U.S. patent application number 16/271770 was filed with the patent office on 2019-12-05 for tunable surface.
This patent application is currently assigned to MASSACHUSETTS INSTITUTE OF TECHNOLOGY. The applicant listed for this patent is Government of the United States as represented by the Secretary of the Air Force, MASSACHUSETTS INSTITUTE OF TECHNOLOGY, Government of the United States as represented by the Secretary of the Air Force. Invention is credited to Wonjae Chol, Robert E. Cohen, Joseph Mark Mabry, Gareth H. McKinley, Anish Tuteja.
Application Number | 20190368073 16/271770 |
Document ID | / |
Family ID | 40229373 |
Filed Date | 2019-12-05 |
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United States Patent
Application |
20190368073 |
Kind Code |
A1 |
Tuteja; Anish ; et
al. |
December 5, 2019 |
TUNABLE SURFACE
Abstract
An article can have a surface with selected wetting properties
for various liquids.
Inventors: |
Tuteja; Anish; (Cambridge,
MA) ; Chol; Wonjae; (Cambridge, MA) ;
McKinley; Gareth H.; (Acton, MA) ; Cohen; Robert
E.; (Jamaica Plain, MA) ; Mabry; Joseph Mark;
(Lancaster, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
MASSACHUSETTS INSTITUTE OF TECHNOLOGY
Government of the United States as represented by the Secretary of
the Air Force |
Cambridge
WRIGHT-PATTERSON AIR FORCE |
MA
OH |
US
US |
|
|
Assignee: |
MASSACHUSETTS INSTITUTE OF
TECHNOLOGY
Cambridge
MA
Government of the United States as represented by the Secretary
of the Air Force
WRIGHT-PATTERSON AIR FORCE BASE
OH
|
Family ID: |
40229373 |
Appl. No.: |
16/271770 |
Filed: |
February 8, 2019 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12599465 |
Aug 23, 2010 |
10202711 |
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PCT/US2008/060176 |
Apr 14, 2008 |
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16271770 |
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60917012 |
May 9, 2007 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
Y10T 428/249921
20150401; D06M 23/08 20130101; D01D 5/003 20130101; Y10T 428/31663
20150401; Y10T 428/24355 20150115; D06M 15/263 20130101; D01F 1/10
20130101; Y10T 428/31504 20150401; D06M 2200/05 20130101; Y10T
428/24612 20150115; Y10T 428/24372 20150115 |
International
Class: |
D01D 5/00 20060101
D01D005/00; D01F 1/10 20060101 D01F001/10; D06M 15/263 20060101
D06M015/263; D06M 23/08 20060101 D06M023/08 |
Goverment Interests
FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
[0002] This invention was made with government support under Grant
No. FA9300-06M-T015 awarded by the Air Force Office of Scientific
Research. The government has certain rights in this invention.
Claims
1-42. (canceled)
43. A method of manufacturing a fabric having tunable wettability,
the method comprising: selecting fluorinated nanoparticles to
create a superhydrophobic, a superhydrophobic, a superoleophilic,
or a superoleophobic surface; coating a polymer with the
fluorinated nanoparticles; forming fibers from the mixed polymer
and fluorinated nanoparticles, the fluorinated nanoparticles
forming microstructures on the fibers configured to influence
contact angle hysteresis; assembling a plurality of the formed
fibers into a fabric.
44. The method of claim 43, wherein selecting the fluorinated
nanoparticles further comprises: selecting a concentration of the
fluorinated nanoparticles, wherein the concentration determines
whether the fabric is superhydrophilic and superoleophobic surface
or superhydrophobic and superoleophilic surface.
45. The method of claim 44, wherein the concentration is less than
0.1 mass fraction nanoparticles.
46. The method of claim 44, wherein the concentration is greater
than 0.1 mass fraction nanoparticles.
47. The method of claim 44, wherein the concentration is greater
than 0.15 mass fraction nanoparticles.
48. The method of claim 44, wherein the concentration is greater
than 0.2 mass fraction nanoparticles.
49. The method of claim 44, wherein the concentration is greater
than 0.25 mass fraction nanoparticles.
50. The method of claim 43, wherein the fluorinated nanoparticles
include a fluorinated silsesquioxane.
51. The method of claim 43, wherein forming the fiber includes
electrospinning.
52. The method of claim 43, wherein coating the polymer with the
fluorinated nanoparticles includes chemical vapor deposition, dip
coating, or chemical solution deposition.
53. A method of modifying the wetting properties of a surface, the
method comprising: exposing the surface to a mixture comprising a
plurality of fluorinated nanoparticles and an organic solvent, the
fluorinated nanoparticles of the plurality configured to form
microstructures on the surface the influence contact angle
hysteresis.
54. The method of claim 53, wherein exposing the surface to a
liquid composition includes chemical vapor deposition, dip coating,
or chemical solution deposition.
55. The method of claim 53, wherein the fluorinated nanoparticles
include a fluorinated silsesquioxane.
56. The method of claim 53, wherein a concentration of the
plurality of fluorinated nanoparticles in the mixture is less than
0.1 mass fraction nanoparticles.
57. The method of claim 53, wherein a concentration of the
plurality of fluorinated nanoparticles in the mixture is greater
than 0.1 mass fraction nanoparticles.
58. The method of claim 53, wherein a concentration of the
plurality of fluorinated nanoparticles in the mixture is greater
than 0.15 mass fraction nanoparticles.
59. The method of claim 53, wherein a concentration of the
plurality of fluorinated nanoparticles in the mixture is greater
than 0.2 mass fraction nanoparticles.
60. The method of claim 53, wherein a concentration of the
plurality of fluorinated nanoparticles in the mixture is greater
than 0.25 mass fraction nanoparticles.
61. The method of claim 53, wherein the surface includes a surface
of a fabric.
62. The method of claim 61, further comprising: stretching the
fabric.
Description
CLAIM OF PRIORITY
[0001] This application is a continuation of U.S. application Ser.
No. 12/599,465, filed Aug. 23, 2010, now U.S. Pat. No. 10,202,711,
which claims priority to PCT Application No. PCT/US2008/060176,
filed Apr. 14, 2008, which claims priority to provisional U.S.
Patent Application No. 60/917,012, filed May 9, 2007, titled
"Tunable Surfaces," each of which is incorporated by references in
its entirety.
