Y. Socol
1
, L. Berenstein
1
, O. Melamed
1
, B. Nitzan
2
, E. Bormashenko
3
, V. Streltzov
3
and
A. Zaban*
1
1
Dept. of Chemistry, Bar-Ilan University, Ramat-Gan 52900 Israel
2
Aprion Digital Ltd., 5c Hatzoran St., New Industrial Area, P.O.Box 8743, Netanya
42505, Israel
3
The Research Institute, College of Judea and Samaria, Ariel 44837 Israel
Abstract
Evaporation of colloidal drops is studied in many contexts including nano-technology: evaporation-driven flow leads to regular annular patterns of nano-particles. We report on a phenomenon of annular pattern formation in drops of water-based suspensions on substrate, which occurs in several seconds, unlike 10 minutes and more in case of evaporation-driven flow.
The phenomenon sometimes leads to highly undesirable effects, but at the same time it may open the way for new applications. The pattern formation can take several seconds only, after that the pattern can be fixed by rapid drying.
The effect is probably caused by Marangoni flow due to gradient of interfacial energy of colloidal particles. The critical conditions are hydrophobic pigment, surfactant’s affinity to the pigment and, probably, surfactant’s moderate strength.
In addition we report here a method for the study of the relative motion of the solvent and pigments in inkjet droplets. Coloring of the solvent with soluble dye of a different spectrum compared with the pigments allows simultaneous monitoring of the solvent and pigments during the drying process using spectral imaging techniques.
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Introduction
Evaporation of colloidal drops is studied in many contexts, such as food industry[1], pharmaceutics[2], fuels [3] etc. Recently, there is a considerable activity in the field of nanotechnology: drop evaporation on substrates lead to regular annular patterns of nano-particles[4-
7], and such patterns are of potential interest in semiconductor technology [5-7]. In all the cited works, the pattern formation took from 10 minutes to 1 hour and more, at constant low evaporation rate. Higher evaporation rate led inevitably to destruction of the patterns.
We report on a phenomenon of annular pattern formation in drops of water-based suspensions on substrate, which occurs in several seconds. The phenomenon leads to highly undesirable effects in printing industry, but at the same time it may open the way for new applications such as mentioned above. Thus the understanding of this phenomenon seems of significant importance. In addition we report here a method for the study of the relative motion of the solvent and pigments in inkjet droplets. Coloring of the solvent with soluble dye of a different spectrum compared with the pigments allows simultaneous monitoring of the solvent and pigments during the drying process using spectral imaging techniques.
Experimental
The study was performed using the Cyan water based ink manufactured by Aprion Digital
Ltd. (Netaniya, Israel) and two different surfactants – SDS and BYK-345. The water-based ink contained 20% dipropylene glycol, 5% ethylene glycol , 5% propylene glycol, 2.4% solids of
Microlith blue 4G WA (Cu-phthalocyanine) manufactured by Ciba Inc. (“pigment”), additional resins and the balance water to perform stable ink with viscosity of 12.6 cP and surface tension of 39.6 dyne/cm.
SDS – sodium dodecyl sulfate CH
3
(CH
2
)
10
CH
3
OSO
3
Na – is a classical short molecule anionic surfactant, while BYK-345 – polyether modified polymethyl siloxane, produced by BYK
Chemie, Germany – is non-ionic surfactant.
The behavior of the drops on 2 substrates – PVC and glass – was studied. For PVC we used
Standard PVC coated ( 90 gsm ) mechanical pulp ( 80 gsm ), manufactured by Forbo CP Ltd.,
UK. For glass – microscope slide of halfwhite glass, from Paul Marienfeld GmbH & Co. KG,
Germany.
The final ink solutions were prepared as follows. First, the surfactant was measured by weight on analytical scales. After this, the ink was added in the amount, which was necessary to obtain the desired weight concentration of the surfactant in the solution. We studied the interval of surfactant concentrations 0 – 3% w/w (prepared solutions had 0.1, 0.5, 1 and 3% w/w). Usually
1 – 3 g of solution was prepared. The prepared solution was then subject to magnetic stirring during 15 min. The prepared suspensions were stable for at least several days.
