Fabrics with tunable oleophobicity
Choi, W. et al. “Fabrics with Tunable Oleophobicity.” Advanced
Materials 21.21 (2009): 2190-2195. © 2009 WILEY-VCH.
Author's final manuscript
Wed May 02 08:00:59 EDT 2012
Attribution-Noncommercial-Share Alike 3.0 Unported
DOI: 10.1002/adma.((please add manuscript number))
Fabrics with Tunable Oleophobicity
By Wonjae Choi,1** Anish Tuteja,2** Shreerang Chhatre,2 Joseph M. Mabry,3 Robert E.
Cohen2* and Gareth H. McKinley1*
(**: these authors contributed equally to this work)
 Prof. Gareth H. McKinley, Wonjae Choi
Department of Mechanical Engineering, Massachusetts Institute of Technology
Cambridge, MA 02139 (USA)
 Prof. Robert E. Cohen, Dr. Anish Tuteja, Shreerang Chhatre
Department of Chemical Engineering, Massachusetts Institute of Technology
Cambridge, MA 02139 (USA)
 Dr. Joseph M. Mabry
Space and Missile Propulsion Division, Air Force Research Laboratory
Edwards Air Force Base, CA 93524 (USA)
*: Corresponding authors
Prof. Gareth H. McKinley (email@example.com)
Prof. Robert E. Cohen (firstname.lastname@example.org)
Acknowledgement. This research has been supported by the Air Force Research Lab (AFRL)
under contract no. FA9300-06M-T015 and the Air Force Office of Scientific Research
(AFOSR) under contract no. FA9550-07-1-0272 and LRIR-92PL0COR. We also thank Prof.
Michael F. Rubner and the Institute for Soldier Nanotechnologies (ISN) at MIT for the use of
various lab facilities. Partial financial support for S.C. starting Sept. 1, 2008 was provided by
the Army Research Office (ARO) through contract no. W911NF-07-D-0004.
Keywords: Superoleophobic, Super-wetting, Super-repellency, Dip-coating, Tunable
Extreme wetting behavior, super-repellency and super-wetting, has recently generated
immense commercial and academic interest[1-10] due to its wide applicability in various fields
including the development of self-cleaning surfaces, liquid-liquid separation membranes,
and anti-fogging films. Various research groups have also tried to develop surfaces that can
effectively switch their surface wetting properties in response to changes in their surrounding
environment. This includes surfaces that alter their wettability in response to changes in
temperature, electrical voltage[15, 16] and mechanical deformation.[17, 18] Because of the
difficulty of making surfaces that are strongly repellent to low surface tension liquids such as
oils and alcohols, most work on switchable wettability has focused on studies with water
droplets (with the exception of recent work of Ahuja et al.). In our previous work we have
demonstrated how the incorporation of re-entrant surface texture, (i.e. a multi-valued surface
topography) in conjunction with surface chemistry can be used to fabricate superoleophobic
surfaces, i.e. surfaces which can support a robust composite (solid-liquid-air) interface and
display contact angles greater than 150° with various low surface tension liquids. More
recently, we have also developed two general design parameters that can aid the systematic
design of omniphobic surfaces that resist wetting by almost any liquid.
In the present work, we analyze the consequences of these non-wetting design
parameters more extensively. Recognizing the role of re-entrant surface features, we first
develop a simple dip-coating process for delivering a conformal coating of fluorodecyl POSS
molecules.[19, 21] This coating enables us to bestow substantially enhanced liquid repellency to
any substrate already possessing suitable textures, such as the lotus leaf, commercial fabrics
and even duck feathers, by promoting the formation of a composite (solid-liquid-air) interface.
Consideration of the geometric scaling of the design parameters suggests that mechanically
deforming a re-entrant structure such as a dip-coated commercial fabric will lead to a dramatic,
but reversible, reduction in the liquid repellency of the surface. Indeed, we observe that a non-
wetting drop (initially sitting on the surface in a composite Cassie-Baxter state) completely
wets into the fabric texture beyond a critical imposed strain, leading to near zero contact angles.
