J
Active Lubricant-Impregnated Surfaces
SACHUSETTS INSTITUTE
OF TECHNOLOGY
by
AUG 15 2014
Karim S. Khalil
B.S.E. Mechanical Engineering and Materials Science
Duke University (2012)
LIBRARIES
Submitted to the Department of Mechanical Engineering
in partial fulfillment of the requirements for the degree of
Master of Science in Mechanical Engineering
at the
MASSACHUSETTS INSTITUTE OF TECHNOLOGY
June 2104
@ Massachusetts Institute of Technology 2104. All rights reserved.
Signature redacted
........
A uthor ..........................
Department of Mechanical Engineering
May 9, 2104
Certified by.................Signature
I
redacted
i~pipa K Varanasi
Doherty Chair in Ocean Utilization
Associate Professor of Mechanical Engineering
Thesis Supervisor
Signature redacted
Accepted by ...............
Dave E. Hardt
Ralph E. and Eloise F. Cross Professor of Mechanical Engineering
Chairman, Department Graduate Committee
2
Active Lubricant-Impregnated Surfaces
by
Karim S. Khalil
Submitted to the Department of Mechanical Engineering
on May 9, 2104, in partial fulfillment of the
requirements for the degree of
Master of Science in Mechanical Engineering
Abstract
This thesis presents the design and testing of actively controlled lubricant-impregnated
surfaces for enhanced droplet mobility and manipulation. Droplet manipulation and
mobility on non-wetting surfaces is of practical importance for diverse applications
ranging from micro-fluidic devices, anti-icing, dropwise condensation and biomedical
devices, however most of the time droplets are moved passively. The use of active external fields has been explored via electric, acoustic and vibrational fields, yet moving
highly conductive and viscous fluids remains a challenge. Magnetic fields have been
used for droplet manipulation, however the fluid is usually functionalized to be magnetic, and requires enormous fields of superconducting magnets when transitioning
to diamagnetic materials such as water.
This thesis presents a new class of active surfaces by impregnating active fluids
such as ferrofluids into a textured surface. Droplets on such ferrofluid-impregnated
surfaces have extremely low hysteresis and high mobility such that they can be propelled by applying relatively low magnetic fields. Our surface is able to manipulate a
variety of materials including diamagnetic, conductive and highly viscous fluids, and
additionally solid particles. The surface's droplet propulsion mechanism is described,
and is demonstrated to operate independently of the fluid or solid's physical properties that normally inhibit motion (such as conductivity, viscosity, magnetization).
In addition, several previous methods for droplet manipulation require pre-fabricated
channels that govern the path of the fluid, however we are able to achieve precise
control of droplets on a free surface along complex paths, which allows for the use of
a single surface for any number of lab-on-a-chip applications and designs.
Thesis Supervisor: Kripa K. Varanasi
Title: Doherty Chair in Ocean Utilization
Associate Professor of Mechanical Engineering
3
4
Acknowledgments
I would first like to thank my advisor Professor Kripa Varanasi. Without his expertise,
creativity and most importantly motivation, I could not have completed this work.
I'd like to thank all the members of the Varanasi Lab - current and past, that
have heavily influenced my work with their relentless attention to detail. It is truly
a pleasure to work alongside all of these amazing people. Specifically the great discussions and support from Hyuk-Min, Dave, Gisele, Divya, Rajeev, Nada, Konrad,
Navdeep and Lauren. A special thanks to Adam, Brian, Srinivas, Sushant and Seyed
for being sources of inspiration for me and showing me how to succeed in the lab.
I would also like thank the MIT-KFUPM Center for Clean Water and Clean
Energy for financial support to complete this work.
I would not be where I am today without my parents. The selfless effort they have
put in to providing for my brothers and I has been a true source of motivation my
entire life, and is what drives me to always follow through with all my goals. Thanks
for everything Mom & Dad. Also, as the youngest of three, I truly have the two best
older brothers one could ask for. Ahmad and Ayman will always be my role models.
Lastly, I want to thank my friends. There is no possible way I can explain how
much each of you mean to me in this short bit of writing. Thank you to all of my
friends from home, Duke, and Boston for being such a big part of my life. To my
Duke Maxwellians: there are way too many of you to list, but I'd like to say that you
remain and will always be among my best friends and I'm ready for the next reunion
trip to wherever. Special thanks to Sameer, Matt, Shaunak, Cohen, Brian, Flav and
Dave, you guys are like brothers to me. Finally, I want to thank you Betsy, your
unquestioning support and comfort along the way keeps me going and has had such
a significant impact on me. I've had the best times with you, and you always know
how and when to help, I can't thank you enough.
5
6
Contents
2
11
1.1
Motivations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
12
1.1.1
Condensation Heat Transfer and Power Generation . . . . .
12
1.1.2
Lab-on-a-chip Droplet Manipulation
13
.
. . . . . . . . . . . . .
Fundamentals of Lubricant-Impregnated Surfaces
15
2.1
. . . . . . . . . . . . . . . . . . . . .
16
2.1.1
Young-Laplace Equation . . . . . . . . . . . . . . . . . . . . .
