drop motion on superhydrophobic fiber mats
TRANSCRIPT
DROP MOTION ON SUPERHYDROPHOBIC FIBER MATS
A Thesis
Presented to
The Graduate Faculty of The University of Akron
In Partial Fulfillment
of the Requirements for the Degree
Master of Science
Gabriel Manzo
December 2011
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DROP MOTION ON SUPERHYDROPHOBIC FIBER MATS
Gabriel Manzo
Thesis
Approved: Accepted: Advisor Department Chair Dr. George G. Chase Dr. Lu-Kwang Ju Committee Member Dean of the College Dr. Bi-min Newby Dr. George K. Haritos Committee Member Dean of the Graduate School Dr. Lingyun Liu Dr. George R. Newkome Date
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ABSTRACT
Superhydrophobic surfaces are defined as having a water contact angle exceeding 150°.
Surface wetting properties of polymers can be enhanced by creating roughness. A simple
and inexpensive way to induce surface roughness is to use the technique of
electrospinning to produce polymeric nanofiber mats. The roughness at the nanoscale of
these surfaces enhances surface properties by lowering the surface energy and increases
the water contact angle. Using the electrospinning technique, three different polymeric
nanofiber mats were produced that were superhydrophobic : poly(vinylidenefluoride-
hexafluoropropylene) (PVDF-HFP), polypropylene, and poly(4-methyl-1-pentene). Since
these surfaces have water contact angles above 150°, water droplets roll across the
surface as near spherical droplets. Experiments were carried out to determine how much
force was required to move water droplets along these surfaces in air and hydrophobic
liquids. Due to the high contact angle, water droplets have low contact area with the
surface which reduces the force required to move the droplets along the surface. It was
determined that the force required to move drops along these hydrophobic surfaces was
less than the force of gravity. Using the force balance, the drag coefficient between the
water droplet and the nanofiber mat was calculated. The drag coefficient was correlated
to drop’s Reynolds number and this correlation can now be used in the force balance to
predict drop motion on superhydrophobic nanofiber surfaces.
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ACKNOWLEDGEMENTS
I would like to express the utmost gratitude to my advisor, Dr. George Chase for
his constant encouragement, support, and words of wisdom throughout the course of this
degree. This thesis work could not have completed without him. The continual guidance
of my committee members, Dr. Lingyun Liu and Dr. Bi-min Newby is highly
appreciated. I also would like to thank the Chemical Engineering Department and
Coalescence Filtration Nanofibers Consortium (CFNC) for providing financial support
towards this degree. CFNC company members also gave valuable insights into
completion of this work during our bi-annual meetings. I would like to thank Frank Pelc
for his technical expertise and helping to design and construct my experimental
apparatus. Next, I would like to express many thanks to all my friends that I have made
working in the lab. It was truly an honor to be in the same research group as them and to
learn all about their different cultures and backgrounds. I have made many great
friendships in my two years working in the lab. Finally, I want to express my sincerest
gratitude to my parents, Mr. and Mrs. Gregory and Mary Ellen Manzo. I would like to
thank my dad for pushing me to be the engineer that I am today. If it wasn’t for your
constant words of advice, and picking me up when the workload seemed overwhelming
and telling me to keep my nose to the grindstone, I would not be here today. And to my
mom, I want to say thank you for your love and care. From calling me to ask how my
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week went to helping wash my clothes and feed me home cooked meals on the weekends
I came home when I was dead tired after a long hard week, I can never say thank you
enough.
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TABLE OF CONTENTS
Page
CHAPTER
I. INTRODUCTION…………………………………………………………………..….1
1.1 Background and overview of work..………………………………………...…….1
1.2 Problem statement…………………………………………………………….…...2
1.3 Objectives...................................................................................................….........3
1.4 Thesis outline...........................................................................................…............4
II. BACKGROUND………………………………………………………………….…...5
2.1 Wettability…………………………………………………………………………5
2.1.1 Spreading Coefficient.....................................................................................5
2.1.2 Young’s Equation..………….…………….…………………..…………….7
2.2 Superhydrophobicity….. ...........................................................................…..........8
2.2.1 Definition of Superhydrophobicity………………………………….………8
2.2.2 Causes of Superhydrophobicity..............................................................…....9
2.2.3 Modeling Superhydrophobic Surfaces……………………………………..9
2.3 Man-made Superhydrophobic Surfaces………………………………………….14
2.3.1 Roughening a Low Surface Energy Material………………………………14
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2.3.2 Making a Rough Substrate and Modifying it with Low Surface Energy
Materials ......................................................................................................15
2.4 Characterization of Superhydrophobic Surfaces...………………………….…...16
2.5 Drop Motion on Fibers…………………………………………………………...17
2.5.1 Drop Motion on a Single Fiber…………………………………………….18
2.5.2 Drop Motion on Superhydrophobic Surfaces …..…………………………19
2.5.3 Drop Motion on Superhydrophobic Fiber Mats……………………………19
III. ELECTROSPINNING OF SUPERHYDROPHOBIC NANOFIBER MATS…........20
3.1 Fundamentals of Electrospinning………………………………………………..20
3.2 Superhydrophobic Nanofiber Surfaces ..………………………………………...22
3.3 Superhydrophobic Polypropylene, Poly(4-methyl-1-pentene) and PVDF-HFP
Nanofiber mats ………………….….....................................................................23
3.4 Contact Angle Hysteresis of Superhydrophobic Electrospun Fiber Mats......…...26
3.5 WCA in Hydrophobic Liquids…………………............................................…...27 3.6 Conclusions………………………………………………………........................30
IV. DROP MOTION ON SUPERHYDROPHOBIC MATS…………………………..31 4.1 Drop Mobility on Superhydrophobic Surfaces....…………….……………….…31
4.2 Gravity Test……………………………….……………………………………...32
4.3 Drag Coefficient Correlation……………………………………………….....….37 4.4 Conclusions………….....................................................................................…....40
V. CONCLUSIONS AND FUTURE WORK…………………………………………..41
5.1 Conclusions………………………………………………………………………41
5.2 Future Work Recommendations…...…………………………………………….42
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5.2.1 Superhydrophobic Fibers as Membrane Filters……………………………43
5.2.2 Curved Superhydrophobic Surfaces……………………………………….44
REFERENCES…………………………………………………….…………...………45
APPENDIX A………………………………………………………………………….51
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CHAPTER I
INTRODUCTION
1.1 Background and overview of work
Wettability is a fundamental property of a solid surface. It is governed by both the surface
chemistry and the geometrical micro and nanostructures of the surface [1-3]. Surfaces with water
contact angles (WCAs) exceeding 150⁰ are defined as superhydrophobic surfaces.
Superhydrophobic surfaces are important in several applications involving water repellency,
biocompatibility, contamination inhibition, and self-cleaning filters [1, 4-7]. In nature, lotus and
silver ragwort leaves are found to have a self-cleaning ability due to their hydrophobic wax
coating of a complicated micro- and nanostructed surface. So far techniques adopted to make
artificial superhydrophobic surfaces can be divided into two categories: modifying a rough
surface by chemicals with low surface energy [1, 8-9] and making a rough surface from low
surface energy materials [1, 10-11].
Recently, the fabrication of fibrous mats with high surface roughness and surface-to-
volume via the electrospinning process has been widely studied [1, 12-13]. Polymer fibers with
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diameters ranging from tens of nanometers to several micrometers can be obtained by applying
high voltage on a viscous polymer solution. Morphology, structure, and properties of fibrous
mats are controllable by adjusting the solution properties and processing parameters such as
solvents, solution viscosities, applied voltage etc [1, 12-15].
Two common types of low surface energy polymers are polyolefins and fluorinated
polymers. However, successful electrospinning of polyolefins such as polypropylene and poly(4-
methyl-1-pentene) has been limited due to their high solvent resistance and very high electrical
resistivity. Recently, polyolefins have been successfully electrospun from multi-component
solvent systems at room temperature [16-18]. Fluorinated polymers such as
poly(vinylidenefluoride-hexafluoropropylene) (PVDF-HFP) have been easier to electrospin and
more widely reported in literature.