This allows us to develop, for the first time, surfaces that exhibit reversible, deformation-
dependent, tunable wettability, including the capacity to switch their surface wetting properties
(between super-repellent and super-wetting) against a wide range of polar and non-polar
When a liquid contacts a textured surface, such as the one shown in Fig. 1a, then,
provided the pressure difference across the liquid-air interface is negligible, the liquid does not
penetrate fully into the pores of the surface texture. Instead, the liquid wets the pore surface
partially until the local angle (?) between the liquid and the textured substrate becomes equal
to the equilibrium contact angle ? (given by Young’s relation) for the three phase contact
line.[23-25] The existence of points on the surface that enables the condition ? = ?  to be
fulfilled is a necessary, though not sufficient condition,[19, 24, 25, 27, 28] for the formation of a
composite interface, such as the one shown in Fig. 1a.
The apparent contact angle ?* for the composite interface that exists under a strongly
non-wetting droplet is typically computed using the Cassie-Baxter relation:
where f1 is the ratio of the total area of solid-liquid interface to a unit projected area of the
textured substrate and f2 is the corresponding ratio for the liquid-air interface. An example
of a natural surface that is able to support a composite interface is shown in Fig. 1b, which
illustrates water (?lv = 72.1 mN/m) droplets ‘beading up’ on the surface of a duck feather. The
feather is composed of a periodic array of micron-scale cylindrical barbules (see SEM in
supporting information). The bright, reflective surface visible underneath the water droplet in
Fig. 1b is a signature of trapped air and the establishment of a composite solid-liquid-air
interface. The formation of this ‘Cassie-Baxter’ state enhances super-repellency by promoting
a high apparent contact angle (?*) and low contact angle hysteresis (defined as the difference
between the advancing and receding contact angles) when f1 << 1.[23, 29-32]
On the other hand, if the liquid fully penetrates into the texture surface, then the
apparent contact angle ?* is determined by the Wenzel relation:
where r is the surface roughness, defined as the ratio between the actual surface area and the
projected area. Since r is necessarily greater than unity, roughness amplifies both the wetting
and non-wetting behavior of materials in the Wenzel regime; i.e. cos?* >> 0 if cos? > 0 and
cos?* << 0 if cos? < 0. A consequence of this dependence on the roughness of the texture is
that, once initiated, the imbibition of a liquid drop into a roughened texture can rapidly lead to
super-wetting because the apparent contact angle ?* ? 0° when r >>1 and ? < 90°.
Development of extremely liquid-repellent surfaces requires the design of substrates
that promote the formation of a composite interface with any liquid. The two important design
characteristics for establishing a composite ‘Cassie-Baxter’ state on a textured surface with a
particular contacting liquid are (i) the magnitude of the apparent contact angle ?* on the
composite interface and (ii) the robustness of the composite interface against external
perturbation. The equilibrium apparent contact angles on the texture can be readily estimated
using the Cassie-Baxter relation (Eq. 1), and in our recent work we developed a dimensionless
design parameter A* to predict the robustness of the composite interface. This robustness
factor represents the ratio between the breakthrough pressure required to cause sufficient
sagging and disruption of the liquid-vapor interface (see Fig. 1c), and a characteristic
reference pressure Pref, given as
(here ? is the fluid
density and g is the acceleration due to gravity). This reference pressure Pref is close to the
minimum pressure difference across the composite interface for millimetric or larger liquid
droplets or puddles, and the breakthrough pressure at which a given composite interface is
disrupted can be computed as
For a texture that is dominated by periodic cylindrical features, such as the duck
feathers shown in Fig. 1 and the fabric surface shown in Fig. 3, the robustness measure A* can
be calculated to be (see supporting information):
R D1 2
Here R is the fiber radius and D is half the inter-fiber gap, as shown in Fig. 1a. Large values
of the robustness factor (A* >> 1) indicate the formation of a robust composite interface, with
very high breakthrough pressures. On the other hand, as A* approaches unity, Pbreakthrough
decreases towards Pref. Thus, a composite interface on any surface for which A* < 1 cannot
maintain its stability against even small pressure differentials across the liquid-air interface,
causing the liquid to penetrate into the textured surface and ultimately to be fully imbibed.