TECHNICAL FIELD
[0003] This invention relates to surfaces having tunable surface
energy.
BACKGROUND
[0004] Surfaces having a nanotexture can exhibit extreme wetting
properties. A nanotexture refers to surface features, such as
ridges, valleys, or pores, having nanometer (i.e., typically less
than 1 .mu.m) dimensions. In some cases, the features can have an
average or rms dimension on the nanometer scale, even though some
individual features may exceed 1 .mu.m in size. The nanotexture can
be a 3D network of interconnected pores. Depending on the structure
and chemical composition of a surface, the surface can be
hydrophilic, hydrophobic, or at the extremes, superhydrophilic or
superhydrophobic.
SUMMARY
[0005] An article can have a surface with selected wetting
properties for various liquids. The surface can include a
protruding portion configured to protrude toward a liquid and a
re-entrant portion opposite the protruding portion. The re-entrant
surface can have negative curvature relative to the space adjacent
that portion of the surface. The protruding portion and the
re-entrant portion can be surfaces of a fiber or surfaces of
microstructures, for example, micronails or reverse micronails. The
microstructures can include a surface texture selected to influence
contact angle hysteresis.
[0006] In general, an article can include a superoleophobic
surface. The superoleophobic surface can include nanoparticles. A
nanoparticle can have a diameter of less than 100 nm, less than 50
nm, less than 40 nm, less than 30 nm, less than 20 nm, or less than
10 nm. The surface of the nanoparticle can be treated with a
hydrophobic material. For example, the nanoparticles can be
halogenated, perhalogenated, perfluorinated, or fluorinated
nanoparticles, for example, perfluorinated or fluorinated
silsesquioxanes. In certain embodiments, the concentration of
nanoparticles can be less than 0.1 mass fraction nanoparticles,
greater than 0.1 mass fraction nanoparticles, greater than 0.15
mass fraction nanoparticles, greater than 0.2 mass fraction
nanoparticles, or greater than 0.25 mass fraction
nanoparticles.
[0007] In another aspect, a method of manufacturing a fabric having
tunable wettability can include selecting a concentration of
nanoparticles to create a superhydropilic, a superhydrophobic, a
superoleophilic, or a superoleophobic surface, forming a fiber from
a mixture including a polymer and the concentration of
nanoparticle, and assembling a plurality of the fibers to form a
fabric. The step of selecting a concentration of nanoparticles can
include choosing the concentration to create a superhydrophilic and
superoleophobic surface or a superhydrophobic and superoleophilic
surface. The fiber can be formed by electrospinning.
[0008] In another aspect, a method of modifying the wetting
properties of a surface includes introducing a component onto the
surface having a protruding portion configured to protrude toward a
liquid and a re-entrant portion opposite the protruding portion.
The step of introducing the component can include depositing a
fiber including a polymer and a plurality of nanoparticles on the
surface or forming a plurality of microstructures on the surface.
The microstructures can be micronails or can include
nanoparticles.
[0009] In another aspect, a method of modifying the wetting
properties of a surface comprising exposing the surface to a liquid
composition including a plurality of nanoparticles.
[0010] Exposing the surface to a liquid composition can include,
for example, chemical solution deposition, or dip coating. The
surface can include a surface of a fabric. The method can include
stretching the fabric.
[0011] The details of one or more embodiments are set forth in the
accompanying drawings and the description below. Other features,
objects, and advantages will be apparent from the description and
drawings, and from the claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] FIG. 1 is a drawing depicting an object with curvature
having both a protrusion surface and a re-entrant surface.
[0013] FIG. 2 is a graph depicting the variation of advancing and
receding contact angles for water on the spin coated surfaces as a
function of the mass fraction of fluorodecyl polyhedral oligomeric
silsesquioxanes (POSS).
[0014] FIG. 3 includes AFM phase images and rms roughness (denoted
as r) of the films corresponding to FIG. 2.
[0015] FIG. 4 is a graph depicting the advancing and receding
contact angles for water on an electrospun surface. The legends are
the same as in FIG. 2.
[0016] FIG. 5 is a representative SEM micrograph for the
electrospun surfaces from data of FIG. 4.
[0017] FIG. 6 is a graph depicting a generalized non-wetting
diagram showing the contact angle of water on the electrospun
surfaces as a function of its value on the spin coated surfaces.
The graph has been divided 4 quadrants. Previous work has shown
that the transition from the Wenzel to the Cassie state occurs in
the III'rd quadrant (also because r>1>.PHI..sub.s). However,
it is seen here that the transition from the Cassie to the Wenzel
state, for the advancing drop, can be delayed well in to the IV'th
quadrant as a results of the surface curvature of the electrospun
surfaces.
[0018] FIGS. 7A-7E are graphs depicting the advancing and receding
contact angles for hexadecane, dodecane, decane, and octane,
respectively, on the electrospun surfaces, as a function of the
fluorodecyl POSS concentration. It is seen that there is a clear
transition from the Wenzel to the metastable Cassie state for each
alkane. The surfaces in the metastable Cassie state have both
advancing and receding contact angles greater than 90.degree., even
though the spin coated surfaces have are always oleophilic for all
fluorodecyl POSS concentrations.
[0019] FIG. 8 is an image of a hexane drop (dyed with oil red O) on
a 44 weight % fluorodecyl POSS electrospun surface.
[0020] FIG. 9 is a graph depicting the height of liquids required
to transition irreversibly from the metastable Cassie state to the
Wenzel state on the surface of a steel grid coated with fibers
containing 44 wt % fluorodecyl POSS. This transition allows the
liquids to flow through the electrospun mat.
[0021] FIG. 10 is a photograph depicting a steel grid coated with
electrospun fibers containing 9.1 wt % fluorodecyl POSS used for
oil/water separation. As many of the electrospun surfaces are
superhydrophobic and superoleophilic, they are ideal for oil-water
separation. Here, octane is indicated using a star while the water
is indicated using an asterisk. It was seen that octane can pass
through the fibers easily while water beads up and stays on top of
the fibers. Other experiments show that a fiber surface already
wetted with octane also prevents water from passing through it.
[0022] FIGS. 11 and 12 are drawings depicting expected liquid-air
interface on the micronail surface. The protruding and re-entrant
surfaces of the micronails are also shown. The surface curvature of
the re-entrant surfaces allows for the Young's equation to be
satisfied even for 0<90.degree., forming a composite interface
with the liquid suspended on both the micronail surface and air.