17
2.1.2
Young Equation . . . . . . . . . . . . . . . . . . . . . . . . . .
17
Surface Thermodynamics . . . . . . . . . . . . . . . . . . . . . . . . .
18
2.2.1
Impregnation and Pinning . . . . . . . . . . . . . . . . . . . .
18
2.2.2
Cloaking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
21
2.2
3
.
Introduction
.
1
Governing Interfacial Relations
Ferrofluid-Impregnated Surfaces
23
3.1
Ferrofluid Properties . . . . . . . . . . . . . . . . . . . . . . . . . . .
23
3.2
Sample Preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . .
24
3.2.1
Impregnation . . . . . . . . . . . . . . . . . . . . . . . . . . .
24
3.2.2
Cloaking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
25
Droplet Manipulation . . . . . . . . . . . . . . . . . . . . . . . . . . .
26
3.3.1
Initial Droplet Actuation Tests
. . . . . . . . . . . . . . . . .
26
3.3.2
High-Magnification Droplet Studies . . . . . . . . . . . . . . .
29
3.3.3
Jetting Transition . . . . . . . . . . . . . . . . . . . . . . . . .
30
3.3.4
Vertical Orientation . . . . . . . . . . . . . . . . . . . . . . . .
32
3.3
7
3.4
4
Potential Applications
. . . . . . . . . . . . . . . . . . . . . . . . . .
34
Conclusions and Recommendations
37
A Experimental Methods and Materials
39
A.1 Fabrication of Surfaces ..........................
A.1.1
Silanization .......
............................
39
39
A.2 Droplet Orientation-Angle Experiments . . . . . . . . . . . . . . . . .
40
A.3 Jetting Experiments
40
. . . . . . . . . . . . . . . . . . . . . . . . . . .
A.4 Vertical Orientation Experiments
. . . . . . . . . . . . . . . . . . . .
40
A.5 Advanced Path Experiments . . . . . . . . . . . . . . . . . . . . . . .
41
43
B Additional Studies
B.1 Sub-Surface Magnet Configurations . . . . . . . . . . . . . . . . . . .
43
B.2 Ice M obility Tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
44
8
List of Figures
1-1
Water rolling off a lotus leaf demonstrating the lotus effect [1]. . . . .
12
1-2
Liquid-Liquid microfluidic system [2]. . . . . . . . . . . . . . . . . . .
13
2-1
Schematic of a superhydrophobic surface. . . . . . . . . . . . . . . . .
16
2-2
Schematic of a lubricant-impregnated surface.
16
2-3
Force balance in horizontal direction of a liquid droplet on a surface.
. . . . . . . . . . . . .
Force balance is used to derive Young's equation.
. . . . . . . . . . .
18
2-4
Impregnation criteria in the presence of a vapor (air). . . . . . . . . .
20
2-5
Impregnation criteria in the presence of a fluid (water). . . . . . . . ..
20
2-6
Diagrams representing positive and negative spreading coefficient So(a)
(cloaking versus non-cloaking surfaces)
. . . . . . . . . . . . . . . . .
21
3-1
Molecular structure of octadecyltrichlorosilane (OTS) . . . . . . . . .
25
3-2
SEM's of dry textured micro post array.
26
3-3
a) Schematic of ferrofluid-impregnated surface b) Photograph confirming cloaking criteria.
. . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . .
3-4
c) Snapshots of droplet propulsion.
3-5
d) Droplet position versus time for experiment.
3-6
a) Area of clumped particles b) Droplet orientation versus Magnet
orientation.
. . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . .
27
28
29
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
30
3-7
a) Jetting experiment. Droplet deformation versus magnet distance. .
31
3-8
b) High-Speed images of fluid jet detaching from cloak/air interface in
the jetting regime. c) Snapshots of droplet deformation as the magnet
is slowly approached to the surface. . . . . . . . . . . . . . . . . . . .
9
32
3-9
Droplet sliding down on a vertical surface. Magnet is able to suspend
a droplet on the surface avoiding shedding by gravity. . . . . . . . . .
33
3-10 Magnet perpendicular to surface brought close enough to observe normalfield instabilities form on surface.
. . . . . . . . . . . . . . . . . . . .
34
3-11 a) Two water droplets (dye used to color droplets) are placed on
ferrofluid-infused surface. Magnet lowered vertically directly between
droplets and causes them to move toward one another. Coalesence of
droplets occurs and is captured just before droplets fully mix. ....
35
3-12 b) Droplets of various fluids and one solid are actuated and displaced
when a permanent magnet is approached to the surface . . . . . . . .
36
3-13 c) Water droplet being dragged along an s-curve as the magnet is held
beneath the surface.
. . . . . . . . . . . . . . . . . . . . . . . . . . .
36
B-1 Water Droplet is moved by magnet that is placed beneath the surface.
44
B-2 Block of ice frozen and placed onto ferrofluid-impregnated surface. . .
