tails are attached to the CNT due to its hydrophobicity. However, the hydrophilic heads of the surfactants orient towards the water (Figure 9-9a). If we assume that the surfactants are attached on the CNT surface uniformly, the CNT and surfactant assembly can be simplified as shown in Figure 9-9b. In the mean-density model, there are two cylindrical groups corresponding to the surfactant head and the CNT. The CNT has uniform carbon atom density, resulting in the same weight as the actual CNT. Similarly, the surfactant head group has uniform density and valence charge. (b) Surfactant CNT Cylindrical surfactan head group 2Rs s Rc Head: As CNT Tail: Ls Figure 9-9. Schematic illustrations of carbon nanotube and surfactant assemblies. The surfactant is composed of a hydrophilic head, which has the cross sectional area of A, (the head radius is R,) and a hydrophobic tail, which has length L. (a) In reality, multiple surfactants are attached on the surface of the CNT. (b) In the simplified model, the surfactant heads are considered a single cylinder. 213 To verify the mean-density model, we compare the total interaction potentials obtained from our model and the molecular dynamics (MD) simulation. In our model, the total interaction potential is calculated by superposing the van der Waals force, the electrostatic force, and the osmotic pressure between two the CNT-surfactant assemblies shown in Figure 9-10b. As a result, the total interaction potential obtained from the mean-density model (Figure 9-1 Oa) agrees well with the MD simulation. Furthermore, our model successfully shows the effect of surfactant concentration on dispersion stability. When the bulk solution concentration is higher than the surfactant critical micelle concentration (CMC), micelles of the surfactant form in solution, maintaining the bulk monomer concentration at CMC. Therefore, the osmotic attraction force becomes significant when the bulk concentration is higher than CMC, lowering the maximum potential barrier, as shown in Figure 9-10b. From this result, we show that the CNT dispersion is unstable when the surfactant concentration is higher than the CMC. Osmotcpress.re . 0 van - o * (b) der Was force between CNTs Electrostatic force osmoticpressure van der Wasis between surfactants Total interaction potential - 20 - 01 0 37 30so 70 90 100 van der Waaft force between CNTs Electrostatic force O M van der Waals between surfactants Total interaction potential * 110 b dlnmJ 401.Wj a 0 ,0 ' (a) -Tf j~~I SX0 60 7 / 6d90 rn uIoJI Figure 9-10. Total interaction potentials between two CNT-surfactant assemblies. The total interaction potential can be obtained by the summation of van der Waals force, electrostatic force, and osmotic pressure. The total interaction potentials of (a) and (b) are obtained with the surfactant coverage ratio of 30 % and the tail stretching ratio of 100 %. Different bulk surfactant concentrations of CMC (8.7x 10- mol/L) and 2xCMC are used in (a) and (b) to compare the effect of micelle formation 214 The mean-density model can show reasonable predictions for the interaction potential between CNT-surfactant assemblies with much less computational time than the molecular dynamic simulations. Furthermore, this model will illuminate the effects of osmotic pressure between the assemblies on dispersion stability. This result will show that more CNT agglomerates form when the surfactant concentration is higher than the CMC, and the most stable CNT-surfactant suspension is obtained around the CMC. 215 216 Bibliography 1. 2. Blossey, R., Self-cleaningsurfaces - virtual realities. Nat Mater, 2003. 2(5): p. 301-306. Min, W.L., B. Jiang, and P. Jiang, Bioinspired Self-Cleaning Antireflection Coatings. 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