This composite interface leads to high contact angles for the
liquid drop on the surface even if 0<90.degree..
[0023] FIGS. 13A and 13B are a set of SEM micrographs depicting two
micronail surfaces having square and circular flat caps,
respectively.
[0024] FIG. 14 is a photograph depicting a droplet of water on top
of SiO.sub.2 micronails. The inter-nail spacing for the surface is
40 .mu.m.
[0025] FIG. 15 is a series of pictures taken for advancing and
receding water droplets on the SiO.sub.2 micronail surface. The
inter-nail spacing for the surface is 10 .mu.m.
[0026] FIG. 16A is a graphical representation of contact angles for
octane on silanized micro-hoodoos as a function of .PHI..sub.s.
[0027] FIG. 16B is a photograph depicting the advancing and
receding contact angles for octane on SiO.sub.2 micronails covered
with a fluorosilane, as a function of .PHI..sub.s. These are the
highest contact angles ever reported for octane on any surface.
[0028] FIGS. 17A-17D are a series of photographs depicting drop of
water (colored with methylene blue) on a lotus leaf surface (FIG.
17A); the surface of the lotus leaf after contact with a drop of
hexadecane (FIG. 17B); drops of water (colored with methylene blue)
on a lotus leaf surface covered with electrospun fibers of PMMA and
44 wt % fluordecyl POSS (FIG. 17C); and drops of hexadecane
(colored with an oil soluble red dye `oil red O`) on a lotus leaf
surface covered with electrospun fibers of PMMA and 44 wt %
fluorodecyl POSS (FIG. 17D).
[0029] FIG. 17E is an SEM micrograph of the lotus leaf surface; the
scale bar is 5 .mu.m.
[0030] FIG. 18A is an SEM image of a honeycomb-like structure of a
superhydrophobic polyelectrolyte multilayer film coated with silica
nanoparticles.
[0031] FIG. 18B is an image of a droplet of water sitting on the
surface of FIG. 18A.
[0032] FIG. 19 is an optical image of a glass slide coated with the
superhydrophobic polyelectrolyte multilayer surface submerged in a
pool of water.
[0033] FIGS. 20A and 20B are an optical image and an optical
micrograph, respectively, showing small water droplets sprayed on a
superhydrophobic surface with an array of hydrophilic domains
patterned using a 1% PAA water/2-propanol solution.
[0034] FIGS. 21A and 21B are schematics illustrating the expected
liquid-vapor interface on two idealized surfaces possessing
different values of .psi.. The gray surface is wetted, while the
surface is non-wetted.
[0035] FIG. 22 is an optical micrograph of silicon micro-post
arrays developed by Cao et al.
[0036] FIG. 23 is a schematic of a surface possessing re-entrant
curvature proposed by Nosonovsky et al.
[0037] FIG. 24 graphically illustrates overall free energy as a
function of the penetration depth (z) for two cases, one where the
surface shown in FIG. 23 is considered to be extremely hydrophobic
(.theta.=150.degree.) and the other when the surface is considered
to be hydrophilic (.theta.=30.degree.).
[0038] FIG. 25 is a graph depicting cos .theta.*.sub.adv(circles)
and cos .theta.*.sub.rec(squares) for water as a function of cos
.theta..sub.adv and cos .theta..sub.rec. The inset shows a scanning
electron microscope (SEM) micrograph for an electrospun surface
composed of PMMA and 9.1 wt % fluorodecyl POSS (reproduced with
permission from Tuteja et al.).
[0039] FIG. 26A is a schematic of the electrospun fibers,
illustrating its important surface characteristics.
[0040] FIG. 26B is a schematic illustrating the important surface
characteristics of the micro-nail surface.
[0041] FIG. 27A is graph showing the change in the Gibbs free
energy density, as a function of apparent contact angle and the
penetration depth (z), for water propagating on a hydrophobic
surface (.theta.=120.degree.) with sinusoidal wrinkles.
[0042] FIG. 27B is a graph showing the change in the Gibbs free
energy density, for hexadecane (.theta.=80.degree.) propagating on
a surface with sinusoidal wrinkles.
[0043] FIG. 27C is a graph showing the change in the Gibbs free
energy density, as a function of apparent contact angle and the
penetration depth (z), for water (.theta.=120.degree.) propagating
on the electrospun PMMA and 44.1 wt % fluorodecyl POSS surface.
[0044] FIG. 27D is a graph showing the change in the Gibbs free
energy density, as a function of apparent contact angle and the
penetration depth (z), for hexadecane (.theta.=80.degree.)
propagating on the electrospun PMMA and 44.1 wt % fluorodecyl POSS
surface. The inset on the graph shows a zoomed in view around
z.about.0.6 to illustrate the local energy density minimization for
the metastable composite interface.
[0045] FIG. 28 is a plot of the robustness parameter (H*) as a
function of the spacing ratio (D*) for octane (.gamma..sub.lv=21.6
mN/m) on various natural and artificial surfaces discussed in the
literature.
[0046] FIG. 29 is a schematic illustration of dip-coating process
of a commercial polyester fabric with insert images of the fabric
before and after the process.
[0047] FIG. 30A is an image of a droplet of hexadecane on an
uncoated duck feather.
[0048] FIG. 30B is an image of a droplet of hexadecane on the same
feather after the feather was dip-coated with a solution of
TECNOFLON and fluorodecyl POSS.
[0049] FIG. 30C is an image of a droplet of hexadecane on an
uncoated, commercially available polyester fabric.
[0050] FIG. 31A is an SEM micrograph of the uncoated polyester
fabric.
[0051] FIG. 31B is an SEM micrograph of the same polyester fabric
after dip-coating with a solution of fluorodecyl POSS.
[0052] FIG. 31C is an SEM micrograph of the same polyester fabric
after dip-coating with a solution of TECNOFLON and fluorodecyl
POSS.
[0053] FIG. 32 is an image of water (.gamma..sub.lv=72.1 mN/m),
methylene iodide (.gamma..sub.lv=50.8 mN/m), hexadecane=27.5 mN/m)
and methanol (.gamma..sub.lv=22.7 mN/m) on the polyester fabric's
surface, after dip-coating with a solution of TECNOFLON and
fluorodecyl POSS.