46
10
Chapter 1
Introduction
The field of interfacial science has evolved greatly over the past decade. Most popularly, the "lotus effect" which has been well studied, allows for droplets to roll easily
along the lotus leaf surface, as seen in Figure 1-1. The leaf is able to stay pristinely
clean despite growing in harsh, muddy conditions through its ability to "self-clean"
in which water droplets roll off the surface dislodging and removing dirt from the
surface.
Droplet mobility and manipulation on non-wetting surfaces has received widespread
attention due to its significance in a variety of applications such as liquid repellency [3-9], anti-icing [7,10-18], dropwise condensation [19-23] and biomedical devices [24-26].
In these applications droplets are normally moved passively, most
often under the force of gravity [4]. Active manipulation of droplets using electric
fields [2, 27] and vibrational fields [28, 29] have been studied, yet moving highlyconductive and viscous fluids remains difficult.
The goal of this work is to create surfaces that utilize active-external force fields to
manipulate free droplets. The fundamentals governing such surfaces interaction with
the external fields is explored, and the results of testing a variety of liquids on the
surfaces are presented. This thesis will focus on the use of magnetic fields. Current
methods for droplet manipulation using external fields are reviewed as well.
11
Figure 1-1: Water rolling off a lotus leaf demonstrating the lotus effect [4].
1.1
1.1.1
Motivations
Condensation Heat Transfer and Power Generation
Currently, steam power cycles account for approximately 50% of the energy that is
produced globally. Condenser surfaces are relied on for providing low temperature
and pressure surfaces at the outlet of the turbines. Yet, as these surfaces are currently
made from high surface energy metals such as titanium, they operate in a filmwise mode, in which a film of water forms and serves as a thermal insulator on the
condenser surface. By designing the surface correctly, condensation can be operated
in a dropwise mode in which the condensed water is shed rapidly from the surface
in the form of individual drops, and one may achieve an increase in heat transfer
coefficient by a factor of more than 10 [30]. This would correspond to an increase in
overall plant efficiency of up to 5%. The surfaces investigated in this thesis could be
later directly studied for enhanced condensation capabilities due to their ability of
on-demand droplet shedding.
12
ground
ground
a.c.
ground
Figure 1-2: Liquid-Liquid microfluidic system [2].
1.1.2
Lab-on-a-chip Droplet Manipulation
Delivering small amounts of liquid is important for diverse applications ranging from
pharmaceutical systems to microfluidic devices for biochemistry and microsystems
engineering [31, 32]. Droplet-based micro-fluidic devices have been found to be beneficial as they can reduce operating pressures and the need for bulky pumps with
significant power consumption [33]. Typical lab-on-a-chip systems include prefabricated microchannels that may constrain the possible pathways to transport droplets
containing materials of interest such as biological cells and proteins or suspended
particles [34]. An area that is drawing much interest of late is the transportation of
droplets on a free surface, but frequently encounter the drawback of material adhesion on the channel walls [35-39]. More recently, liquid-liquid microfluidic systems
were successfully applied utilizing an alternating or constant electric field for liquid
transport as shown in Figure 1-2 [2].
Yet the overall flexibility of a surface of method remains a challenge. The goal
of this thesis is to present a surface with the ability to manipulate a large range of
materials with no pre-determined design or function such that one could achieve any
number of lab-on-a-chip applications or designs with the use of a single surface.
13
14
Chapter 2
Fundamentals of
Lubricant-Impregnated Surfaces
Non-wetting surfaces containing impregnated lubricating liquids have recently been
shown to exhibit superior non-wetting behavior compared to typical superhydrophobic surfaces.
Lubricant-impregnated surfaces display low contact angle hysteresis
[40-46], self-cleaning [47], dropwise condensation [48,49] anti-icing [50-52] and antifouling properties [53-57].
On a traditional superhydrophobic surface, a stable air pocket is trapped within
the surface (Figure 2-1) which creates a shear-free interface that diminishes the contact of the surface with the liquid that is being repelled.
Maintaining stable air
pockets, however, can be challenging as they can collapse by an externally applied
pressure or even diffuse away into the liquid over time. It has been shown that by
introducing a lubricant into a rough surface (Figure 2-2), stabilizing it by capillary
forces arising from the microscopic texture, liquid droplets can move with ease, exhibiting a contact angle hysteresis of approximately 10.
These surfaces make up a unique four phase system (the repelled liquid, the surrounding gas, the lubricant oil, and the rough solid), and how they interact ultimately
determines the contact line behavior of the liquid being repelled. The way with which
the contact line of the liquid behaves on the surface will ultimately govern droplet
pinning and hence its mobility on the surface.
15
Repelled Liquid
Figure 2-1: Schematic of a superhydrophobic surface.
Repelled Uquid
Figure 2-2: Schematic of a lubricant-impregnated surface.
2.1
Governing Interfacial Relations
The fundamental equations of interfacial science are key to understanding how a
droplet will rest on a surface. For the case of superhydrophobic surfaces, or lubricantimpregnated surfaces, the interaction of all phases (fluid, solid, vapor) are governed
by the interfacial forces, particularly at small scales (submillimeter).