Obtaining information about the solvent and the pigment separately poses a problem since the solvent is transparent. In order to cope with the problem we added a color dye Ethyl Red ( 1-(4-
Diethylaminophenylazo) benzoic acid, CAS 76058-33-8 ) to the solvent. Since Ethyl Red is not solvable in water, 50 g/L solution of Ethyl Red in ethanol was prepared, and this solution was intermixed with the original ink preparation in proportion 1:1 (Vol.). The behavior of this mixture was undistinguishable from the behavior of the original (Aprion Cyan + 3% SDS) preparation. Drop images were taken with two narrow bandwidth-pass filters of 450 and 600 nm, corresponding to absorption maxima of Ethyl Red and Aprion Cyan correspondingly. Thus, the
450 nm filter images carry information about the pigment, and 600-nm filter – about the solvent.
To simulate the inkjet printing process, the paint was injected onto the substrate by means of a glass pipette tip, which is vibrated with ultrasonic frequency. The drops’ diameter on substrate was 30-50 μm. The image frames are read via microscope by CCD camera and stored in computer as pixel-to-pixel maps of light intensity during the drop evolution till its shape being
"frozen". The methodic, including the algorithm of the 3D visualization, is reported elsewhere
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[8].
Results
For the initial ink composition (no surfactant added) on PVC substrate, the spatial shape of the drop remains more or less the same during the evolution. However, with 3% SDS w/w added, the drop behavior is significantly different: after 1 second it begins to form a non-monotonic structure with valley in the center. This behavior is illustrated at Figs. 1 and 2 where the left and right columns contain 3D and color based 2D spatial visualization of the distribution of the pigment (see [8] for details). One can readily see, that the spatial shape of the drop remains more or less the same for the initial ink composition (Fig. 1). The same picture was observed with solutions containing 0.1%, 0.5% and 1% SDS w/w. In contrast to this drying pattern, when the ink contained 1.5–3% SDS w/w the behavior is strikingly different. The time evolution of drops made from this ink, presented in Figure 2 shows after 1 second the beginning of a nonmonotonic structure formation yielding a “valley” in the center of the drop.
Printing on glass, the results were the same: solutions containing 0, 0.1, 0.5 and 1% SDS w/w behaved in an usual way producing uniform drops as at Fig.1, while 3% SDS solution developed annular structure as at Fig.2.
The described ring-formation effect was totally absent when BYK-345 was added to the basic ink instead of SDS.
The results of separate imaging of pigment and solvent are presented at Fig. 3. Here, a 2D presentation of two colors in the same microscope view are presented. One color relates to the pigments dispersed in the ink (Fig. 3, right) while the other relates to the dye dissolved in the solvent (left). One can easily see that while the pigments form the rings, the dye (solvent) distribution has regular drop-like shape.
Discussion
Often ring stains are produced from liquid drops due to capillary flow, when the pigment particles are drawn towards the border by the flow of the solvent [4-7]. As already mentioned in the introduction, this process takes minutes and more, and is very improbable here (the pattern is formed in seconds, long before the evaporation is over). However, our investigation provided direct evidence against the above mechanism. Since the solvent is evaporated and the added dye is non-volatile, the latter should have been accumulated near the drop border and be imaged in the same way as the pigment distribution. We do not see this picture, i.e. the dye is regularly distributed all over the initial drop surface. We are urged therefore to dismiss this ring-formation mechanism and state, that the pigment particles are aggregated towards the drop boundary in the liquid support phase.
It may be supposed that the pattern formation is connected to loss of stability (coagulation) of the suspension by DLVO mechanism [9,7]. In this case, different effect of SDS and BYK-345 can be explained by the fact that SDS, dissociating in water and increasing the ionic strength, leads to coagulation. BYK-345, being non-ionic, does not cause coagulation. However, we should remember that the suspensions are stable for days if not sprayed to drops of several tens of μm. Therefore, we should seek for some surface effects that lead to the pattern formation.