1.2 Problem Statement
The goal of this work is to use the electrospinning technique to produce
superhydrophobic nanofiber mats. Three polymers, polypropylene, poly(4-methyl-1-pentene),
are electrospun to produce polymeric nanofiber mats. These nanofiber mats then have their WCA
measured using the Drop Shape Analyzer from Kruss and fiber diameters measured using the
SEM. Fiber diameters were varied by increasing the concentration of polymer in the
electrospinning solution to determine if the WCA was strongly dependent on the fiber diameter.
WCA hysteresis was also measured to determine if the nanofiber mats were truly
superhydrophobic.
In industrial applications, porous superhydrophobic nanofiber surfaces provide unique
properties which prove useful in oil-water separations. Since the nanofiber mats are both porous
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and oleophilic, they can be used to separate oil-water emulsions where the concentration and size
of the water drops are too small to separate by conventional mechanical ways such as gravity
settling and centrifuging. For these applications, conventional WCA measurements do not apply.
Typically, WCA measurements are made with air as the surrounding fluid. For oil-water
separations, a hydrophobic liquid is the surrounding fluid and the appropriate WCA for these
cases is when the WCA of the nanofibers is measured when they are immersed in another liquid.
Finally, the water drops must be continually cleared off the surface or the nanofiber
surface will become blinded by water and the filter will plug. A simple experiment was designed
to calculate the minimum force to cause water drops to move on the superhydrophobic nanofiber
surfaces and to develop a correlation for the drag coefficient between the drops and the
nanofibers to be used in the force balance to model filter performance.
1.3 Objectives
The objectives of this work include:
1. Electrospin superhydrophobic nanofiber mats
2. Determine how fiber diameter affects superhydrophobicity
3. Determine if nanofiber mats are still superhydrophobic when immersed in a liquid
4. Design simple gravity experiment
a. Measure minimum tilt angle to move water drops
b. Sine of minimum tilt angle related to minimum force
c. Measure steady-state velocity
5. Develop correlation for drag coefficient between water drops and nanofibers
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a. Use force balance, minimum tilt angle, and steady velocity to calculate drag
coefficient between drop and nanofibers
b. Correlate drag coefficient to drop’s Reynolds Number
c. Separate correlations for different Bond Number regions
1.4 Thesis Outline
This thesis is organized into five chapters. Chapter II gives a review of literature about
wettability and superhydrophobic surface theories. Chapter III discusses electrospinning of
superhydrophobic nanofiber mats, contact angle hysteresis and differences of water contact angle
on a solid when submerged in a liquid compared to a solid in air and how to measure the WCA
when submerged in a liquid. Chapter IV outlines the design and execution of the gravity
experiment, how to measure the minimum tilt angle, how to calculate the steady-state velocity
and contains the drag coefficient correlation obtained by using the force balance. Chapter V has
the overall conclusions from this work and discusses recommendations for continuation of this
work.
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CHAPTER II
BACKGROUND
2.1 Wettability
Wetting is the ability of a liquid to maintain contact with a solid surface, resulting from
intermolecular interactions when the two are brought together. The degree of wetting
(wettability) is determined by a force balance between adhesive and cohesive forces.
2.1.1 Spreading Coefficient
The wetting criterion is defined based on the spreading parameter S, which measures the
difference between the surface energy of the dry substrate and wet substrate.
[ ] [ ]wetsubstartedrysubstrate EES −= (2.1)
( )lgγγγ +−= slsgS (2.2)
where the termssgγ , slγ and lgγ are the surface tensions at the solid/air, solid/liquid, liquid/air
interfaces, respectively.
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Figure C.3. The interfacial surface tensions at the solid, liquid and air interfaces.
The two wetting regimes are defined as:
Complete wetting: S > 0
The complete wetting is defined by the spreading parameter when positive. This shows
that the liquid completely spreads (θ=0) to lower the surface energy of the liquid. Finally, the
liquid forms a film on the solid substrate as a result of a balance between the molecular and
capillary forces.
Partial wetting: S < 0
In this case the drop maintains its spherical shape on the substrate and does not spread
forming a contact angle θ with the surface. The liquid is highly wetting when θ <90⁰ and highly
non-wetting θ>90⁰ [19]
Solid Substrate
Liquid
Air
slγ
lgγ
sgγ
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2.1.2 Young’s Equation
The wettability of flat surface expressed by its water contact angle (WCA), is given by
Young’s equation:
cos � = ���������� (2.3)
where γSV, γSL, and γLV represent the interfacial surface tensions with S, L, and V referring to
solid, liquid and vapor respectively. The water contact angle results from the thermodynamic
equilibrium of the free energies at the solid-liquid-vapor interface [20].
Flat surfaces can be split into two categories: hydrophobic (WCA > 90°) and hydrophilic
(WCA < 90°). For more complete characterization, two types of WCAs are used: static and
dynamic. For flat surfaces, measured static WCA are close to the predicted WCA by Young’s
equation. Dynamic WCAs are non-equilibrium contact angles. Static WCAs are obtained by the
sessile drop method, where water drops are placed on a surface and the contact angle value is
measured using a goniometer. Dynamic WCAs are measured during growth (advancing WCA
θA) and shrinkage (receding WCA θR) of a water drop. The difference between θA and θR is
defined as the contact angle hysteresis. Contact angle hysteresis is a reflection of the surface’s
homogeneity: the lower the hysteresis the more homogeneous the surface is [20-22].
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2.2. Superhydrophobicity
Superhydrophobicity is a unique aspect of the wetting regime situated at the far end of the
hydrophobic wetting regime. The next couple of sections discuss the fundamentals of
superhydrophobicity.
2.2.1 Definition of Superhydrophobicity
One aspect of wetting, situated at the hydrophobic extreme of the above wettability range,that
has recently received considerable interest is superhydrophobicity [23-32]. Superhydrophobic
surfaces are defined as having a WCA > 150⁰. Drops that rest on superhydrophobic surfaces
retain a nearly spherical shape and have high mobility [23-24, 27, 33]. Due to greatly decreased
contact angle hysteresis, drops easily move around when small forces are applied, such as by
slightly tilting the substrates or blowing gently across the surfaces. Superhydrophobic properties
lead to an interesting side-effect: if water drops are brought onto a superhydrophobic material by
rain or spraying, the moving drops pick up and thus remove dust and dirt particles that are
loosely situated on the surface. This is known as “self-cleaning.” [23, 34-35]
From various natural materials, superhydrophobicity has been a known phenomenon for
thousands of years. The most prominent superhydrophobic surface in nature is that of the lotus
leaf. SEM studies of the lotus leaf’s surface linked its water-repellent properties to the
pronounced roughness of the leaf’s surface which is colloquially termed “lotus effect.”
Superhydrophobic surfaces are fairly common in nature: numerous other plant species exhibit
similar characteristics. Also, many animals are equipped with superhydrophobic body parts. The
water strider would drown if it wasn’t for its superhydrophobic legs. [36-44]
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2.2.2 Causes of Superhydrophobicity
From a theoretical standpoint, two parameters work together in determining the wetting
properties of a material: one factor is the chemical composition of the surface and the other
factor is the surface roughness. Depending on the interplay between these two characteristics,
one of two distinct wetting states manifests itself: in the first case frequently referred to as
Wenzel wetting, drops penetrate and engulf the surface features. The second case, called Cassie
or composite wetting, drops rest on top of the roughness features with air trapped underneath.
Cassie wetting is often observed when hydrophobic surfaces have high degrees of roughness.
Superhydrophobic properties are associated exclusively with the Cassie state of wetting: in this
regime, the fact that the drop’s footprint is partially in contact with air leads to a balling up of the
drop to a more or less spherical shape and frequently an increase in droplet mobility. These two
models are the most common ways to predict the effect of surface roughness on surface
wettability [39, 46-50].