16
2.1.1
Young-Laplace Equation
The Young-Laplace equation relates the pressure difference across a curved surface of
two static phases
AP
R1
(2.1)
+
R2
where R1 and R 2 are the radii of curvature of the curved surface and -y is the surface
tension (or more formally the interfacial tension between the two phases) [58]. This
equation allows us to understand that along any curved surface, there will be a pressure difference between the concave and convex sides that is related to the interfacial
tension. This pressure difference can be used to understand several phenomenon associated with interfacial science and capillarity, including the classical capillary rise
experiment where the capillary pressure of the curved fluid interface is balanced by
a rise in fluid height (and gravitational energy).
2.1.2
Young Equation
Young's equation relates the contact angle of a resting droplet on a surface to the
interfacial energies of the 3 phases.
YICOS 6 = -Ysv - 7sl
(2.2)
where -y is the interfacial tension between the 3 phases (1-liquid, v-vapor, s-solid) [58].
This equation can be explained as a horizontal force balance of the surface energies
of all the phases for a droplet resting on a surface, as seen in Figure 2-3.
One assumption that is inherent in the derivation of the Young equation is that
it assumes a perfectly flat and rigid surface. This is not valid for even fairly smooth
surfaces, and most certainly not for micro-textured surfaces like those studied in this
work. This means that a drop will assume a wide spectrum of contact angles when
placed on surfaces with varying morphologies. This is normally accounted for by
considering a range of contact angles for a given surface (advancing and receding
17
.Y'v
ysv
Figure 2-3: Force balance in horizontal direction of a liquid droplet on a surface.
Force balance is used to derive Young's equation.
contact angles). The difference between these surfaces is referred to the contact angle
hysteresis of a given surface, which is generally a gauge of the droplet adhesion on a
surface.
2.2
Surface Thermodynamics
It has been shown that the thermodynamically stable state of a lubricant impregnated
in a rough solid depends on both the spreading coefficients of the two fluids, as well
as the texture geometry [40]. When designing a lubricant-impregnated surface, it is
important to understand what state will result from the choice of a lubricant and
solid in order to properly repel the desired liquid.
2.2.1
Impregnation and Pinning
A microtextured surface lends itself to a critical contact angle below which the oil
will successfully impregnate the texture and remain held by capillary forces. The
lubricant will impregnate a texture surface if,
Oos(a)
18
; Oc
(2.3)
where 0os(a) is the contact angle of lubricant oil (o) on the smooth solid (s) in the
presence of air (a) and 0, is the critical contact angle for impregnation, given by:
C= cos-[(1
Here,
# is the fraction of the
- #)/(r - #)]
(2.4)
projected area of the textured surface that is occupied
by a solid and r is the ratio of total surface area of the textured surface to its projected
area. In the case of square micro-posts with width a, edge-to-edge spacing b and height
#
is given by
# = a2 /(a +
b) 2 andr= 1 + 4ah/(a + b) 2
.
h,
Secondly, the degree to which the lubricant fills the textured solid will govern the
contact line pinning dynamics of a repelled liquid. One must not only consider the
interfacial behavior of the oil in air, but also in the presence of the repelled liquid
say water (w). As initially stated, the lubricant will impregnate the texture if its
contact angle on the solid in the presence of air is below a critical contact angle. It
is worth mentioning that the micro-post tops will remain uncovered or dry unless
Oos(a)
=
0. In this case the solid posts are fully encapsulated by the lubricant. This is
the only condition with which the oil will fully cover the solid. This is an important
consideration because the oil serves as a cushion to reduce the contact area of the
droplet and the solid. Yet if the post tops remain dry, they serve as pinning sites for
the droplet on the solid (See Figure 2-4 and 2-5 for all possible impregnated states in
the presence of air and water and the criteria for each case).
Yet these considerations do not fully account for the system at stake.
As the
repelled liquid is placed on the surface, a new phase introduces new interfacial tensions
to consider. In order for the water to not displace the oil our new condition is:
0os (w) <; 0c
(2.5)
where the new oil-solid contact angle is measured in the presence of water rather than
air. If this condition is met, the oil will remain in the texture as water is placed on
top. As discussed earlier, the texture posts remain dry and will act to pin the water
droplet, unless they can be fully encapsulated by oil. This alters our encapsulation
19
oosta> >0 c
dry
impregnated, dry posts
impregnated, encapsulated
0<0
osta>
o osta> =
<0 c
0
Figure 2-4: Impregnation criteria in the presence of a vapor (air).
o*sCw)
impaled
impregnated, dry posts
>0 c
0<0 OS(w) <0 C
0
=0
impregnated, encapsulated
Figure 2-5: Impregnation criteria in the presence of a fluid (water).
criteria for under the water droplet to os(W) = 0.
Using the knowledge given above, one may design and predict how well a lubricantimpregnated surface will shed a particular liquid.