A possible direction to explain the effect may be related to the liquid-air surface tension changes induced by the surfactant addition. However, BYK-345 affects the surface tension liquid-air much stronger, than SDS: one can readily see this at Fig. 4 which presents the measured surface tension as function of the surfactant concentration. And despite the lower surface tension, the described ring-formation effect was totally absent when BYK-345 was added to the basic ink instead of SDS. We have therefore to dismiss this explanation.
The phenomenon is probably due to Marangoni flow driven by surface tension gradient [9-
12]. The segregation mechanism seems to be two-stage: pigment migration to the free surface, and then segregation to the edges. We note, that Cu-phthalocyanine is hydrophobic, i.e. has
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positive surface tension in respect to water: its molecule has rather large organic chains that have low affinity to polar molecular bonds of the water. Therefore SDS, having an organic chain, has high affinity to Cu-phthalocyanine (affinity of non-polar organic bonds), and should act as a surfactant for pigment-solvent interface [9]. The same may be relevant for BYK-345, though details of its structure are not disclosed. However, distributions of SDS and BYK-345 in the drop volume should be very different. One can see at Fig. 4, that small addition (<0.5%) of BYK-345 decreases the surface tension (ST) nearly to saturation, and further addition has nearly no effect.
As for SDS, its addition decreases ST gradually. It is therefore plausible to assume, that BYK-
345 forms a thin surface layer and is nearly uniformly distributed below. And SDS forms concentration gradient, with concentration gradually increasing towards the liquid-air interface.
So pigment-solvent surface tension (p-s ST) gradient is formed: the p-s ST is lower in the drop bulk and decreases towards the surface. The interfacial energy (E) of a pigment particle is proportional to the surface tension ( s): E=A s , where A is a pigment particle surface area. In presence of surface tension gradient ds/dr, each particle is subject to a driving force [13] F= d E/dr
= A ds/dr . The pigment particles are pumped therefore to the drop surface, when sufficient amount of SDS is added.
It should be noted here that that the pigment particle velocity is inversely proportional to the liquid viscosity. As solvent evaporates, the viscosity increases. Therefore there should be some critical value of surface tension gradient ds/dr that leads to visible changes. For lower values of this gradient, particles do not have enough time to migrate till their movement is "frozen" by increased viscosity (therefore after the pattern is formed, the evaporation rate may be significantly increased e.g. by hot air flow; industrial efficiency is therefore much higher). The simplest assumption about the surface tension dependence on the surfactant concentration (c): s = s o
– s
1
c, yields ds/dr= s
1 d c/dr. Within the discussed approach concentration gradient d c/dr is proportional to concentration (c) itself, therefore the effect is observed only for rather high surfactant concentration.
When the pigment particle reaches the surface, it is driven towards the edges– e.g. since the drop curvature (and therefore capillary tension) are higher in the center for drops on hydrophilic substrates (as in our case).
It should be stressed here once more, that after the annular pattern is formed and the pigment particle movement is "frozen" by increased viscosity, the evaporation rate may be significantly increased by hot air flow, infrared or microwave drying. The pattern is formed in matter of several seconds and can be dried several seconds later. The cost-efficiency may be much higher than in case of slow evaporation-driven pattern formation, that takes at least many minutes.
Conclusion
The ring-like pattern of high-SDS water-based ink drops is caused by pigment particles aggregation to the drop boundary, probably due to pigment-solvent surface tension (ST) gradient.
For preparing industrial suspensions, surfactants and other additives should be chosen taking into account their effect on pigment-solvent surface tension. This understanding should be of great importance for designing new industrial ink compositions. On the other hand one can use this approach for deliberate rings’ formation (including 2D structures) or spatial material separation, e.g. in semiconductor industry. Such pattern formation can take several seconds only, after that the pattern can be fixed by rapid drying. We note that the critical conditions are hydrophobic pigment, surfactant’s affinity to the pigment and, probably, surfactant’s moderate strength, which leads to a gradual depth distribution and constant ST gradient.
We would like to thank Dr. A. Arinstein and Dr. O. Gendelman and especially Dr. Y.