2.2.3 Modeling Superhydrophobic Surfaces
Wenzel Model
Wenzel observed that roughness leads to an amplification of the wetting properties of a suface if
a drop wets the surface in such a way that it follows the roughness features. Here, the area of the
drop’s solid-liquid interface is enlarged by a factor of r. Wenzel proposed that the contact angle
on the rough surface, θW, can be calculated:
cos �� = � cos � (2.4)
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In eqn 2.2, the surface roughness r is defined as the ratio of the actual over the apparent surface
area of the substrate. According to eqn 2.2, a hydrophobic surface will become more
hydrophobic with increasing degrees of roughness, while a hydrophilic surface will become
more hydrophilic if the same type of roughness is present. While Wenzel wetting can lead to
WCA in the superhydrophobic regime, the fundamental nature of this type of wetting causes
water drops to adhere to the surface and limits drop mobility. Since the water drop is intrinsically
involved with the surface roughness, this surface does not have anti-adhesive properties [46].
Figure 1: In Wenzel wetting, the drop is intrinsically touching the irregular features that make up
the roughness of the macroscopic surface.
Cassie-Baxter Model
The second model for roughened surfaces is the Cassie-Baxter model. Cassie and Baxter looked
at the effects of chemical heterogeneities in the surface on the equilibrium contact angle. In this
model, averaging over the surface energies of the respective area fractions, φi, led to the
following expression.
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cos �� = ∑ �� cos ��� (2.3)
��� = 1�
In eqn 2.3, θC is the equilibrium contact angle on the heterogeneous surface and θi is the contact
angle belonging to the corresponding area fraction. On rough materials with very hydrophobic
surface chemistries and pronounced degrees of roughness, drops often prefer to rest on top of the
roughness features, with air trapped underneath. This can be modeled as a composite surface
composed of solid and air, where the air composite parts can be considered perfectly non-wetting
(θ=180⁰). Hence, equation 2.3 becomes
cos �� = ��cos�� + 1� − 1 (2.5)
In eqn 2.4, φ is now the solid area fraction i.e., the fraction of the drop’s footprint in contact with
the solid. Superhydrophobic surfaces with Cassie wetting have some distinct differences than
superhydrophobic surfaces with Wenzel wetting. Since in Cassie wetting water drops sit on top
of the surface roughness instead of intrinsically involved, water drops have more mobility [27,
38, 47-48, 50].
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Figure 2: In Cassie wetting, the drop rests on top of the roughness with air pockets underneath.
Comparison of Wenzel and Cassie Models
In the limit as the roughness factor approaches a critical value, the Wenzel formula predicts a
total drying of the surface (θ=180⁰), which is not possible because of the contact that must exist
between a drop and the surface. Similarly, it is expected the Wenzel equation to hold for solids
that are slightly hydrophobic: then, air pockets, which imply that many liquid/vapor interfaces of
high surface energy exist, should not be favored. Therefore, the Wenzel model should be obeyed
as the contact angle increases until a critical contact angle is reached, where there is a transition
to the Cassie state. This critical contact angle is solved by equating eqns 2.2 and 2.4.
cos �� = ��������� (2.6)
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This critical contact angle represents the point at which the minimum energy state transitions
from Wenzel wetting to Cassie wetting. A comparison between the interfacial energies
associated with the Wenzel and Cassie wetting confirms that air pockets should be favored only
if θ is larger than θc [51-52].
On some roughened surfaces, drops in the Wenzel and Cassie states can coexist, with the actual
wetting state depending on how the respective drop was deposited. In reality, kinetic barriers
may lead to a stabilization of drops in a wetting situation that is not the minimum-energy state.
As a consequence, drops in both wetting states may appear on one and the same surface.
Patankar studied the Cassie-to-Wenzel transition theoretically for a model surface composed of
regularly arranged posts. For a hydrophobic surface, he found the initial impalement of a drop on
the post structure to be associated with an increase in interfacial energy. Energy was then
recovered as the drop made contact with the bottom of the post surface and liquid-air was by
liquid-solid interface.
A transition of drops from the Cassie to the Wenzel state can be induced by exerting pressure on
the drops, vibrating the substrate, applying an electrical voltage, or having droplets evaporate.
All these methods lead to an increased Laplace pressure ∆P across the drops liquid-air interfaces,
which can thus be seen as the driving force for the transition. ∆P and the radius of the meniscus
are related by the Laplace equation
∆� = �� (2.7)
A transition occurs once the meniscus is so strongly curved that either a) direct contact with the
bottom of the surface is made while it still pinned to the surface roughness or b) the advancing
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angle on the sidewalls of the roughness features is reached so that the meniscus can slide down
into the roughness structure [53-56].
2.3 Man-made Superhydrophobic Surfaces
Techniques to make superhydrophobic surfaces can be simply divided into two categories:
making a rough surface from low surface energy material and modifying a rough surface with a
material of low surface energy.
2.3.1 Roughening a low surface energy material
Fluorinated polymers are of particular interest due to their extremely low surface energies.
Roughening these polymers directly leads to superhydrophobicity. Zhang et al. reported a simple
way to achieve a superhydrophobic film by stretching a PTFE film: the extended film consisted
of fibrous crystals with large void spaces which was believed responsible for the
superhydrophobicity. Due to their limited solubility, many fluorinated materials have not been
used directly but cross-linked or blended with other materials to make superhydrophobic
surfaces. Yabu and Shimonmura prepared a porous superhydrophobic membrane by casting a
fluorinated block polymer solution under humid environment [57-61].
Silicones are another class of materials with low surface energies. Because of its intrinsic
deformability and hydrophobic property, PDMS is a commonly used silicone to make
superhydrophobic surfaces by various methods. Jin et al. used a laser etching method to make a
rough PDMS surface containing micro-, submicro- and nanocomposite structures which yielded
a superhydrophobic surface and sliding angles less than 5⁰. Sun et al. recently reported a
nanocasting method to make superhydrophobic PDMS surfaces. The first made a negative
PDMS template using a lotus leaf as an original template and then used the negative template to
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make a positive PDMS template which was a replicate of the original lotus leaf. The positive
PDMS template had the same surface structures and superhydrophobicity as the lotus leaf.
Another way to exploit the low surface energy of PDMS is to use a block copolymer such as PS-
PDMS. Ma et al. made a superhydrophobic membrane in the form of a nonwoven fiber mat by
electrospinning a PS-PDMS block copolymer blended with PS homopolymer. The
superhydrophobicity was attributed to combination of enrichment of PDMS component on fiber
surfaces and the surface roughness due to small fiber diameters [62-66].
Although fluorocarbons and silicones are known as hydrophobic materials, nature achieves non-
wetting and self-cleaning using paraffinic hydrocarbons. Recently, several groups have
demonstrated superhydrophobic surfaces made from organic materials. Lu et al. proposed a
simple and inexpensive method to produce a highly porous superhydrophobic surface of
polyethylene by controlling its crystallization behavior. Jiang et al. showed that by
electrospinning and electrospraying a PS solution in DMF they obtained a superhydrophobic film
composed of porous microparticles and nanofibers [67-69].
2.3.2 Making a rough substrate and modifying it with low surface energy materials
Methods to make superhydrophobic surfaces by roughening low surface energy materials are
mostly one-step processes and have the advantage of simplicity. Making a superhydrophobic
surfaces by a totally different strategy, i.e. making a rough substrate first and then modifying it
with a low surface energy material, decouples the surface wettability from the bulk properties of
the material and enlarges potential applications of superhydrophobic surfaces. There are many
ways to make rough surfaces such a mechanical stretching, etching, lithography, sol-gel
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processing, layer-by-layer assembling. However, the focus of this work is mainly on using the
first method to make superhydrophobic surfaces [70-73].
2.4 Characterization of Superhydrophobic Surfaces
The sessile drop method, where a small drop of liquid is placed on a surface, is currently
the most popular technique for characterization of superhydrophobic surfaces. The drop is
analyzed from the side view and modern systems use a camera and computer to detect drop
shape and extract the contact angle.
The size of the drop must be chosen such that the effect of evaporation on the drop shape can be
neglected; this condition limits the minimum drop size. However, the drop must be small enough
so that gravitational forces do not significantly deform the drop; this is usually satisfied if the
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drop’s diameter is below the capillary length of the respective liquid. The capillary length, λC, is
defined as
� = ����� (2.8)
where ρ is the liquid density and g is the gravitational acceleration. For drop diameters below λC,
surface forces dominate; the capillary length for water is 2.7 mm and represents the maximum
size for unperturbed water drops.