20
S
>0
Sow(a)
< 0
Lubricant Cloak
Figure 2-6: Diagrams representing positive and negative spreading coefficient Sow(a)
(cloaking versus non-cloaking surfaces)
2.2.2
Cloaking
It has been shown that depending that lubricant oil may in fact spread over and
"cloak" the droplet of repelled liquid. The criterion for whether cloaking occurs is
given by the spreading coefficient of the oil on water in the presence of air:
Sow(a)
=Ywa
-
_Ywo - 'Yoa
(2.6)
where -y is the interfacial tension between the phases defined earlier and w referring to water as the repelled liquid. If
of repelled liquid, whereas
silicone oil for which
Sow(a)
Sow(a)
Sow(a)
> 0 the oil will cloak the droplet
< 0 implies no cloaking will occur. For example,
~ 6mNm- 1 has been shown to be a cloaking lubricant.
Certain lubricants have been investigated that have negative spreading coefficients
such as ionic liquid (1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide)
21
for which Sow(a) ~ -5mNm-
1.
These two cases are displayed in the schematics of
Figure 2-6. Cloaking is important to consider not only for understanding the true
physical state of the system, but also for the progressive loss of the cloaked lubricant
through entrainment in the water droplets as they are shed from the surface.
22
Chapter 3
Ferrofluid-Impregnated Surfaces
The use of magnetic fields for droplet manipulation has also been recently studied [59-61], but encounter similar issues regarding the magnetic properties of the fluid,
and normally need the enormous magnetic fields of superconducting magnets to operate on diamagnetic fluids such as water. Here we present a lubricant-impregnated
surface that is able to manipulate a variety of different liquids including diamagnetic, electrically conductive and even solid particles, using a rough surface that is
impregnated with ferrofluid.
3.1
Ferrofluid Properties
Ferrofluids, a colloidal suspension of ferromagnetic nanoparticles
(~
10 nm) in a car-
rier fluid stabilized by surfactant, have been studied extensively, but only rarely in its
ability to manipulate free droplets [62-64]. The surface treatment of the ferromagnetic particles prevents agglomeration due to short range van der Waals forces, and
Brownian motion prevents particle sedimentation in both gravitational and magnetic
fields. In the absence of an applied field, the particles are randomly oriented giving the fluid no net magnetization. Both paramagnetic and ferromagnetic materials
align their magnetic dipole moment parallel to the direction of the applied field, yet
ferromagnetic materials display a strong magnetic interaction between neighboring
molecules, while paramagnetic materials display only weak interactions.
23
Magnetization theory for ferrofluids is derived utilizing the assumptions that ferrofluids consist of non-interacting monodomain magnetic dipoles. The Langevin relation for paramagnetic behavior describes the tendency for the dipole moment to align
itself with the applied field, while also introducing the counteracting thermal energy,
which acts to randomize the spatial orientation of the dipole moment:
= L(a) = coth(a) - 1
M8a
a =
,uomH
kbT
(3.1)
(3.2)
where M is the magnetization, M, is the saturation magnetization of the ferrofluid
which corresponds to all dipoles being aligned with the field, A 0 is the permeability of
free space, m is the magnetic dipole moment of a particle, H is the applied magnetic
field intensity and kbT is the thermal energy [62,64
3.2
3.2.1
Sample Preparation
Impregnation
A textured superhydrophobic surface was fabricated using typical photolithographic
techniques along with deep-reactive-ion etching (DRIE). Square micro posts were
created with side length, edge-to-edge spacing and height 10 pm (See Figure 3-2).
The micro post array was then treated with a coating of octadecyltrichlorosilane
(OTS) to render it hydrophobic. As described earlier, the critical contact angle with
which a fluid will impregnate the texture depends on a few geometric factors of the
array. The critical contact angle:
0c = cos- [(1 - #)/(r - 0)]
24
(3.3)
CI\ CI
C1" i'
Figure 3-1: Molecular structure of octadecyltrichlorosilane (OTS).
for the above described system requires a contact angle of 650. The ferrofluid used
(EMG901, Ferrotec,Inc.) exhibited an equilibrium contact angle of 20' on a flat
silicon substrate treated with OTS, which confirms the criteria for the ferrofluid impregnated the 10pm x 10pm square post array used to carry the liquid.
3.2.2
Cloaking
The spreading coefficient of the ferrofluid on water in the presence of air will determine
whether the ferrofluid will rise out of the texture and cloak the water droplet. The
spreading coefficient:
Sow(a) ='Ywa - ""Ywo -- Yoa
(3.4)
for the ferrofluid is in fact positive which would predict cloaking behavior as a water
droplet is placed on the surface.
Due to the dark color of the ferrofluid lubricant containing the magnetic nano
particles, the cloaking film was observed and confirmed visually as shown in the
photograph (Figure 3-3b).
25
Figure 3-2: SEM's of dry textured micro post array.