[0054] FIG. 33 is a photograph of methanol, methylene iodide,
hexadecane, and water on a polyester fabric surface after the
fabric was dip-coated with a solution of TECNOFLON and fluorodecyl
POSS.
[0055] FIGS. 34A and 34B are a series of photographs illustrating
the sequential results of stretching the fabric of FIG. 33.
[0056] FIGS. 35A-35C are schematics illustrating the key
geometrical parameters for fibers and the micro-nail surfaces.
[0057] FIGS. 36A-37B are electron micrographs showing various
design aimed at controlling the contact angle hysteresis.
[0058] FIG. 38 is a graph depicting a Zisman plot for various spin
coated PMMA and fluoroPOSS films.
DETAILED DESCRIPTION
[0059] Surface geometry can create superoleophobic surfaces. It is
believed that any superoleophobic surface has to make use of a
geometry in which the surface has a protrusion portion and a
re-entrant portion. Referring to FIG. 1, an article 10 can have a
protrusion surface and a re-entrant surface. The article can
include a core 15 and a coating 20. The core 15, the coating 20, or
both, can include a plurality of nanoparticles which can further
modify the properties of the surface.
[0060] In addition, fabrics with tunable wettability, produced in a
single step by electrospinning two components, a polymer and a
fluorinated nanoparticle. The process can be used to create
superhydrophilic, superhydrophobic, superoleophilic or
superoleophobic surfaces (i.e., surfaces having a contact angle
greater than 150.degree. with alkanes such as hexadecane, decane
and octane) by only changing the concentration of the
nanoparticles. In general, higher the nanoparticle concentration,
the lower the surface energy. This flexibility can allow surfaces
having multiple desirable properties to be produced, for example, a
surface that is both superhydrophobic and superoleophilic. Such a
surface has been produced and is an excellent oil-water
separator.
[0061] The produced fabrics can also be used as coatings on a wide
range of rigid substrates such as metals, ceramics or bricks and
glass, as well as, flexible substrates like paper and plastic. The
fabric can be formed on directly the surface of the substrate or
formed on a transfer medium and subsequently transferred to the
surface of the substrate. The surface energy of the coating can be
controlled to provide resistance or repellency to all liquids
including water and alkanes or to specifically repel only a few
liquids like water or alcohols.
[0062] The methods and surfaces described here can have certain
advantages and improvements over other methods of surface
modification. For example, super-oleophobic surfaces, i.e.,
surfaces which are resistant to even the lowest surface tension
liquids like decane and octane, can be produced. A re-entrant
surface curvature can be an essential feature for creating a
superoleophobic surface. It is likely that any super-oleophobic
surface produced by any method will have to make use of this
geometry.
[0063] Fabrics with tunable wettability can be produced in a single
step by electrospinning. The wettability of the fabric is easily
controlled by changing the concentration of the nanoparticles. This
flexibility allows for the production of surfaces having multiple
desirable properties, for example a surface that is both
superhydrophobic and superoleophilic.
[0064] There are a number of different commercial applications for
the various types of surfaces produced in this work. The surfaces
can be a portion of any article, including a vehicle, equipment, a
tool, construction material, a window, a flow reactor, a textile,
or others. A few applications for each surface include the
following.
[0065] Superhydrophobic surfaces can be used to produce articles
having anti-icing and/or anti-fogging properties, which can make
them an ideal coating for airborne and ground-borne vehicle
applications. Also, the superhydrophobic surfaces can be
self-cleaning, i.e., water droplets simply roll of them, dissolving
and removing any dust or debris present on the surface. Hence, they
would be ideal as coating on windows, traffic lights etc. Other
applications include prevention of adhesion of snow to antennas,
the reduction of frictional drag on ship hulls, anti-fouling
applications, stain-resistant textiles, minimization of
contamination in biotechnological applications and lowering the
resistance to flow in microfluidic devices.
[0066] Superhydrophobic and superoleophilic surfaces can be ideal
for oil-water separation, which has a number of useful
applications, including waste water treatment and cleaning up oil
spills. Other applications include cleaning of ground water, oil
well extractions, biodiesel processing, mining operations and food
processing.
[0067] Superoleophobic surfaces can be resistant to dust, debris
and fingerprints. This would make them ideal as coating on lenses,
computer screens, tablet computers, personal data assistants and
other handheld devices. Superoleophobic surfaces can also be used
as anti-graffiti self-cleaning surfaces. Superoleophobic surfaces
can also be of great use in the petroleum industry. For example,
various surfaces that are attacked by the petroleum products could
be lined with these superoleophobic coatings, preventing their
degradation, for example, providing swell resistance to organic
materials on fabrics. Also, superoleophobic linings can be used as
a drag reducer in various pipelines.
[0068] A number of surfaces in nature use extreme water repellency
for specific purposes; be it water striding or self-cleaning. A
number of surfaces encountered in nature are superhydrophobic,
displaying water (surface tension .gamma.=72.1 mN/m) contact angles
(WCA) greater than 150.degree., and low contact angle hysteresis.
The most widely-known example of a superhydrophobic surface found
in nature is the surface of the lotus leaf. It is textured with
small 10 .mu.m to 20 .mu.m sized protruding nubs which are further
covered with nanometer size epicuticular wax crystalloids. See, for
example, W. BARTHLOTT et al., "Purity of the sacred lotus, or
escape from contamination in biological surfaces," Planta, Vol. 202
(1997) 1-8. Numerous studies have shown that it is this combination
of surface chemistry plus roughness on multiple scales--micron and
nanoscale that imbues super hydrophobic character to the lotus leaf
surface. The effects of surface chemistry and surface texture can
be controlled to create high levels of oil-repellency and
superoleophobic behavior.
[0069] Two distinct models, developed by Cassie and Wenzel, are
commonly used to explain the effect of roughness on the apparent
contact angle of a drop sitting on a surface. See, for example, A.