For the case of superhydrophobic wetting, the previous approximations breaks down: as
the contact angle of a drop on a surface increases, the circular patch which forms the contact area
between liquid and solid becomes smaller. The gravitational force exerted by a drop of a given
volume is thus concentrated onto a progressively smaller area. Ultimately, for a contact angle of
180⁰, the contact area would vanish and the pressure on the contact area would become infinite,
which is physically unattainable. Instead, the contact line is pushed outwards and the drop is
deformed from the ideal spherical cap shape even for drop diameters below λC. Therefore the
effect of gravity should not be neglected when drops are characterized on superhydrophobic
surfaces [26, 74-75].
2.5 Drop Motion on Fibers
Few papers discuss the motion of individual drops attached to fibers. The study of drop motion
on fibers is of scientific and economic interest for many possible applications like printing,
coatings, drug delivery and release, and filters to remove or neutralize harmful chemicals or
particulates from air streams. Gas convection induced drop motion in fibrous materials occurs in
coalescing filters, clothes dryers, textile manufacturing, convection ovens, and dewatering of
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filter cakes. Droplet removal can significantly reduce drying costs by reducing the free moisture
contained in fibrous materials prior to applying thermal drying techniques. For superhydrophobic
fibrous surfaces, studying drop motion is important in modeling how these surfaces can be used
as membrane filters. Drops need to be cleared from the surface to prevent blinding of the surface
by a thin film of water [76-79].
2.5.1 Drop Motion on a Single Fiber
Dawar and Chase investigated drop motion on a single fiber by applying a drag force on the drop
by passing air by the drop while it was attached to the fiber. By defining a drag coefficient and
applying dimensional analysis, they determined that the drag coefficient was a function of the
Reynold’s number and the Capillary number. They also determined that the drag coefficient was
a power-law function of these two dimensionless numbers using the Buckingham-Pi method. By
varying the applied drag force and measuring the velocity of the drop as it moved along the fiber,
they were able to apply the force balance to calculate the drag coefficient between the drop and
the fiber. Running multiple experiments using different flow regimes and fiber wettability, they
were able to generate a correlation for the drag coefficient as a function of the Reynolds and
Capillary numbers. Drag coefficient correlations are very useful because they can then be used in
models to where dimensionless numbers are used instead of absolute quantities. Dimensionless
values are a great way to model phenomena because they are independent of the individualities
of specific systems and therefore the model can be adapted to a wide range of systems [80-81].
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2.5.2 Drop Motion on Superhydrophobic Surfaces
Bhushan et al. investigated drop motion on superhydrophobic surfaces by measuring a tilt angle
needed to move a drop along the surface. In essence, this is using a force balance again to
characterize how drops move on a surface. Unlike what Dawar did, Bhushan did not apply a drag
force from a moving fluid but rather used gravity as the applied force to cause drops to move on
the surface. By measuring the tilt angle from the horizontal, the applied force needed to move a
drop can be calculated by taking the sine of the angle. Also, Bhushan did not define a drag
coefficient between the drop and the surface but rather defined an adhesion force and measured a
coefficient of friction. Since they were working with solid surfaces rather than porous fiber
surfaces, it is more standard to define a coefficient of friction rather than a drag coefficient [82].
2.5.3 Drop Motion on Superhydrophobic Fiber Mats
The best way to characterize drop motion on superhydrophobic fiber mats would be to combine
aspects from both Dawar and Bhushan’s work. Practically speaking, it is much easier to measure
the amount of applied force by using a tilt angle (gravity) than designing an experiment where a
moving fluid is imparting a drag force on the drop. However, in terms of modeling a fiber
surface, it is better to define a drag coefficient between the drop and fiber surface rather than
defining a adhesion force and coefficient of friction. By combining using a tilt angle experiment
to measure the amount of applied force needed for a drop to move and defining a drag coefficient
between the drop and the fiber surface in the force balance, the best characterization for drop
motion on superhydrophobic fiber surfaces can be achieved.
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CHAPTER III
ELECTROSPINNING OF SUPERHYDROPHOBIC NANOFIBER MATS
3.1 Fundamentals of Electrospinning
Electrospinning is a unique approach using electric forces to produce fine fibers. Electospinning
is a process by which a polymer solution or melt can be spun into smaller diameter fibers using a
high potential electric field. Electrospun fibers have small pore size and high surface area. The
advantages of the electrospinning process are its technical simplicity and adaptability.
Constructing an electrospinning setup is simple: it consists of a high voltage source, a syringe
pump with tubes to carry the solution from the syringe to the spinnerette, and a conducting
collector like aluminum. The collector can be made of any shape such as a flat plate, rotating
drum, etc. A schematic of the electrospinning process is shown in Figure 3.1.
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Figure 3.1: Schematic of electrospinning process
Polymer solution or melt used for electrospinning is forced through a syringe pump to form a
pendant drop of the polymer at the tip of the needle. High voltage is applied to the polymer
solution inside the syringe by charging the metallic needle, inducing free charges into the
polymer solution. These charged ions move in response to the applied electric field towards the
electrode of opposite polarity, thereby transferring tensile forces to the polymer liquid. At the tip
of the needle, the pendant hemispherical polymer drop takes a cone like projection in the
presence of the electric field. And, when the applied voltage reaches a critical value needed to
overcome the surface tension of the solution, a jet of liquid is ejected from the cone tip.
Most charge carriers in organic solvents and polymers have lower mobilities and the charge is
expected to move through the solution for larger distances only if given enough time. After
initiation from the cone, the jet undergoes chaotic bending instability and the electric field directs
it toward the grounded collector, which collects the charged fibers. As the jet travels between the
charged needle and the grounded collector, the solvent evaporates, leaving behind dry fibers on
22
the collector. For low viscosity solutions, the jet breaks up into droplets (electrospraying), while
for higher viscosity solutions it travels as fiber jets [83-87].
3.2 Superhydrophobic Nanofiber Surfaces
Research activity on the electrospinning of nanofibers has been successful in producing
submicron range fibers from different polymeric solutions and melts. Polymers with attractive
chemical, mechanical and electrical properties such as high conductivity, high chemical
resistance, and high tensile strength have been spun into ultrafine fibers by the electrospinning
process, and their application potential in areas like filtration, optical fibers, etc. have been
examined.
In this work, another property of polymers is examined: surface energy. In order to create
superhydrophobic surfaces, a low surface energy material must be chosen and the surface must
be roughened. In recent times, nanofibers have attracted the attention of researchers due to their
pronounced micro and nano structural characteristics which lead to the roughness needed to
create superhydrophobic surfaces. More importantly, high surface area, small pore size, and the
ability to use these nanofibers in membrane separations make this type of superhydrophobic
surface very interesting to a wide array of industries.
Two common classes of polymers are known to be very hydrophobic: polyolefins and
fluorinated polymers. Polyolefins are very difficult to electrospin due to their high resistance to
solvents and until very recently have not be successfully electrospun. However, recently Lee et.
al. developed a method to electrospin polypropylene and poly(4-methyl-1-pentene) nanofibers
from a multi-solvent complex at room temperature. Many fluorinated polymers are also difficult
to electrospin due to their resistance to being dissolved: for instance Teflon has a water contact
23
angle of 120⁰ but cannot be electrospun because it is impossible to dissolve. However, one
fluorinated polymer, PVDF-HFP, is readily dissolved in acetone at room temperature and easily
electrospun.
3.3 Superhydrophobic Polypropylene, Poly(4-methyl-1-pentene) and PVDF-HFP Fiber Mats
In this work, three polymers were electrospun to determine if superhydrophobic surfaces could
be produced by the electrospinning process. The three polymers chosen were polypropylene
(PP), poly(4-methyl-1-pentene) (PFMOP) and PVDF-HFP. These three polymers were chosen
due to their high water contact angles (WCA). Each polymer’s WCA is listed in Table 3.1.
Table 3.1: WCA for polymers used in electrospinning process
Polymer WCA (⁰) PP 108
PFMOP 108 PVDF-HFP 90
Each polymer was electrospun from four different solutions of different polymer concentrations
to see if the effect of the fiber diameter affects the hydrophobicity of the electrospun fiber mat. It
is well known that by increasing the polymer concentration in the electrospinning solution
increases the average fiber diameter of the electrospun fiber mat. For each polymer, the three
primary electrospinning parameters (flowrate, voltage, and gap distance) were held constant. The
electrospinning conditions are listed in the Appendix.