It is also useful to define another dimensionless parameter, D*, that parameterizes the
geometry of the re-entrant features. For a texture consisting of uniform cylindrical features as
shown in Fig. 1a, the ratios f1 and f2 in the Cassie–Baxter relationship (Eq. 1) become
. Substituting these expressions into Eq. 1
and factoring out a dimensionless spacing ratio D*= (R + D)/ R enables us to express the
apparent contact angle in terms of a purely geometric factor and a contribution involving the
equilibrium contact angle on a flat surface:
?????= ? ++?
Higher values of D* correspond to a higher fraction of air in the composite interface (f2 in Eq.
1), and consequently an increase in the apparent contact angle ?* for a given liquid. Care must
be taken in developing appropriate expressions for D* and A* for surfaces with variable
surface texture parameters or multiple scales of roughness such as a duck feather or a lotus
leaf (see supporting information for further details).[34, 35]
To achieve high apparent contact angles with low contact angle hysteresis and a robust
composite interface, we seek to maximize the two design parameters D* and A*
simultaneously. However, for a periodic geometry of cylindrical features, these two design
factors are strongly coupled. Increasing the value of D*, by either increasing D or reducing R
(with the other geometric variable held fixed), leads to a decrease in the values of A* (see Eq.
3). This coupling is further highlighted in Fig. 2 which shows a general design chart for a
typical oil (rapeseed oil; ?lv = 35.7 mN/m, ? = 86o) on a surface with cylindrical fibers,
illustrating the variation in the robustness factor A* with the variation in the spacing ratio D*.
The solid lines in the graph correspond to A* values computed by changing the inter-fiber
distance (D) while maintaining the fiber radius (R) constant. Each solid line corresponds to a
different value of the fiber radius R, varying between 1 mm – 1 nm.
Evaluating the magnitude of the robustness factor A* also explains why rapeseed oil
spontaneously penetrates the texture of many naturally re-entrant superhydrophobic surfaces
such as a duck feather or a lotus leaf (see inset of Fig. 3a, D* ? 58; also see supporting
information). In each case, A* << 1 as shown in Fig. 2. To enable these surfaces to support a
composite interface with various low surface tension liquids, it is essential to increase the
magnitude of the robustness factor A*. For a fixed surface texture, such an enhancement can
be induced most readily by markedly lowering the surface energy of the solid, leading to
increased values of the equilibrium contact angle ? (based on the Young’s relation).
In our recent work,[19, 21] we discussed the synthesis and application of a new class of
polyhedral oligomeric silsesquioxane (POSS) molecules, in which the silsesquioxane cage is
surrounded by 1H,1H,2H,2H-heptadecafluorodecyl groups. These molecules are referred to as
fluorodecyl POSS. The high concentration of perfluorinated carbon atoms in the alkyl
chains leads to an extremely low solid surface energy for these molecules (?sv ? 10 mN/m).
As a comparison, the surface energy of Teflon® is ?sv ? 18 mN/m. To provide a conformal
and flexible coating of fluorodecyl POSS molecules on any preformed substrate possessing
re-entrant texture, we have developed a simple dip-coating procedure using the fluorodecyl
POSS and a thermoplastic elastomeric binder (see experimental methods). After dip-coating,
the equilibrium contact angle for rapeseed oil on a smooth glass slide increases to ? = 86°;
compared to ? ? 0° on a clean uncoated glass slide (see supporting information for contact
angle measurements with selected polar and non-polar liquids on a dip-coated glass slide).
Dip-coating also increases the value of the robustness factor for rapeseed oil on a duck feather
and a lotus leaf to A* ? 4.2 (assuming R = 1 µm, D = 20 µm) and A* ? 26 (assuming Rnub = 2.5
µm, Dnub = 5 µm) respectively (also see Fig. 2). Note that because the lotus leaf does not
possess a cylindrical surface texture, the robustness factor A* for the lotus leaf cannot be
computed using Eq. 3 but requires a more complex expression. As a result of the high
values of the robustness factor, a dip-coated duck feather is able to support a composite
interface even with hexadecane (?lv = 27.5 mN/m; A* = 3.3) as shown in Fig. 1d.