B. D. CASSIE et al., "Wettability of porous surfaces," Trans.
Faraday Soc., Vol. 40 (1944) 546-551; and R. N. WENZEL, "Resistance
of solid surfaces to wetting by water," Ind. Eng. Chem., Vol. 28
(1936) 988-994. The Wenzel model recognizes that surface roughness
increases the available surface area of the solid, which
geometrically increases the contact angle for the surface according
to:
cos .theta.*=r cos .theta. (1)
here .theta.* is the apparent contact angle, r is the surface
roughness, and .theta. is the equilibrium contact angle on a smooth
surface of the same material. The Cassie model, on the other hand,
proposes that the superhydrophobic nature of a rough surface is
caused by air remaining trapped below the water droplet. This
results in a composite interface with the drop sitting partially on
air. Thus, the contact angle is an average between the value of the
fluid-air contact angle (i.e., 180.degree.) and .theta.. If
.PHI..sub.s is the fraction of the solid in contact with water, the
Cassie equation yields:
cos .theta.*=-1+.PHI..sub.s(1+cos .theta.) (2)
[0070] Thermodynamic arguments can be used to determine whether a
rough hydrophobic surface will stay in the Wenzel or the Cassie
state. See, for example, A. MARMUR, "Wetting on Hydrophobic Rough
Surfaces: To Be Heterogeneous or Not To Be?" Langmuir, Vol. 19
(2003) 8343-8348; and M. NOSONOVSKY, "Multiscale Roughness and
Stability of Superhydrophobic Biomimetic Interfaces," Langmuir,
Vol. 23 (2007) 3157-3161. Previous work has shown that if a series
of substrates with progressively increasing equilibrium contact
angles is considered, a transition from the Wenzel to the Cassie
state should ultimately be observed on the corresponding rough
surfaces. See, for example, A. LAFUMA et al., "Superhydrophobic
states," Nat. Mater., Vol. 2 (2003) 457-60. The threshold value of
the critical equilibrium contact angle (.theta..sub.c) for this
transition can be obtained by equating eqns. 1 and 2:
cos .theta..sub.c=(.PHI..sub.s-1)/(r-.PHI..sub.s) (3)
[0071] Because r>1>.PHI..sub.s, the critical angle,
.theta..sub.c, is necessarily greater than 90.degree., and thus
.theta.>90.degree. is required to create superhydrophobic
surfaces. This is readily achievable using siloxanes or fluorinated
surfaces and a wide variety of superhydrophobic surfaces have now
been created. However, these arguments also explain why researchers
so far have not been successful in making superoleophobic surfaces,
i.e., surfaces with contact angles about 150.degree. for mobile
alkane oils such as decane (.gamma.=23.8 mN/m) or octane
(.gamma.=21.6 mN/m). For a smooth surface to have an equilibrium
contact angle about 90.degree. with a liquid alkane, the surface
would need to have a surface energy about 5 mN/m. See, for example,
K. TSUJII et al., "Super oil-repellent surfaces," Angewandte
Chemie-International Edition in English, Vol. 36 (1997) 1011-1012.
Zisman et al. reported that the surface free energy decreased in
the order
--CH.sub.2>--CH.sub.3>--CF.sub.2>--CF.sub.2H>--CF.sub.3,
and the lowest solid surface energies reported to date are in the
range of approximately 6 mN/m (for a hexagonally closed pack
arrangement of --CF.sub.3 groups on a surface). See, for example,
W. A. ZISMAN, "Relation of the equilibrium contact angle to liquid
and solid construction," In: Contact Angle, Wettability and
Adhesion, ACS Advances in Chemistry Series. (ed. Fowkes, F. M.)
(American Chemical Society, Washington, D C., 1964); and T. NISHINO
et al., "The lowest surface free energy based on --CF.sub.3
alignment, Langmuir, Vol. 15 (1999) 4321-4323.
[0072] Surface curvature can be used as a third factor, apart from
surface energy and roughness, to modify surface wettability. The
surface curvature (apart from surface chemistry and roughness), can
be used to significantly enhance liquid repellency, as exemplified
by studying electrospun polymer fibers containing very low surface
energy perfluorinated nanoparticles (FluoroPOSS). Increasing the
POSS concentration in the electrospun fibers can systematically
transcend from superhydrophilic to superhydrophobic and to the
superoleophobic surfaces (exhibiting low hysteresis and contact
angles with decane and octane greater than 150.degree.).
[0073] A surface has a re-entrant portion surface (or negative
curvature) as shown in FIG. 1, which enhances the
resistance/contact angle with any liquid. The curved surface, for
example, the cross section of a sphere or a fiber, always provides
a point along its length such that Young's equation
cos .theta.=(.gamma..sub.sv-.gamma..sub.sl)/.gamma..sub.lv
where .gamma. refers to the interfacial tension and s, l, and v
refer to the solid, liquid and vapor phases, respectively, is
satisfied at the air-liquid-solid interface (contact
angle=equilibrium contact angle) even if 0<90.degree.. See, for
example, M. OWEN et al., "Surface active fluorosilicone polymers,"
Macromol. Symp., Vol. 82 1994) 115-123; A. MARMUR, supra; and M.
NOSONOVSKY, supra. Thus, the re-entrant surface leads to the drop
sitting partially on air with high overall contact angles (Cassie
state). This Cassie state is however metastable as the total energy
of the system decreases significantly when the liquid advances and
completely wets the surface leading to a homogeneous interface.
See, for example, M. NOSONOVSKY, supra. It should be mentioned that
the lower the value of .theta., the more the liquid wets the curved
surface, leading to higher contact angle hysteresis, even with the
composite interface. Thus, a surface in the Cassie state does not
necessarily have low hysteresis, as is widely believed. Surfaces
without curvature or having only a protruding surface cannot lead
to a composite interface if .theta.<90.degree., as the Young's
equation is not satisfied at any point, other than for complete
wetting.
[0074] Consider the schematics shown in FIGS. 21A and 21B, which
depict the expected solid-liquid-vapor profile for a liquid with
.theta. .about.70.degree. on two different surfaces. If
.theta.<.psi., as in FIG. 7A, the net traction on the
liquid-vapor interface is downwards, thereby facilitating the
imbibition of the liquid into the solid structure, leading to a
fully-wetted interface. On the other hand, if .theta.>.psi., as
shown in FIG. 21B, the net force is directed upwards, thereby
supporting the formation of a composite interface. See, for
example, L. CAO et al., "Design and fabrication of microtextures
for inducing a superhydrophobic behavior on hydrophilic materials,"
Langmuir, Vol. 23 (2007) 4310-4314, which is incorporated by
reference in its entirety. In other words, either of these surfaces
can support the formation of a composite interface provided
.theta.>.psi., (see, e.g., A. Tuteja et al., "Designing
superoleophobic surfaces," Science, Vol. 318 (2007) 1618-1622; M.