Guzman for extremely fruitful discussions.
References
[1] Adhikari, B Howes, T Bhandari, BR Truong, V. Experimental studies and kinetics of single
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drop drying and their relevance in drying of sugar-rich foods: A review. INTERNATIONAL
JOURNAL OF FOOD PROPERTIES 3(3) 323-351 (2000)
[2] Gibaud, S, Bonneville, A , Astier, A. Preparation of 3,4-diaminopyridine microparticles by
solvent-evaporation methods. INTERNATIONAL JOURNAL OF PHARMACEUTICS 242 (1-
2) 197-201 (2002)
[3] Daif, A Bouaziz, M, Grisenti, M. Vaporization of binary fuel mixture droplets in a thermal wind tunnel. JOURNAL OF THERMOPHYSICS AND HEAT TRANSFER, 12(1) 107-113
(1998).
[4] R. D. Deegan, O. Bakajin, T.F. Dupont, et al. Capillary flow as the cause of ring stains from dried liquid drops. Nature 389, 827 (1997)
[5] Maenosono S, Dushkin CD, Saita S, et al. Growth of a semiconductor nanoparticle ring during the drying of a suspension droplet. LANGMUIR 15 (4): 957-965 FEB 16 1999
[6] Sommer, AP, Ben-Moshe, M, Magdassi, S. Size-discriminative self-assembly of nanospheres in evaporating drops. JOURNAL OF PHYSICAL CHEMISTRY B 108(1) 8-10
(2004)
[7] Mori, Y, Zaitsu, K Particle deposition in evaporating droplets of polystyrene latex
suspension on hydrophilic and hydrophobic substrates, JOURNAL OF CHEMICAL
ENGINEERING OF JAPAN 37(5) 657-661 (2004).
[8] Socol Y, Berenstein L, Melamed O, et al. Method for in situ measurements of ink jet printed ink drops J IMAGING SCI TECHN 48 (1): 15-21 JAN-FEB 2004
[9] Adamson A.W, Gast A. P. Physical Chemistry of Surfaces, 6-th ed., John Wiley and Sons,
1997.
[10] P.G. de Gennes, A. Aradian, E. Raphaël A scaling theory of the competition between interdiffusion and cross-linking at polymer interfaces Macromolecules, 35 4036-4043 (2002)
[11] S.Ryazantsev P. Gupalo. Thermocapillary motion of a liquid with a free surface with nonlinear dependence of the surface tension on the temperature. Fluid Dynamics , 23(5):752-757,
1989.
[12] Lavrenteva OM, Leshansky AM, Berejnov V, Nir A. Spontaneous interaction of drops, bubbles and particles in viscous fluid driven by capillary inhomogeneities. Ind. Eng. Chem. Res.
41 (3): 357-366 FEB 6 2002
[13] Landau, Lifshitz. Statistical Physics. Butterwoth-Hinemann, Oxford, 2000.
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sec
Time,
0
3
10
Fig. 1 Evolution of an ink drop (Aprion Cyan, no surfactant added) on PVC substrate. A typical uniform structure is formed.
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Time, sec
0
1
10
50
Fig. 2 Evolution of an ink drop (Aprion Cyan) with 3% SDS w/w added. PVC substrate. An annular structure is formed with no pigment in the center.
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Fig. 3. Solvent (left) and pigment (right) spatial distribution in the drop. The pictures were taken with 2 different narrow-band optical filters. Solvent was made visible by adding Ethyl Red dye (see text). Annular structure can be readily seen for the pigment (right), but not for the solvent.
Fig. 4. Influence of SDS (black) and BYK-345 (blue) on the surface tension ink-air. BYK-345 has stronger influence, but it does not cause the annular structure formation.
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Fig.5. Proposed mechanism of annular structure formation. 1-st stage: non-uniform solvent evaporation creates gradient of non-volatile surfactant concentration (higher at the droplet edges), and therefore surface tension gradient (STG) – ST is lower at the droplet edges. 2-nd stage: STG drives the hydrophobic pigment particles towards the edges, forming the annular structure.
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