24
Table 3.2: WCA for electrospun PP fiber mats
Average Diameter (nm) WCA (°)
279 170±2
404 171±2
363 170±3
996 170±2
939 166±3
1506 168±3
2836 166±2
Table 3.3: WCA for electrospun PFMOP fiber mats
Average Diameter (nm) WCA (°)
499 161±2
558 159±3
501 162±4
794 158±2
781 162±2
2230 155±2
25
Table 3.4: WCA for electrospun PVDF-HFP fiber mats
Average Diameter (nm) WCA (°)
248 168±2
257 165±3
451 158±2
677 157±2
From Tables 3.2, 3.3 and 3.4, it can be seen that superhydrophobic fiber mats were successfully
electrospun from PP, PFMOP and PVDF-HFP polymer solutions. Not all of the electrospun fiber
mats can be considered nanofiber mats by the strict definition of nanofibers, however each fiber
mat had sufficient roughness at the nano and mico level to yield superhydrophobic surface
properties. Another factor that contributed to the surface roughness was that the electrospun
fibers were not smooth fibers but rather beaded fibers. The beads on the fibers introduce another
dimension of roughness that also aids in the hydrophobicity of the fiber mat surfaces. SEM
images of the fiber mats can be seen in Appendix A. Also from the above tables, it can also be
seen that the hydrophocity of PP and PFMOP fiber mats are independent of the fiber diameter
but the hydrophobicity of PVDF-HFP fiber mats decreases as the fiber diameter increases. This
is most likely due to the fact that the PP and PFMOP polymers are more hydrophobic than
PVDF-HFP and therefore the hydrophobicity of PVDF-HFP is more sensitive to changes in
surface roughness than PP and PFMOP. More than likely, if fiber mats of PP and PFMOP with
larger diameter fibers were electrospun, the same trend as PVDF-HFP would be observed.
26
3.4 Contact Angle Hysteresis of Superhydrophobic Electrospun Fiber Mats
As mentioned in Chapter 2, the difference between the maximum (advancing) and mimimum
(receding) contact angle is called the contact angle hysteresis. In general, the static contact angle
is nearly identical to the advancing contact angle, so by determining the contact angle hysteresis
and receding contact angle, the fiber mat surfaces can be characterized as truly superhydrophobic
in all cases. Also, contact angle hysteresis can lead to insights on surface heterogeneity,
roughness and mobility. Contact angle hysteresis measurements were made on all of the
nanofiber mats and the results are listed in Tables 3.5-3.7.
Table 3.5: CA hysteresis for PFMOP fiber mats
Fiber Diameter (nm) WCA (⁰) Hysteresis (⁰) 499 162±2 3 501 161±3 4 794 162±2 5 2230 155±2 7
Table 3.6 CA hysteresis for PP fiber mats
Fiber Diameter (nm) WCA (⁰) Hysteresis (⁰) 279 171±2 4 404 170±2 4 939 166±3 5 2836 166±3 7
27
Table 3.7: CA hysteresis for PVDF-HFP fiber mats
Fiber Diameter (nm) WCA (⁰) Hysteresis (⁰) 248 168±2 2 257 165±2 1 451 158±3 1 677 157±3 8
From Tables 3.5, 3.6 and 3.7, each of the nanofiber mats had contact angle hysteresis less than
10⁰, which by standard convention means the surface roughness is nearly homogeneous. Since
the electrospinning process produces non-woven fiber mats with a random orientation of fibers
with different diameters and beads, it is not surprising that the surface roughness is homogeneous
and the contact angle hysteresis is low. Also, as mentioned previously, the static contact angle
minus the hysteresis yields the receding or minimum contact angle of the fiber mat surface.
Almost all of the electrospun fiber mats produced had minimum WCAs that were still in the
superhydrophobic, which proves that the electrospinning process can produced stable,
homogeneous superhydrophobic surfaces. In Chapter 4, it will be demonstrated that the low
contact angle hysteresis leads to high drop mobility.
3.5 WCA in Hydrophobic Liquids
In the previous sections, it has been demonstrated that stable superhydrophobic fiber mats can be
created using the electrospinning process. However, all WCA measurements were made with the
surface surrounded by air. If these surfaces were to be used in membrane separations to remove
small quantities of water from another liquid which water is immiscible in, the above WCA
measurements are not an appropriate characterization of the fiber surface. For a surface
submerged in another fluid, the form of Young’s equation to predict the contact angle remains
28
the same, however the surface tension must be replaced with the interfacial tension between the
two liquids. Also, the assumptions made in the Cassie-Baxter model used as the basis to
determine if man-made superhydrophobic surfaces are possible still hold because a liquid that is
immiscible with water has a contact angle of 180⁰. The only concern about whether the fiber
mats will still be superhydrophobic when submerged in a liquid is that the interfacial tension
between water and an immiscible liquid is much lower than the surface tension of water.
Therefore, one fiber mat from each of the polymers was selected for a WCA measurement when
submerged in a liquid.
Measuring the WCA of a surface when submerged in another liquid is much more tedious. To
perform the measurement, a device is needed that can simultaneously hold a liquid but also have
transparent walls so that the image of the drop resting on the surface can be detected by the
instrument’s camera used to make the WCA measurement. A spectrophotometric cell is perfect
for this type of application and can be seen in Figure 3.2
Figure 3.2: Spectrophotometric cell used for making WCA measurements of surfaces submerged
in a liquid
29
Next, a glass slide was cut to the appropriate dimensions to fit inside the cell and fibers were
electrospun on the slide. The slide was placed inside the cell and the cell was filled with the
desired liquid. WCA measurements were performed as mentioned before and the results are
listed in Table 3.8.
Table 3.8: WCA of fiber mats submerged in liquids
(Polymer) Water Contact Angle (water drop size µL) Fiber
Diameter (nm)
Air Hexadecane
Viscor oil Diesel 279 (PP) 170±3 (5) 165±3 (5) 166±3 (5) 168±3 (23)
499 (PFMOP) 169±2 (5) 171±2 (12) 165±4 (21) 171±2 (25) 257 (PVDF-
HFP) 165±2 (5) 166±3 (12) 166±3 (12) 165±2 (25)
From Table 3.8, it can be seen that the fiber mats are still superhydrophobic when submerged in
three different liquids. However, there are some key differences in how the measurements were
made that need to be addressed. In the previous WCA measurements, a standard drop size of 5
μL was used. However, when trying to get a drop of water to detach from the needle onto to the
surface when both are submerged, some surface-liquid combinations required different size
drops for this to be possible. Therefore, not all measurements were made with the same drop size
which could possibly introduce the effects of gravity on the drop shape and influence the
apparent position of the three-phase contact line if drops were too large. All efforts were made to
use the smallest diameter drop that would detach from the needle in making the measurements.
Along similar lines, when a drop is resting on the surface submerged in a liquid, buoyant effects
are non-trivial and should also be accounted for. Buoyant effects reduced the effect of gravity
and could affect what drop size is needed to neglect body forces compared to interfacial forces.
30
More investigation is needed to completely understand these effects. The physical properties of
the immiscible liquids are listed in the Appendix. Also, WCA measurements in all five liquids
were made using 5 µL drops on all PVDF-HFP fiber mats and the results can be seen in the
Appendix.
3.6 Conclusions
Using the electrospinning process, several superhydrophobic fiber mats were produced using
three different polymers: PP, PFMOP and PVDF-HFP. For PP and PFMOP, it was determined
that the surfaces’s hydrophobicity was independent of the fiber diameter in the range tested while
for PVDF-HFP the hydrophobicity decreased slightly as the fiber diameter increased. Contact
angle hysteresis measurements showed all electrospun fiber mats were stable superhydrophobic
surfaces with hysteresis less than 10⁰. Finally, it was determined that the fiber mats were still
demonstrated superhydrophobicity when submerged in various liquids which increases the
applications these surfaces can be used in.