NOSONOVSKY, supra; and C. W. EXTRAND, "Model for contact angles and
hysteresis on rough and ultraphobic surfaces," Langmuir, Vol. 18
(2002) 7991-7999; each of which is incorporated by reference in its
entirety) while any liquid for which .theta.<.psi. will
immediately yield a fully-wetted interface.
[0075] The presence of re-entrant texture (or
.omega.<90.degree.) in the surface illustrated in FIG. 21B
allows for the formation of a composite interface and thus
extremely high apparent contact angles even if
.theta.<90.degree.. Silicon micro-post arrays possessing
re-entrant texture (See, e.g., FIGS. 13A, 13B, and 21B) display
superhydrophobicity, even though the equilibrium contact angle for
water on the silicon surface was .theta.=74.degree..
[0076] Nosonovsky analyzed the stability of composite interfaces on
a range of surfaces having different roughness profiles and
suggested that the creation of a stable composite interface on any
rough surface requires a local minimum in the overall free energy
diagram and dA.sub.sld.theta.<0. See M. NOSONOVSKY, supra, which
is incorporated by reference in its entirety. Here dA.sub.sl is the
change in solid-liquid contact area with the advancing or receding
of the liquid, accompanied by a change in the local contact angle,
d.theta.. Based on this criterion, Nosonovsky proposed a
liquid-repellent structure of rectangular pillars, covered with
semi-circular ridges and grooves as shown in FIG. 23. Because of
the presence of re-entrant curvature at various local regions on
this structure (where 0.degree.<y<90.degree.), this surface
provides the possibility of obtaining a composite interface with
any liquid for which q>0.degree. (see, e.g., A. TUTEJA, supra,
which is incorporated by reference in its entirety). FIG. 24 shows
the computed free energy as a function of the penetration depth of
the liquid-vapor interface (z), for a hydrophilic (q=30.degree.)
and a hydrophobic (q=150.degree.) surface having the same texture
as shown in FIG. 23. It is possible to form a composite interface
(around z.about.1.5) on the hydrophilic surface (leading to
extremely high apparent contact angles), even though the
equilibrium contact angle for this surface is only 30.degree..
However, this composite interface configuration is not the true
equilibrium state as the fully wetted interface (around z.about.4)
leads to a lower overall free energy. However, it is clear that the
correct choice of surface texture can lead to the formation of
metastable (energetically trapped) composite interfaces, and
extremely high contact angles, even though the solid surface by
itself may be hydrophilic. See, for example, S. HERMINGHAUS,
"Roughness-induced non-wetting," Europhys. Lett., Vol. 52 (2000)
165-170; A. TUTEJA, supra; A. MARMUR, supra; N. A., PATANKAR, "On
the modeling of hydrophobic contact angles on rough surfaces,"
Langmuir, Vol. 19 (2003) 1249-1253; and B. H E et al., "Multiple
equilibrium droplet shapes and design criterion for rough
hydrophobic surfaces," Langmuir 2003, 19, (12), 4999-5003; each of
which is incorporated by reference in its entirety. Thus,
superoleophobic surfaces can be prepared even when limited to
materials exhibiting q<90.degree. with various low surface
energy alkanes.
[0077] Based on the above considerations, oleophobic surfaces were
prepared electrospinning polymer-nanoparticle composite fibers. The
fibers possess the re-entrant surface by virtue of their curvature,
and hence have enhanced resistance to wetting by liquids. The
details for the materials and the process used are as follows.
[0078] Nanoparticles can include inorganic nanoparticles. One or
more of the nanoparticle can be modified to have a hydrophobic
surface. The nanoparticles can be halogenated, perhalogenated,
perfluorinated, or fluorinated nanoparticles, for example,
perfluorinated or fluorinated silsesquioxanes. The halogenated,
perhalogenated, perfluorinated, or fluorinated nanoparticles can be
surface modified with organic moieties having between 1 and 20
carbon atoms, in particular, C2-C18 alkyl chains, which can be
substituted or unsubstituted. The nanoparticles can have an average
diameter of less than 50 nm, less than 40 nm, less than 30 nm, less
than 20 nm, between 1 nm and 10 nm, or between 1 nm and 5 nm,
inclusive. The nanoparticles can have a surface area to volume
ratio of greater than 1 nm.sup.-1, greater than 2 nm.sup.-1, or
greater than 3 nm.sup.-1.
[0079] A new class of hydrophobic fluorinated polyhedral oligomeric
silsesquioxanes (POSS) molecules has been developed in which the
rigid silsesquioxane cage is surrounded by fluoro-alkyl groups
(details for the synthesis are provided as supplementary
information). A number of different molecules with different
organic groups including 1H,1H,2H,2H-heptadecafluorodecyl (referred
to as fluorodecyl POSS) and 1H,1H,2H,2H-tridecafluorooctyl
(fluorooctyl POSS) have now been synthesized, and this class of
materials is denoted generically as fluoroPOSS. The fluoroPOSS
molecules contain a very high surface concentration of fluorine
containing groups, including --CF.sub.2 and --CF.sub.3 moieties.
The high surface concentration and surface mobility of these
groups, as well as the relatively high ratio of --CF.sub.3 groups
with respect to the --CF.sub.2 groups results in one of the most
hydrophobic and lowest surface energy materials available today.
See, for example, M. J. OWEN, supra. A spin coated film of
fluorodecyl POSS on a Si wafer has an advancing and receding
contact angle of 124.5.+-.1.2.degree., with an rms roughness of 3.5
nm. Blends of a moderately hydrophilic polymer, poly(methyl
methacrylate) (PMMA, M.sub.w=540 kDa, PDI.about.2.2) and
fluorodecylPOSS can be used in various weight ratios to create
materials with different surface properties. Other polymers can be
used in place of or in combination with other polymers. By varying
the mass fraction of fluoroPOSS blended with various polymers, the
surface energy of the polymer-fluoroPOSS blend can be
systematically changed. This ability can afford control over the
equilibrium contact angle of the blends and provide a mechanism for
systematically studying the transition from the Wenzel to the
Cassie state on rough surfaces made from the blends.
[0080] FIG. 2 shows the advancing and receding contact angle values
of a spin coated blend of PMMA and fluorodecylPOSS on a Si wafer
(the rms roughness of the various films is also mentioned in FIG.