In Fig. 3b we show an SEM micrograph highlighting the surface texture of a dip-
coated lotus leaf. A comparison with the surface morphology of an uncoated lotus leaf (Fig.
3a) shows that all of the surface details, even features in the sub-micron range are preserved
after dip-coating. The inset in Fig. 3b shows that the dip-coating process turns the surface of
the leaf oleophobic, allowing it to support a robust composite interface with rapeseed oil and
display large apparent contact angles (?* = 145°).
Another general class of textured substrates possessing re-entrant curvature is
commercial fabric.[37-40] Fig. 3c shows an electron micrograph of a commercial polyester
fabric (Anticon 100 clean-room wipe). The inset in Fig. 3c shows that a droplet of hexadecane
completely wets the surface of the as-received polyester fabric. Fig. 3d shows the surface
morphology of a polyester fabric dip-coated with fluorodecyl POSS. It is clear that all of the
surface details of the polyester fabric are preserved after dip-coating. The inset in Fig. 3d
shows the elemental mapping of fluorine on the dip-coated fabric surface using energy
dispersive X-ray scattering (EDAXS). Dip-coating provides a conformal coating of
fluorinated molecules on the fabric surface. The liquid repellency of the dip-coated fabric is
highlighted in Fig. 3e which shows that this fabric is able to support a composite interface and
display high apparent contact angles (?* ? 140o) even with octane (?lv = 21.7 mN/m). Taking
the geometry of the large fiber bundle as the dominant texture (Rbundle = Dbundle = 150 µm,
Rfiber = 5 µm, Dfiber = 10 µm, D* = 6, ? = 60°), we obtain a value of the robustness factor A* =
2.5 for octane. The measured contact angles on various dip-coated surfaces with selected polar
and non-polar liquids are provided in the supporting information.
It should however be noted that when the equilibrium contact angle ? < 90°, the
composite interface is necessarily metastable,[4, 19, 20, 25, 41-43] i.e. it corresponds to a local
minimum in the overall Gibbs free energy for the system, while the fully-wetted Wenzel state
corresponds to the global minimum.
To obtain even higher apparent contact angles with very low surface tension liquids, it
is necessary to increase the value of the spacing ratio D* (see Eq. 4). For fabrics, this can be
readily achieved by equi-biaxial stretching such that the imposed strain is the same in both
directions. Here strain is defined as ? =
, where L and L0 represent the stretched
and unstretched dimensions of the surface respectively. Stretching a fabric sample (such that
the inter-fiber spacing D is increased for a fixed value of the fiber radius R) increases the
spacing ratio to Dstretched
(), leading to an increase in the value of the apparent
contact angles. Based on the Cassie-Baxter relation (Eq. 4), the apparent contact angles on
the stretched fabric can then be computed as:
sin? + (? ??)cos?
1+ ?(1+ cos?unstretched
However, as noted earlier, for a cylindrical geometry the two design parameters D* and A* are
strongly coupled (see Eq. 3). As a result, increasing the inter-fiber spacing D causes more
severe sagging of the liquid-air interface, leading to lower values of the robustness factor A*.
Thus, with increasing values of D*, we expect to initially see an increase in the value of
apparent contact angles ?*, followed by a sudden transition to the Wenzel state once A*
decreases to a value close to unity.