3.3
3.3.1
Droplet Manipulation
Initial Droplet Actuation Tests
Approaching a cylindrical permanent magnet (K&J Magnetics) to the surface and
droplet attracts the magnetic particles to the region of highest magnetic field intensity,
26
ferrofluid cloak
magnetic nanop
cles
oil-based carrier fluid
h
Figure 3-3: a) Schematic of ferrofluid-impregnated surface b) Photograph confirming
cloaking criteria.
along the centerline of the nearest magnetic pole. Macroscopically, this acts to distort
the ferrofluid-air interface towards the magnet, thus translating the droplet along the
surface towards the magnet. This is made possible by the extremely low contact-angle
hysteresis (~1 ). As discussed in Chapter 2, the lubricant, solid, water interaction
depends on the most stable thermodynamic state that the system can attain. When
characterizing the pinning forces on the water droplet, the exposure of the micro post
array tops needs to be considered. In order for the lubricant to fully coat the post
tops in air,
Oos(a) =
0. But, as measured earlier, the ferrofluid has a finite contact
27
C
Figure 3-4: c) Snapshots of droplet propulsion.
angle on OTS coated silicon (~20) which means the post tops will be exposed to
the air. Yet, to explain the low contact angle hysteresis observed by a water droplet
moving along this surface, we measured the contact angle of the ferrofluid on the OTS
Si substrate in the presence of water to be ~Oo. This would explain that beneath
the water droplet, the post tops are in fact being covered by the ferrofluid and act
to lubricate the motion of the droplet along the surface, thus leading to virtually no
pinning forces.
The permanent magnet was approached to the surface horizontally, and the droplet
was propelled across the surface towards the magnet. The droplet position versus time
28
d
E
14-
E 12
C
.o 10U,)
0
8
(
6
L2
4
2
0
5
10
15
20
time/ s
Figure 3-5: d) Droplet position versus time for experiment.
was tracked and recorded, and shows that the droplet is in face accelerating towards
the magnet, or the region of highest magnetic field intensity.
3.3.2
High-Magnification Droplet Studies
Now to better understand the interaction of the magnetic field and the cloaked
droplets, we imaged single droplets resting on the surface at high magnification. In
our experiments, we first approached the cylindrical magnet directly from above the
droplet. The droplet is seen to deform toward the magnet, and the magnetic particles
crowd to form local cone-like structures. The magnetic attractive force is locally bal-
anced by restoring interfacial forces that act to hold these particles in the cloak. In
this regime when the magnet is far enough, or the magnetic field is low, the cone-like
structures move along the droplet when the magnet is moved. To further illustrate
this point, as the region of highest magnetic field is moved, the cone-like structures
move to that region (See Figure 3-6). This serves to act as a droplet propulsion mech-
anism in which the cloaked film is able to distort and propel free droplets along the
surface. As more particles are attracted towards the magnet, more particles move to
29
a
b
e-magnet
e-droplet orientation_
90-
-0
080-
0
50-
-
/
270D 60(D 40-
O
230,97
S20-
10
(
0
10
20
30
40
50
60
70
80
90
e-magnet
Figure 3-6: a) Area of clumped particles b) Droplet orientation versus Magnet orientation.
regions of higher field intensity, ultimately causing droplet acceleration.
3.3.3
Jetting Transition
Cloaked droplets continue to deform along with the ferrofluid until the magnet reaches
a critical distance at which the magnetic attractive force of the particles is greater
than the interfacial forces that are stabilizing them in the cloak. The particles then
physically detach from the film and agglomerate on the surface of the magnet, thus
marking the onset of the "jetting" transition that has been previously reported.
[65]
Applying scaling analysis, one can roughly predict the required condition for which
jetting transition occurs.As shown the natural scaling for the interfacial deflection of
the ferrofluid:
h*2
where h
2
2
XM*
R*
6
187ryL*
(35)
~ O(10 3 m) is a characteristic interfacial distortion during a jetting
30
a
E
N
-
,,02.5
0~0
2
7.5
2
8
'
9
8.5
9.5
0I
10
GM
10.5
magnet distance d / mm
Figure 3-7: a) Jetting experiment. Droplet deformation versus magnet distance.
experiment, /u 0
=
1.257 * 10- 6m * kg
*
s~2* A 2 is the permeability of free space, mag-
netic susceptibility x = 6.79, permanent magnet magnetization M* ~ O(10 6 A/m),
radius of the magnet R* = 3 * 10- 3m, characteristic volume of clumped particle
area V*
O(10-
0 m3
), interfacial tension
= 0y.022N/m, and magnet distance
from unperturbed interface L*. [65] Solving the above equation for L* utilizing the
given values for the other quantities, we can predict a critical magnet distance of
L* ~O(10- 2m) for the jetting transition to occur, which we confirm experimentally.
A high-speed camera was used to visualize this phenomenon. The droplet height was
recorded as the distance between the magnet and the surface was decreased. We observe two different regimes in the droplet deformation as shown in the plot in Figure
3-7. When the distance between the magnet and droplet was larger than the critical
jetting length, the droplet deforms smoothly and reaches an equilibrium deformation
(Figure 3-8c).
When the distance is shorter, the particles begin to jet from the cloak, and the
31
b
C
Figure 3-8: b) High-Speed images of fluid jet detaching from cloak/air interface in
the jetting regime. c) Snapshots of droplet deformation as the magnet is slowly
approached to the surface.
droplet height or deformation begins to oscillate due to the continuous forming and
detaching of liquid jets (Figure 3-8b). This accounts for the jagged data points on the
left portion of the graph. This observation illuminates a new design consideration for
these surfaces. As the particles jet from the cloak, fluid and particles are permanently
removed from from the lubricant layer (and attaching themselves to the permanent
magnet or magnetic field source). Continued use of the system without considering
the jetting transition would ultimately reduce the lubricant layer low enough to remove the surface's slippery qualities. Therefore, this thesis provides the guidelines
for designing droplet manipulation systems that apply sufficient magnetic fields to
translate droplets below the critical jetting transition distance to avoid lubricant loss.