31
CHAPTER IV
DROP MOTION ON SUPERHYDROPHOBIC FIBER MATS
4.1 Drop Mobility on Superhydrophobic Surfaces
As discussed in Chapter 2, many superhydrophobic surfaces allow water drops to roll very easily
along their surfaces with minimal amounts of applied force required. On many superhydrophobic
surfaces it only takes a slight tilting of the surface or a gentle blowing of air to cause drops to roll
along the surface. For example, the lotus leaf can keep itself clean because a slight incline in its
leaves is enough for drops to roll off its surface and the moving drops collect dirt from the leaf’s
surface and carries the dirt away from the surface. Superhydrophobic surfaces exhibit high drop
mobility for two main reasons. First, due to the high WCA on these surfaces, the drops have low
area contact which reduces the drag between the drop and the surface. Second, many of the
materials used to create superhydrophobic surfaces have a low coefficient of friction which also
reduces the drag between the drop and the surface. In this work, the minimum force required to
move drops along superhydrophobic fiber mats is determined and then the force balance is used
to develop a correlation calculate the drag coefficient between the drop and the fiber surface.
This correlation can then be used in models to predict filter performance using these surfaces.
32
4.2 Gravity Test
To determine the minimum amount of applied force required to move a water drop along a
superhydrophobic surface, a simple gravity experiment was designed as seen in Figure 4.1. A
super-hydrophobic nanofiber mat is attached to the lever arm whose angle from the horizontal
can be adjusted and controlled to within 1⁰ by the device in Figure 4.1. A water drop of 30 µL, is
then placed by a needle onto the test surface and the surface is tilted away from the horizontal
until the water droplet rolls of the surface.
Figure 4.1: Gravity test experimental apparatus.
Since the only applied force causing the water drop to move in this simple experiment is
gravitational force, determining the minimum angle from the horizontal needed to cause a water
drop to roll of the surface yields the minimum force needed to overcome the drag force holding
the drop to the surface. PFMOP, PP and PVDF-HFP nanofiber mats were electrospun at
different conditions so that the surfaces have a range of water contact angles. The minimum tilt
angle was measured on each surface. Modifications in the electrospinning process were needed
to make fiber mats with lower WCA in order to compare how the hydrophobicity of a surface
33
affects drop mobility. If the same fiber mats discussed in Chapter 3 were used, it would be
impossible to study the effect of surface hydrophobicity on drop mobility because all the surfaces
had relatively the same WCA. The electrospinning conditions and fiber diameter of the mats
used in the this experiment are contained in the Appendix. Figures 4.2, 4.3, and 4.4 show the
results for the tilt angle experiment.
Figure 4.2: Minimum tilt angle results for PFMOP fiber mats
Figure 4.3: Minimum tilt angle results for PP fiber mats
10121416182022242628
140.0 145.0 150.0 155.0 160.0
Tilt A
ngle
WCA
Tilt Angle vs. WCA for PFMOP
Tilt Angle
58
11141720232629323538
135.0 140.0 145.0 150.0 155.0 160.0 165.0 170.0
Tilt A
ngle
WCA
Tilt Angle vs WCA for PP
Tilt Angle
34
Figure 4.4: Minimum tilt angle results for PVDF-HFP fiber mats
From Figures 4.2, 4.3 and 4.4, the general trend for all polymeric fiber mats is the minimum tilt
angle needed for water droplets to roll of the surface decreases as the WCA increases. These
results are not very surprising. As the WCA for the fiber surface increases, the fraction of the
drop in contact with the fibers decreases as according to the Cassie-Baxter model. The drag force
acting on the drop is surface force and as the WCA increases, the contact area between the drop
and the fibers decreases. As this happens, the drag force between the drop and fibers decreases
and drops are able to move with less applied force. Therefore the results of this experiment are
another validation that the Cassie-Baxter model is an appropriate model for fibrous
superhydrophobic surfaces. These results also indicate that moving drops on these surfaces(drops
moving along the surface under the influence of drag from another moving fluid) should have a
lower drag force between the drop and the fiber surface as the WCA increases.
While the above results are applicable for separations with air as the carrier fluid, for separations
involving removal of water from other liquids, calculating the required force to move drops on
02468
101214161820
155 160 165 170
Tilt A
ngle
WCA
Tilt Angle vs WCA for PVDF-HFP
Tilt Angle
35
superhydrophobic nanofiber mats submerged in a liquid is more appropriate. To do this, the same
gravity test is used as previous explained with a slight modification as seen in Figure 4.5
Figure 4.5: Experimental apparatus for gravity test in liquids
As seen from Figure 4.5, a Plexiglas cell was designed to hold a superhydrophobic fiber surface
submerged in a liquid. A channel the width of a half a microscope glass slide was machined into
a solid piece of Plexiglas with sufficient depth so that when the cell is angled the liquid would
not spill out. This cell was then clamped to the arm of the miter gauge as seen above in Figure
4.5. A custom sized glass slide covered with superhydrophobic fibers was placed on the bottom
of the cell and the cell was filled with a liquid to a height of 2 cm. A water drop was placed on
the submerged superhydrophobic fiber surface and the minimum tilt angle needed to make the
drop roll along the surface was measured. The steady-state velocity was also measured. The
experimental results are listed in Table 4.1.
36
Table 4.1: Minimum tilt angle results for fiber mats submerged in liquids
Tilt Angle
Heptane Hexadecane Toluene Viscor Diesel
PFMOP 3⁰ 5⁰ 3⁰ 5⁰ 4⁰
PP 6⁰ 8⁰ 3⁰ 3⁰ 4⁰
PVDF-HFP 3⁰ 5⁰ 8⁰ 4⁰ 5⁰
There are some notable differences from this experiment and the previous results. First, unlike
when fiber surfaces are exposed to air, the WCA of the surfaces when submerged in a liquid are
independent of the fiber diameter and the polymer type. So this eliminate correlation of the tilt
angle to the WCA values we have in our experiment. Also, if we examine the results from Table
4.1, there is no direct correlation to type of fiber surface and the liquid it is submerged in and the
tilt angle needed to move the drops along the surface. All that can be deduced from the data is
that water drops have high mobility on these fiber surfaces submerged in a liquid because all the
tilt angles are less than 10⁰. However, it was observed that drops moved at different velocities in
each experimental run. The velocities of the drops submerged in liquids were slower than the
velocities of the drops submerged in air as expected because of the smaller tilt angle to initiate
movement, because of buoyancy reduction of the total net gravitational force, and because of the
higher viscosity and density of the liquids compared to air. The velcocity of the drop at steady-
state is directly related to the drag between the drop and the fiber surface but also the drag
37
between the drop and the surrounding fluid. Drop velocities for each experiment can be seen in
Appendix A.
4.3 Drag Coefficient Correlation
The drag force is related to the kinetic energy per unit of volume through the drag coefficient by
the following equation
�� = �������� (4.1)
At steady-state, the force balance around the drop reduces to
���� = ��������� + �
��������� (4.2)
In Eq. 4 2, FNET is the net applied force causing the drop to move along the surface and is
calculated by the following equation.
���� = �� − ����� sin (4.3)
Also in Eq. 2, the two terms on the left side of the equation are the two drag force terms adapted
from Eq. 1. The first term is the drag between the drop and the fibers and the second term is the
drag between the drop and the surrounding fluid. For the first term, the area of contact between
the drop and the fiber surface can be determined from the force balance on the inside of the drop.
This force balance, in Eq.4.4, relates gravity to the internal pressure of the drop.
��� = �� − ����� (4.4)
The pressure inside the drop can be calculated using equations 4.5 and 4.6.
∆� = � − �� = �� (4.5)
38
�� = ��ℎ (4.6)
In Eq. 4.5, γ is the interfacial tension and R is the radius of the drop.
For the drag coefficient in the term for the drag between the drop and surrounding fluid,
correlations for flow around a sphere are used where
�� = �� ��� (4.7)
�� = ������
(4.8)
Ultimately, Cf is directly dependent on two dimensionless numbers: the Reynolds number and
the Bond number. The Reynolds number for the drop is given by
�� = ������
(4.9)
The Bond number is a measure of the importance of surface tension forces compared to body
forces. There are two different regimes: the first regime is Bond numbers less than 1 where
interfacial tension forces dominate. The second regime is Bond numbers greater than 1 where
body forces dominate. The Bond number is given by Eq. 4.10
�� = ∆����� (4.10)
Two of the liquids had Bond numbers less than 1 and the other two liquids had Bond numbers
equal to one. So the drag coefficient was correlated to the Reynolds number for each regime
separately. The experimental results are plotted in Figures 4.6 and 4.7.