The changes in the apparent advancing and receding contact angles for dodecane (?lv =
25.3 mN/m, ? = 70o) on a dip-coated polyester fabric, as a function of the applied strain are
shown in Fig. 4a. As expected from Eq. 5, initially there is an increase in both the apparent
advancing and receding contact angles with increasing strain due to the increasing fraction of
trapped air underneath the liquid droplet. Fig. 4b shows the variation with applied strain in the
values of both the robustness factor A*, and the spacing ratio D* for dodecane on the dip-
coated fabric surface. Comparing the responses in Fig. 4a and 4b, we see a rapid transition
from a composite or Cassie-Baxter interface to a wetted interface with both
when A* ? 1. Threshold values of A* for the transition from the Cassie regime to the Wenzel
regime were measured to be consistently close to unity (varying between A* = 1.0 and A* =
1.7) for various polar and non-polar liquids. Thus, biaxial stretching provides a simple
mechanism for systematically adjusting the wetting properties of the dip-coated polyester
fabric. To our knowledge, this is the first demonstration of tunable superoleophobicity on any
An example of switchable oleophobicity of the fabric surface is shown in Fig. 4c.
When a droplet of decane (?lv = 23.8 mN/m, ? = 60o) is placed on an unstretched (D* = 6) dip-
coated fabric, the relatively high value of the robustness factor (A* = 2.8) enables the fabric to
support a composite interface with the decane droplet and thereby display high advancing
apparent contact angles
?? (see Fig. 4d). However, when the fabric is biaxially
stretched to a strain of 30% (D* = 7.8), the robustness factor is reduced to almost unity (A* ?
1.4), and the fabric is fully wetted by the decane, with
? 0° (see Fig. 4e).
When the strain on the fabric is released, the initial surface texture is restored.
Returning to the original value of the inter-fiber spacing D leads once more to high values of
the robustness factor A*. This allows the fabric to once more support a composite Cassie-
Baxter interface and display high apparent contact angles with a new droplet of decane placed
at any location on its surface, except the spot where the initial liquid drop has been imbibed.
The fabric is able to regenerate its oleophobic properties at the original wetted spot as well
after a simple evaporative drying process. The data in Fig. 4c shows the apparent contact
angles with decane at a single fixed location on the dip-coated polyester fabric. The first
contact angle measurement is performed on the unstretched fabric. Next, the fabric is
stretched until D* increases to a value of 7.8 and correspondingly A* decreases to a value of
1.4, leading to complete wetting. The strain is then removed and the fabric is placed in a
vacuum oven to dry at 100°C for 20 minutes. This is followed by another contact angle
measurement at the same location. Next, the fabric is stretched again and the cycle is repeated.
It is clear from Fig. 4c that biaxial stretching enables the fabric to easily switch its wetting
characteristics from perfectly oil-wetting to non-wetting, easily and reproducibly. Further,
EDAXS elemental mapping (see supporting information) and microscopy on the dip-coated
fabrics after the sixth cycle indicates that there is no apparent degradation in the fluorodecyl
POSS coating after repeated stretching. Similar switchable wettability can also be achieved
with ethanol (?lv = 22.1 mN/m), methanol (?lv = 22.7 mN/m) and dodecane (?lv = 25.3 mN/m)
by stretching up to 20%, 30% and 60% strain respectively (see supporting information for
data on switchable wettability with methanol).
From the definitions of the design parameters A* and D* in Eqs. 3 and 4, it is clear that
for the same value of the spacing ratio D*, different liquids possess different values of the
robustness factor A* due to differing values of surface tension, capillary length and
equilibrium contact angle for each liquid on the fluorodecyl POSS-coated surface. Thus, while
the dip-coated fabric may support a composite interface with a particular contacting liquid (A*
>> 1), another liquid with a lower surface tension may fully wet the fabric (A* ? 1). In
addition, as we have shown in Fig. 4b, biaxial stretching of the fabric provides a simple
mechanism to tune the robustness factor A* and correspondingly adjust the conditions under
which a liquid will wet and permeate into the fabric, or remain beaded up on the surface.