3.3.4
Vertical Orientation
Experiments were performed to see how droplets could be arrested on vertically oriented surfaces. As the surfaces exhibit extremely low droplet adhesion and contact
angle hysteresis, droplets are easily shed at high surface tilt angles. Yet when a magnet is brought perpendicularly to the surface, as in Figure 3-9, the attraction between
the droplet and the magnet is strong enough to suspend the droplet on the vertical
32
Figure 3-9: Droplet sliding down on a vertical surface. Magnet is able to suspend a
droplet on the surface avoiding shedding by gravity.
surface. The active actuation of the surface allows for highly switchable non-wetting
to wetting behavior and can be tuned by the applied magnetic field.
Upon removal of the droplet one can see that once the magnet is close enough to
the surface, the ferrofluid begins to form spike-like structures directly on the surface
(Figure 3-10).
These structures, which one typically observes when a magnet is
approached sufficiently close to a flat ferrofluid surface (normal-field instabilities),
have been thoroughly studied in the past [62]. The shape of the ferrofluid-air interface
distorts itself to minimize the total energy of the system. The spikes of the ferrofluid
move out along the magnetic field lines and distort the fluid, which is also being
resisted by gravity and surface tension. The spikes and valleys decrease magnetic
33
Figure 3-10: Magnet perpendicular to surface brought close enough to observe normalfield instabilities form on surface.
energy, yet are increasing gravitational energy by raising fluid, and surface energy by
creating a larger surface area of the fluid. The equilibrium shape of the ferrofluid
for a given magnetic field will be determined by the balance of these energies. This
description is consistent with the concept that spikes will only form above a critical
magnetic field strength, which we observe experimentally.
3.4
Potential Applications
To highlight the potential technological impact of this type of active surface we present
experiments including free droplet coalescence, movement of viscous and conductive
fluids as well as solid particles, and moving objects on complex curved paths. Simply
introducing a magnet centered between two droplets may induce coalescence. The
region of highest magnetic field intensity, directly beneath the magnet, attracts both
droplets to center under the magnet, leading to coalescence (Figure 3-11).
Additionally, the motion of conductive liquids (1M NaCl aqueous solution) and
highly viscous dielectric liquids (100% glycerol) was achieved. The manipulation of
low surface tension fluids could be achieved with this method utilizing the appropriate
immiscible lubricant, which can be a challenge with typical superhydrophobic surfaces
34
a
Figure 3-11: a) Two water droplets (dye used to color droplets) are placed on
ferrofluid-infused surface. Magnet lowered vertically directly between droplets and
causes them to move toward one another. Coalesence of droplets occurs and is captured just before droplets fully mix.
unless they have complex textures. [6] The proposed material along with magnets can
also be used for handling solid objects. It is possible to translate a 5 mm glass bead
using this material and a magnet. All of these experiments can be seen in Figure
3-12.
Finally, for lab-on-a-chip style applications, pre-fabricated microfluidic channels
or embedded electrodes [66] would be used to move and mix droplets along complex paths, yet this surface's droplet actuation mechanism naturally does not have
constraints on path geometries (Figure 3-13). Therefore, this ferrofluid-impregnated
35
b
Figure 3-12: b) Droplets of various fluids and one solid are actuated and displaced
when a permanent magnet is approached to the surface.
C
Figure 3-13: c) Water droplet being dragged along an s-curve as the magnet is held
beneath the surface.
surface provides a framework for free surface manipulation of a broad range of liquids
with various physicochemical, electrical, and magnetic properties.
36
Chapter 4
Conclusions and Recommendations
To conclude, the focus of this thesis was the design and testing of active lubricantimpregnated surfaces for the manipulation of droplets. A silicon micro post array was
treated with octadecyltrichlorosilane (OTS) and then impregnated with a ferrofluid.
The resulting surface interacts dynamically with applied non-uniform fields by permanents magnets. Droplets placed on the surface would deform and begin to displace
towards the magnet, taking advantage of the low droplet adhesion and contact angle
hysteresis of the impregnated surface.
The mechanism behind droplet propulsion was further studied as single droplets
were inspected under high-magnification and high-speed video. The ferrofluid is able
to cloak up onto the droplet, and then encounter particle-magnet attraction, which
would then induce droplet motion along the surface. This mechanism does not rely
on the typical properties of materials that can inhibit active methods such as conductivity, viscosity or magnetization.
This research is largely motivated by a need for droplet manipulation methods
that do not have limitations on the type of fluid being transported. Low-surface tension fluids are difficult to shed in a dropwise manner, such as in condensers, which
is one area that this type of surface could provide a direct solution. In addition,
this method's droplet actuation mechanism naturally does not have constraints on
path geometries which would prove to be useful in lab-on-a-chip applications where
pre-fabricated microfluidic channels or embedded electrodes are normally required.