39
�� = �4�10� ± 1�10�� ����.���±�.����
� = .9461
Figure 4.6: Drag coefficient correlation for Bo <1.
�� = �5�10� ± 2�10�� ����.����±.�����
� = .9932
Figure 4.7: Drag coefficient correlation for Bo=1
1
10
100
1000
10000
100000
1 10 100
C f
Re
Cf vs. Re
CfPower (Cf)
110
1001000
10000100000
1 10 100 1000
C f
Re
Cf vs. Re
CfPower (Cf)
40
From Figures 4.6 and 4.7, the drag coefficient correlations for both Bond number regimes are
very similar. They both follow the very familiar trend for drag coefficient correlations, as the
Reynolds number increases, the drag coefficient decreases exponentially. Also, as typical with
most drag coefficient correlations, the drag coefficient follows a power law function of the
Reynolds number. However, the drag coefficient for the regime where the Bond number is less
than one is significantly higher than for the regime where the Bond number is equal to one.
4.4 Conclusions
It was determined that the superhydrophobic fiber surfaces generated by the electrospinning
process have high water droplet mobility. Water drops move along the surface with very small
amounts of applied force. From experimental data, it was determined that the amount of force
needed to move drops is only a fraction of one gravitational force. Also, it was determined that
the amount of force needed to move water drops on fiber surfaces submerged in liquids is less
than the amount of force needed to move water drops on fiber surfaces exposed to air. However,
drops move at much slower velocities when submerged in a liquid due to the greater drag force
the liquid imposes on the drop. Since the velocities were much slower, they were measurable and
evaluated in a force balance to calculate the drag coefficient between the drop and the surface.
This drag coefficient correlation can now be used in models to predict how these surfaces will
perform in membrane separations.
41
CHAPTER V
CONCLUSIONS AND FUTURE WORK
5.1 Conclusions
Using the electrospinning process, several superhydrophobic fiber mats were produced using
three different polymers: PP, PFMOP and PVDF-HFP. For PP and PFMOP, it was determined
that the surfaces’s hydrophobicity was independent of the fiber diameter in the range tested while
for PVDF-HFP the hydrophobicity decreased slightly as the fiber diameter increased. Contact
angle hysteresis measurements showed all electrospun fiber mats were stable superhydrophobic
surfaces with hysteresis less than 10⁰. Finally, it was determined that the fiber mats were still
demonstrated superhydrophobicity when submerged in various liquids which increases the
applications these surfaces can be used in.
It was determined that the superhydrophobic fiber surfaces generated by the electrospinning
process have high water droplet mobility. Water drops move along the surface with very small
amounts of applied force. From experimental data, it was determined that the amount of force
needed to move drops is only a fraction of one gravitational force. Also, it was determined that
the amount of force needed to move water drops on fiber surfaces submerged in liquids is less
than the amount of force needed to move water drops on fiber surfaces exposed to air. However,
drops move at much slower velocities when submerged in a liquid due to the greater drag force
the liquid imposes on the drop. Since the velocities were much slower, they were able to be
42
measured and used in the force balance to calculate the drag coefficient between the drop and the
surface. This drag coefficient correlation can now be used in models to predict how these
surfaces will perform in membrane separations.
5.2 Future Work Recommendations
It has been proven that superhydrophobic fiber surfaces can be created by using the
electrospinning process. These surfaces have very high levels of hydrophobicity and are very
stable. One suggestion for future work would be to investigate the effect of the geometry of the
fiber mat and its effect on wettability. All of the electrospun superhydrophobic fiber surfaces
studied in this work were flat surfaces. It may be possible to increase the hydrophobicity of fiber
surfaces if they were curved in nature. One such surface would be a fiber tube. Due to these two
facts, water droplets have very high mobility and correlations have been developed to predict
how they will move on these surfaces. Due to high drop mobility and low drag force between the
drop and the fibers, it is expected that these surfaces would perform well as membrane separators
for oil-water emulsions. The second suggestion for future work would be to investigate how
efficiently these superhydrophobic fiber surfaces separate oil-water emulsions. Since these fiber
surfaces tend to also be oleophilic and porous, they are ideal candidates to be used to separate
oil-water emulsions that cannot be separated by traditional means.
43
5.2.1 Superhydrophobic Fibers as Membrane Filters
Currently there are several ways to separate oil-water emulsions: gravity settling, centrifuging,
coalescence etc. However, for very fine oil-water emulsions with water drops less than 50
microns, traditional methods used to separate oil-water emulsions begin to fail. With such small
water drop sizes, the settling velocity of water drops becomes very small which inhibits gravity
settling of very fine water drops. Gravity settling uses the fact that water is denser than oil and
drops will settle to the bottom and the two phases can be separated by decanting. However, if the
water drops do not settle to the bottom or take very long to settle, these water drops will still
remain in the oil phase when decanted after gravity settling. Also, very small water drops are
also harder to radially accelerate than larger drops. Centrifuging uses the density difference
between water and oil to radially accelerate water drops outward towards the centrifuge’s wall.
Since very fine water drops are not radially accelerated fast enough and cannot get to the
centrifuge’s wall fast enough, they remain with the oil phase. Since our superhydrophobic fiber
mats are both porous and oleophilic, they have the possibility of being used as a membrane filter
to remove very small water drops from oil. It is expected that our superhydrophobic fiber mats
have very small pore sizes due to the very small diameters and small water drops should not be
able to penetrate the pores due to the pressure drop across the membrane. Also, for most
emulsions, even the smallest water drops should still be larger than the diameter of the fibers. If
the diameter of the drop approaches the size of the fibers, the superhydrophobic effect will be
lost and the drops wettability will return to that of the contact angle between water and the
polymer itself. It is not expected that this will happen and even the smallest water drops will
bead up the same as larger drops on these fiber mats and all the benefits of superhydrophobicity
can be utilized such as high drop mobility due to the reduced drag between the drops and the
44
fiber surface. Even if it is discovered that small water drops “stick” to the surface or penetrate the
pores to remain in the oil phase, a coalescer can be used to increase the drop size before it
encounters the superhydrophobic fiber mat. These superhydrophobic fiber mats also need to be
utilized in cross-flow filtration to avoid blinding of the surface by a thin film of water. If enough
water drops collect on the surface without being cleared, they will coagulate into a thin water
film that will block the surface from oil and prevent flow causing plugging of the filter.
5.2.2 Curved Superhydrophobic Surfaces
Another interesting aspect of superhydrophobic surfaces to be studied is the effect of curvature
on superhydrophobicity. Does the curvature of a surface make a surface more or less
hydrophobic? Does the surface curvature enhance water droplet mobility? Making tubular
superhydrophobic fiber mats via the electrospinning process may be challenging but the benefits
of these types of superhydrophobic media make them very enticing. If it can be determined that
tubular fiber mats have increased hydrophobicity and droplet mobility compared to their
rectangular fiber mat counterparts, these superhydrophobic tubular fiber mats may prove to be
superior membrane filters for oil-water emulsions that rectangular surfaces. For one, tubular
shapes have lower pressure drop per unit of cross sectional area than do rectangular shapes. If the
same separations can be achieved with a lower pressure drop, tubular filters will be more
efficient than their rectangular counterparts.
45
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APPENDIX
A.1 Electrospinning Conditions for Superhydrophobic Fiber Mats
The electrospinning conditions for PP and PFMOP were selectively chosen based off previous
work done by Lee et al [17]. However, it was discovered that our electrospinning set-up required
slightly different conditions to produce fibers than the ones used in previous work.
Electrospinning conditions were varied until a continuous network of fibers was produced with
no spraying or dripping. Electrospinning conditions for PVDF-HFP were based off work done in
collaboration with another research lab here at the University of Akron. Once the correct
electrospinning conditions for each polymer were determined, they were held constant while the
concentration of polymer in the electrospinning solution was varied. In the cases of PP and
PFMOP, the concentrations were limited due to viscosity. Table A.1 lists the electrospinning
conditions for each polymer.