3). It can be seen that the addition of fluorodecyl POSS
systematically changes the receding contact angle of the surfaces
from 69.degree.-123.degree.. The inset on the figure shows the
shapes of water droplets on the surfaces with varying concentration
of fluorodecylPOSS as well as the AFM phase images of the surfaces.
Comparing the phase images of pure PMMA and 1.9 wt %
fluorodecylPOSS suggests a large amount of surface migration of the
POSS particles, as can be expected from the low surface energy
material. This surface migration causes significant enhancements in
the contact angle of the blend at very low mass fraction of
POSS.
[0081] Smooth surfaces (maximum rms roughness of about 4.4 nm;
maximum advancing water contact angle=123.degree.) can be created
by spin coating. The corresponding rough surfaces for the system
can be created by electrospinning (see, for example, M. L. M A et
al., "Electrospun poly(styrene-block-dimethylsiloxane) block
copolymer fibers exhibiting superhydrophobicity," Langmuir, Vol. 21
(2005) 5549-5554) solutions of fluorodecyl POSS and PMMA from
Asahiklin-AK225 (Asahi Glass Co.) solvent. The density of fibers
can be modified, selected, or otherwise adjusted to allow fluid to
contact one or more fibers at one time depending on the sag of the
bottom of a drop of fluid. FIG. 4 shows the contact angle variation
as a function of mass fraction of POSS for an electrospun mat of
the same PMMA-fluorodecyl POSS blend at the same mass fractions as
FIG. 2. The inset on the figure shows a typical scanning electron
microscope (SEM) micrograph for the various systems. There is no
observable change in the micron scale structure with increasing
mass fraction of POSS as observed using the SEM. It can be seen
that the process of electrospinning has provided enough roughness
(and porosity) to the surface to turn it superhydrophobic for all
POSS concentrations above about 10 wt %. The graph also shows the
maximum contact angle for the PMMA-POSS blend on a flat surface
(123.degree.). An interesting observation can be made for the
advancing contact angles of the pure PMMA and 1.9 wt % POSS
electrospun surfaces. It is seen that the advancing contact angles
for both these cases are greater than 90.degree., even though the
advancing contact angles on a flat surface (spin coated) are less
than 90.degree.. It is thus possible to generate very hydrophobic
rough surfaces, with high advancing contact angles, even though
their corresponding smooth surfaces are hydrophilic.
[0082] A number of different researchers have seen similar effects
with unusual hydrophobicity or oleophobicity obtained from rough
materials whose corresponding smooth surfaces are hydrophilic or
oleophilic, and have so far been unable to explain these unexpected
results (the surfaces should be in the Wenzel state leading to
contact angles less than .theta.). See, for example, K. TSUJII,
supra; S. SHIBUICHI et al., "Super water- and oil-repellent
surfaces resulting from fractal structure," J. Colloid Interface
Sci., Vol. 208 (1998) 287-294; W. CHEN et al., "Ultrahydrophobic
and Ultralyophobic Surfaces: Some Comments and Examples," Langmuir,
Vol. 15 (1999) 3395-3399; and Z. MEIFANG et al., "Superhydrophobic
surface directly created by electrospinning based on hydrophilic
material," J. Mater. Sci., Vol. 41 (2006) 3793. This unusual effect
is further explored in FIG. 6 which shows a plot of the apparent
contact angle (.theta..sub.apparent) on the rough electrospun
surface as .theta. for the corresponding smooth (spin coated)
surface is varied by changing the blend composition. It can be seen
that the transition from the Cassie to the Wenzel state for these
systems does not occur as the contact angle is progressively
reduced to 90.degree.. It is thus possible to generate very
hydrophobic rough surfaces, with high advancing contact angles,
even though their corresponding smooth surfaces are hydrophilic.
However, these textured surfaces exhibit high contact angle
hysteresis (the receding contact angles are much lower than
.theta., indicative of being in the Wenzel state). Liquid droplets
deposited on the fiber surfaces are trapped in a nonwetting state,
as they advance, due to the severe surface curvatures of the
electrospun fibers (with diameters 100 nm to 500 nm). For low POSS
concentrations (less than 2 wt %) the re-entrant surfaces (see FIG.
9) of the fibers results in high advancing contact angles,
indicative of being in the Cassie state, however, separate
experiments show that this Cassie state is metastable, as water
droplets dropped from a certain height can wet the surface. It can
also be seen here (as in FIG. 4) that the electrospun surfaces
transition become truly superhydrophobic
(.theta..sub.apparent>150.degree.) for all POSS concentrations
above 10 wt %. For example, the transition energy between the
Cassie and Wenzel states can increase with the concentration of
POSS and the electrospun fiber mat becomes truly superhydrophobic
(with advancing and receding contact angles of 161.+-.2.degree.) at
POSS concentrations above 10 wt %. The inset in the figure shows a
superhydrophobic electrospun surface submerged in water. The
submerged superhydrophobic surface acts like a mirror (due to the
total internal reflection of light caused by the presence of a
layer of air in between the superhydrophobic surface and water)
displaying a reflection of the object placed in front of it. The
surface remains superhydrophobic with a stable mirror even after
being submerged in water for over a week.
[0083] This effect is further explored in the form of a general
wetting diagram, FIGS. 6 and 21A, in which the apparent advancing
and receding contact angles for water on the rough electrospun
surfaces for various PMMA-fluoroPOSS blend concentrations are
plotted as a function of the corresponding advancing and receding
contact angles on smooth (spin-coated) surfaces. By increasing the
mass fraction of the fluoroPOSS molecules blended with PMMA, it is
possible to systematically lower .gamma..sub.sv for the
polymer-fluoroPOSS blend, thereby allowing us to access this entire
parameter space with a single liquid (water). It can be seen from
the figure that a few data points lie in the lower right quadrant
(IV) of this diagram. These surfaces correspond to hydrophilic
substrates that are rendered hydrophobic, purely by re-entrant
topography.
[0084] The electrospinning process is described in more detail
here. PMMA was purchased from Scientific Polymer Products, Inc.,
while the fluorodecyl POSS nanoparticles were obtained. See, for
example, J. M. MABRY et al., "Hydrophobic Silsesquioxane
Nanoparticles and Nanocomposite Surfaces," IN: ACS Symposium
Series, The Science and Technology of Silicones and
Silicone-Modified Materials, Eds: S. J. Clarson et al. (2006)
290-300. Both the polymer and the nanoparticle were dissolved in a
common solvent, Asahiklin AK-225 (Asahi glass co.) in this case, at
a concentration of about 5 wt %. The solution was then electrospun
using a custom-built apparatus as described previously (see, for
example, S. SHIBUICHI, supra) with the flow rate, plate-to-plate
distance and voltage set to 0.05 mL/min, 25 cm, and 20 kV,
respectively.