37
This would allow for the use of a single surface for any number of lab-on-chip applications and designs. Therefore, ferrofluid-impregnated surfaces provide a framework
for free surface manipulation of a broad range of liquids with various physiochemical,
electrical, and magnetic properties.
38
Appendix A
Experimental Methods and
Materials
A.1
Fabrication of Surfaces
Square microposts were etched in silicon using a standard photolithography process
followed by deep reactive ion etching. Each post has a width, height, and edge-to-edge
spacing of 10pm. The samples were then cleaned in a Piranha solution and treated
with a low-energy silane. The samples were infused with ferrofluid by dip coating
followed by nitrogen gas purging to remove any excess fluid not held by capillary
forces.
A.1.1
Silanization
A self-assembled monolayer of octadecyltrichlorosilane (OTS) was deposited by solution deposition onto the silicon substrates. This process involved dissolving the OTS
into a solvent (toluene) and then reacting this mixture with a water/toluene emulsion. Once the water comes into contact with the OTS molecules in the surrounding
toluene, the reaction begins. A monolayer of OTS is then deposited onto the surface
while the mixture is kept under vigorous sonication. Inevitably due to gravity, reacted molecules will sediment during the reaction process and form extra layers on
39
top of the monolayer. After 2 minutes of reaction, the sample is taken out of the
reaction bath and rinsed in acetone. The acetone rinse is meant to remove the excess
OTS layers laying on top of the monolayer. Once the surface was dry, it exhibited
hydrophobic properties (advancing/receding angles are 1100
/
96' respectively for a
flat silicon substrate).
A.2
Droplet Orientation-Angle Experiments
A 10pA drop was deposited on one of our surfaces. A cylindrical magnet (K&J Magnetics) of radius 3 mm was slowly approached to the droplet at several angles relative to
the droplet vertical centerline. Droplet deformation was then analyzed using ImageJ
software analysis.
A.3
Jetting Experiments
A 10ld drop was deposited on one of our surfaces. A cylindrical magnet (K&J Magnetics) of radius 3 mm was slowly lowered directly over the droplet using an x-y controlled stage. The droplet height and magnet distance from the unperturbed interface
were tracked using ImageJ as the attraction began to deform the droplet towards the
magnet. A high-speed camera was used to capture the jetting phenomena (Photron
SA1).
A.4
Vertical Orientation Experiments
A 10pl drop was deposited on one of our surfaces that was attached vertically to
an x-y micro-controller stage. A cylindrical magnet (K&J Magnetics) of radius 3
mm was attached to a separate x-y micro controller stage such that it could be
controllably approached horizontally and perpendicularly to the surface. Droplets
were then allowed to slide down the vertical surface, and the magnet distance from
the surface was varied such that eventually the droplet was completely arrested upon
40
falling.
A.5
Advanced Path Experiments
A 10pl drop was deposited on one of our surfaces. A cylindrical magnet (K&J Magnetics) of radius 3 mm was held beneath the surface so that the system could be
imaged from above. Photoshop was used to overlay the first and last frame to show
drops final position, and to illustrate the s-path the droplet moved along.
41
42
Appendix B
Additional Studies
B.1
Sub-Surface Magnet Configurations
As displayed in the advanced path experiments, the magnet configuration used on
the surface-droplet can significantly alter the behavior of both the surface and the
droplet. This section offers some insight and some brief observations when the magnet
was approached from beneath the surface.
As a magnet is approached from beneath the ferrofluid-impregnated surface, normalfield instabilities are observed. These shark-fin needle like instabilities are well studied
and documented in ferrofluids [62]. As previously discussed in this thesis, a mechanism for droplet propulsion when the magnet is approached from above the surface
is the lubricant-cloak on the droplet. When the magnet is approached from below,
as seen in the Figure B-1, the water droplet is pulled from beneath and distorted
along its contact line seemingly by the ferrofluid spikes. The majority of this thesis
discusses the ability for a magnet from above to distort the cloak and droplet, yet
it is noted that droplets can be manipulated from distortions along the contact line
which is observed when a magnet is approached from beneath the surface.
43
Figure B-1: Water Droplet is moved by magnet that is placed beneath the surface.
B.2
Ice Mobility Tests
Some preliminary tests with ice were taken on ferrofluid-impregnated surfaces. Due to
complications with the Peltier stage used to cool the water droplet, the ice was frozen
44
on a separate surface and then placed onto a room temperature surface ferrofluidimpregnated surface. Although the ice would begin to melt immediately upon contact
with the surface, it still offers an insightful experiment and observation on how this
surface interacts with semi-solid materials.
The complications of the Peltier stage
can be attributed to an embedded magnetic stirrer. The embedded magnet would
interact with the ferrofluid and cause it to cease up when the surface was placed on
the stage, rendering the surface unusable.
45
Figure B-2: Block of ice frozen and placed onto ferrofluid-impregnated surface.
46
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