Fig. 5 highlights the strain-induced sequential wetting of a dip-coated polyester fabric
with a series of different liquid drops by exploiting this sensitivity to the surface tension and
equilibrium contact angle of the contacting liquid. The unstretched dip-coated fabric shown in
Fig. 5a (D* = 6) is able to support a composite interface with four different low surface
tension alkanes; octane (?lv = 21.7 mN/m), decane (?lv = 23.8 mN/m), dodecane (?lv = 25.3
mN/m) and hexadecane (?lv = 27.5 mN/m). Starting from this initial state, we continuously
increase the applied strain on the fabric. At a strain of 15% the spacing ratio reaches a value of
D* = 6.9, and the lowest surface tension oil (octane,
? 1.7) transitions to a fully wetted
Wenzel state (Fig. 5b), while the other three liquids maintain a Cassie-Baxter composite
interface with the dip-coated fabric. Thus, the stretched fabric can demonstrate remarkably
different wetting properties with liquids having a surface tension difference of only ??lv ~ 2
mN/m between them. Additional strain increases the spacing ratio further; once it reaches a
value of D*= 7.8, the decane droplet (
? 1.4) wets the fabric surface (Fig. 5c), while
dodecane and hexadecane still maintain a non-wetting composite interface. Further stretching
of the fabric (until D* = 9.6) causes even the dodecane drop (
? 1.0) to be imbibed into
the fabric (Fig. 5d).
In conclusion, in this work we have developed a simple and benign dip-coating
process that enables us to provide a flexible and conformal coating of extremely low surface
energy fluorodecyl POSS molecules on any surface, including those that inherently possess
re-entrant texture, such as duck feathers, lotus leaves or commercially available fabrics. The
synergistic effect of roughness, re-entrant topography of the substrate, and the low surface
energy of fluorodecyl POSS molecules, enables the dip-coated texture to support a composite
interface even with very low surface tension liquids. We have also used two design
parameters D* and A* to provide an a priori estimate of both the apparent contact angles as
well as the robustness of the composite Cassie-Baxter interface. The design framework
outlined in this paper enables us to identify and develop suitably textured surfaces which can
have their surface wettability characteristics systematically adjusted. By combining this
understanding with a dip-coating process that provides a conformal and flexible fluorinated
coating, we can reversibly switch the wettability behavior of fabric surfaces between super-
wetting and super-repellent with a wide range of polar and non-polar liquids using simple
mechanical deformation. Biaxial stretching of a dip-coated, commercial polyester fabric can
cause an appreciable increase in both the apparent advancing and receding contact angles as
predicted by changes in the spacing ratio D*. As a result of the strong coupling between the
two design parameters A* and D* for a typical woven fibrous geometry, stretching commercial
fabrics can lead to a dramatic reduction in the robustness of the composite interface that is
supported with a given contacting liquid. Indeed, beyond a critical strain (at which the
robustness A* decreases to values close to unity), the contacting liquid spontaneously
penetrates the fabric texture, leading to near zero contact angles. However, because of the
conformal and flexible characteristics of the fluorodecyl POSS coating, mechanical unloading
and drying of the fabric enables the super-repellency of the sample to be regained repeatedly.
Dip-coating: For the dip-coating process, we first prepare a solution of 50 wt% fluorodecyl
POSS and Tecnoflon® (BR9151) in Asahiklin AK-225 (Asahi glass company) at an overall
solid concentration of 10 mg/ml. Tecnoflon is a commercial fluoro-elastomer manufactured
by Solvay-Solexis. The use of Tecnoflon as a polymeric binder prevents the crystallization of
fluorodecyl POSS, and yields a more conformal and elastomeric coating. Next, the substrate
to be dip-coated is immersed in the fluorodecyl POSS-Tecnoflon solution. After 5 minutes,
the substrate is removed from the solution and placed to dry in a vacuum oven for 30 minutes
at a temperature of 60oC.
Biaxial Stretching: The fabric stretching experiments were carried out using a custom built
biaxial stretcher. Multiple markings were made on the fabric surface, with an inter-marking
separation of 5 mm on the unstretched fabric. Next, as the fabric was stretched, we measured
the separation between the markings to provide an accurate measurement of strain.
Contact angle measurements: The contact angles for various liquids were measured using a
contact angle goniometer, VCA2000 (AST Inc.). The advancing contact angle was measured
by advancing a small volume of the probing liquid (typically 2-4 µl) on to the surface, using a
syringe. The receding contact angle was measured by slowly removing the probing liquid
from a drop already on the surface. For each sample a minimum of four different readings
were recorded. Typical error in measurements was ~ 2°.
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