Table A.1: Electrospinning conditions
Polymer
Flowrate
(mL/hr)
TCD
(cm)
Voltage
(kV)
PP 25 25 25
PFMOP 20 20 20
PVDF-HFP 15 15 30
52
A.2 Fiber Distribution of Superhydrophobic Fiber Mats
In Chapter 3, the hydrophobicity of fiber mats was related to the average fiber diameter of each
mat. However, all of the electrospun fiber mats did not have a uniform fiber size distribution.
Tables A.2, A.3, and A.4 list the fiber distribution for each electrospun fiber mat tested.
Table A.2: Fiber size distribution for PP fiber mats
Average Diameter
(nm)
Standard Deviation
(nm)
Diameter Range (nm)
279 88 100-500 404 188 100-900 363 267 100-1600 996 466 200-2000 939 403 200-2300 1506 511 600-3100 2836 883 900-5100
Table A.3: Fiber size distribution for PFMOP fiber mats
Average Diameter
(nm)
Standard Deviation
(nm)
Diameter Range (nm)
499 246 100-1300 558 216 100-1200 501 384 200-2000 794 312 400-1400 781 450 200-1900 2230 1249 500-6500
53
Table A.4: Fiber size distribution for PVDF-HFP fiber mats
Average Diameter
(nm)
Standard Deviation
(nm)
Diameter Range (nm)
248 164 100-800 257 116 100-700 451 191 100-1000 677 327 200-1900
From Tables A.2, A.3, and A.4, the fiber size distribution for large diameter fibers is has a wider
range than for smaller fibers. This is due mainly to the fact that electrospinning fiber mats from
more concentrated solutions produces a wider diameter distribution in the resultant fiber mat
than electrospinning from lower concentration solutions. Figures A.1a and A.1b show SEM
images of the superhydrophobic fiber mats.
Figure A.1: a) SEM image of PVDF-HFP superhydrophobic fiber mat b) SEM image of PP
superhydrophobic fiber mat.
From the above figures, the superhydrophobic fiber mats were not smooth fibers but rather
beaded fibers which added another layer of roughness to improve the wettability properties of the
surface.
54
A.3 Physical Properties of Immsicible Liquids
Five immiscible liquids were used in several different experiments. These liquids were used to
test the superhydrophobic nature of the fiber mats when submerged and also used to measured
the minimum tilt angle needed for water drops to move on a submerged fiber mat. Table A.5 lists
the physical properties of the five immiscible liquids.
Table A.5: Physical Properties of Immiscible Liquids
Liquid
Density
(kg/m3)
IFT
(mN/m)
Viscosity
(kg/m-s) Bo
Hexadecane 773 53 0.00152 0.67
Toluene 867 36 0.00059 0.58
Heptane 684 50 0.00039 0.99
Viscor 832 16 0.00207 1.64
Diesel 850 25 0.00762 0.94
A.4 WCA on PVDF-HFP surfaces submerged in liquids
In Chapter 3, it was mentioned that there was difficulty using the standard 5 µL drop in
measuring the WCA of fiber surfaces submerged in liquids. Previously, the needle was
submerged in the liquid before the water was dispensed and 5 µL drops would not detach onto
the surface. However, if a 5 µL drop is dispensed from the needle before it is submerged in the
liquid, once the drop comes into contact with the immiscible liquid, the drop detaches and
gravity causes the drop to fall through the liquid and rest on the submerged fiber mat surface.
Using this method, one can use the standard 5 µL to make WCA measurements on submerged
55
fiber surfaces. Table A.6 lists the WCA values for all PVDF-HFP fiber mats when submerged in
each of the five liquids
Table A.6: WCA of submerged PVDF-HFP fiber mats
Fiber Diameter
(nm) Viscor
Oil Diesel Heptane Hexadecane Toluene 248 167±2 158±1 166±2 162±2 168±2 257 167±2 157±1 166±1 159±2 169±3 451 165±2 158±1 162±1 161±1 159±3
From Table A.6, the results show that the PVDF-HFP fiber mats retain superhydrophobic
wetting charactersistics even with smaller drops. The results are also consistent with the data
listed in Chapter 3.
A.5 Area of Contact Between Drop and Fibers
In Chapter 4, a force balance at the bottom of the drop was proposed to calculate the area of
contact between the drop and the fibers. Figure A.2 shows the force balance
Figure A.2: Depiction of force balance at bottom of the drop
As seen from Figure A.2,the pressure on the inside of the drop times the area of contact between
the drop and surface equals the difference between gravity and buoyancy forces acting on the
drop. As discussed in Chapter 4, this is one method of calculating the contact area between the
Fg
Fb PiAc
56
drop and the solid surface needed to solve the force balance to determine the drag coefficient
between the drop and the solid surface. As proof of concept that this method is correct, there is
another approach to solving for the contact area as depicted in Figure A.3.
Figure A.3: Depiction of alternate force balance at bottom of drop.
From Figure A.3, the interfacial tension times the length of the contact line is the force that
offsets the difference between gravity and buoyancy forces acting on the drop. From Chapter 4
��� − ������ = 2���� sin (A.1)
∆ = ��� (A.2)
��� − ������ = ����∆ sin (A.3)
If we combine Eqns. A.3 and 4.4, the following relation is obtained.
��� = ����∆ sin (A.4)
If the area of contact is defined as
�� = ���� (A.5)
2πRcγ R
Rc
57
and the pressure difference is approximately equal to the inside pressure, then by combining
Eqns. A.4 and A.5 yields the relationship that must be satisfied for both force balances to be
equivalent.
� sin = �� (A.6)
In Eqn. A.6, Rc is the radius of contact, R is the radius of the drop and θ is the contact angle
between the drop and the surface. From known quanties, the above relation in Eqn. A.6 is true.
A.5 Raw Data from Liquid-Liquid Tilt Angle Experiment
In Chapter 4, the drag coefficient between the drop and the fiber surface was calculated by
solving the force balance for the drag coefficient. To solve the force balance, the tilt angle
required to move a drop along the surface and the steady-state velocity had to measured. Table
A.6, A.7, and A.8 lists the raw data used in these calculations.
58
Table A.6: Raw data for liquid-liquid gravity experiment on PFMOP superhydrophobic fiber
mats
Immiscible Liquid Tilt Angle (°) Velocity (m/s) Diesel 4 0.0056 Diesel 3 0.0083 Diesel 3 0.0083 Viscor 5 0.0038 Viscor 5 0.0063 Viscor 4 0.0050
Heptane 3 0.0167 Heptane 4 0.0167 Heptane 3 0.0167
Hexadecane 5 0.0050 Hexadecane 4 0.0056 Hexadecane 4 0.0050
Toluene 3 0.0083 Toluene 3 0.0071 Toluene 3 0.0083
59
Table A.7: Raw data for liquid-liquid gravity experiment on PP superhydrophobic fiber mats
Immiscible Liquid Tilt Angle (°) Velocity (m/s) Diesel 4 0.0071 Diesel 5 0.0071 Diesel 3 0.0056 Viscor 3 0.0063 Viscor 4 0.0125 Viscor 3 0.0050
Heptane 14 0.0167 Heptane 4 0.0125 Heptane 4 0.0100
Hexadecane 8 0.0083 Hexadecane 3 0.0083 Hexadecane 7 0.0071
Toluene 3 0.0100 Toluene 3 0.0083 Toluene 3 0.0071
60
Table A.8: Raw data for liquid-liquid gravity experiment on PVDF-HFP superhydrophobic fiber
mats
Immiscible Liquid Tilt Angle (°) Velocity (m/s) Diesel 3 0.0035 Diesel 6 0.0033 Diesel 4 0.0067 Diesel 4 0.0040 Viscor 3 0.0060 Viscor 5 0.0071 Viscor 3 0.0044 Viscor 4 0.0046
Heptane 3 0.0060 Heptane 4 0.0083 Heptane 18 0.0200 Heptane 6 0.0300
Hexadecane 4 0.0055 Hexadecane 10 0.0100 Hexadecane 4 0.0083 Hexadecane 6 0.0100
Toluene 3 0.0020 Toluene 13 0.0167 Toluene 6 0.0080 Toluene 7 0.0150