[0085] The re-entrant surfaces of the electrospun fibers can also
be used to make extremely oleophobic surfaces (in the metastable
Cassie state), (i.e., these electrospun surfaces are also strongly
oleophobic (with advancing contact angles about 140.degree. and
receding contact angles greater than 100.degree. for Octane)), even
though all of the corresponding spin coated surfaces are
oleophilic, at all POSS concentrations. FIGS. 7A-7D shows the
advancing and receding contact angles for the electrospun surfaces
for a series of alkanes (Hexadecane, Dodecane, Decane and Octane).
The maximum contact angles on the spin coated surfaces for each of
the alkanes is also shown. It can be seen that in many cases both
the advancing and receding contact angles for the electrospun
surfaces are much greater than 90.degree.. A transition from the
Wenzel to the metastable Cassie state, with increasing POSS
concentration, can also be observed for each alkane. This
transition systematically shifts to a higher POSS concentration
(lower surface energy) with the decreasing surface tension of the
liquid, suggesting that the strength of the metastability is
inversely proportional to both the substrate surface energy and the
liquid surface tension.
[0086] An interesting application for the electrospun materials can
be derived by studying the data in FIGS. 4 and 7A-7E and noticing
that many of the electrospun surfaces are superhydrophobic and
superoleophilic (alkane contact angle of about 0.degree.). Thus,
these surfaces are ideal for separating mixtures/dispersion of
alkanes and water. FIG. 10 shows a steel wire mesh coated with
fibers containing 9.1 wt % POSS, which acts as a membrane for
oil-water separation. Octane droplets (indicated using a star) are
easily able to pass through the membrane while water droplets
(indicated using an asterisk) bead up on the surface.
[0087] The metastability strength for the electrospun fiber
surfaces is directly measured by electrospinning the PMMA and POSS
fibers directly on to a steel wire mesh (with pore size of 1
mm.sup.2) and measuring the height of liquid required to
`breakthrough` the metastable Cassie surface of the fibers. This
breakthrough height is shown in FIG. 9 for fibers containing 44 wt
% POSS. It can be seen that these fibers are extremely stable and
do not transition to the Wenzel state even when submerged under 110
mm of Hexadecane. Notably, apart from Octane, all of the other
liquids started leaking from the edges of the container used to
suspend the liquids at the heights specified in FIG. 9 (pressing
the container edges on the surface of the fibers damages them),
while the rest of the fiber surface remained
oleophobic/hydrophobic. Hence, the true breakthrough heights are
expected to be much greater than those mentioned here.
[0088] Herminghaus first pointed out that many leaves in nature
display superhydrophobic properties, even though their flat contact
angles are less than 90.degree., recognizing this unusual effect to
be a direct result of the re-entrant surfaces (he refers to them as
surfaces with overhangs, like the micronail structure described
below). See, for example, S. HERMINGHAUS, supra. Herminghaus also
contended that the superhydrophobic state of the leaves was not the
true equilibrium state (which should be the Wenzel state), and a
transition from this `metastable` state to the true equilibrium
state could be made by submerging the leaf in water to a certain
depth. Based on the re-entrant geometry, as well as the
metastability of the re-entrant electrospun fibers, SiO.sub.2
micronails, i.e., pillars with large flat caps (FIGS. 11 and 12),
were fabricated using lithographic chemical etching (details of the
micronail synthesis are provided in the methods section). A number
of different micronail surfaces with inter-nail spacing varying
between 10 .mu.m-40 .mu.m were fabricated, in order to vary the
fractional surface coverage .PHI..sub.s. The micronail height and
cap width were held fixed at 7 .mu.m and 20 .mu.m, respectively,
while the cap thickness was kept at about 300 nm. SEM micrographs
of two model micronail surfaces are shown in FIGS. 13A and 13B.
[0089] As an alternative to micronails, the microstructure can be a
reverse micronail, in which the base is broader than the top, and
the top has a re-entrant portion on the surface. The
microstructures can be spaced periodically, for example, in square
or hexagonal patterns. The spacing between microstructures and
height can be selected to avoid liquid contact with the substrate
upon with the microstructures are built. In certain circumstances,
the re-entrant portion of the surface has negative curvature
relative to the space between microstructures. In an alternative
method of forming the microstructures, a material can be used as a
template or porophore to create microstructures on a surface of a
substrate. The microstructures can be patterned in a periodic or
aperiodic manner.
[0090] FIGS. 11 and 12 show representations of the liquid-air
interface on the micronail surface (the thickness/width ratio for
the pillar caps is exaggerated). As the distance between the nails
is small in comparison to the capillary length, the effect of
gravity is negligible and assuming the liquid-air interface to be a
horizontal plane, as shown in the figure. The curved surface of the
micronails always provides a point along its length such that the
Young's equation (see, for example, T. YOUNG, "III. An essay on the
cohesion of fluids," Philos. Trans. R. Soc. London, Vol. 95 (1805)
65, is satisfied at the air-liquid-solid interface (see, for
example, A. MARMUR, supra and M. NOSONOVSKY, supra) (contact
angle=equilibrium contact angle) even if .theta.<90.degree..
Thus, the re-entrant surface leads to the drop sitting partially on
air with high overall contact angles (Cassie state). This Cassie
state is however metastable as the total energy of the system
decreases significantly when the liquid advances and completely
wets the pillars and fills the space between them, leading to a
homogeneous interface. See, for example, M. NOSONOVSKY, supra. It
should be mentioned that the lower the value of 0, the more the
liquid wets the pillar surface, leading to higher contact angle
hysteresis, even with the composite interface. Thus, a surface in
the Cassie state does not necessarily have low hysteresis, as is
widely believed. Pillars without curvature or with a protruding
surface cannot lead to a composite interface if
.theta.<90.degree., as the Young's equation is not satisfied at
any point other than at the bottom of the pillars (complete
wetting).
[0091] To demonstrate the importance of re-entrant curvatures in
the electrospun fiber mats, model SiO.sub.2 micropillars with large
flat caps were also fabricated using lithographic chemical etching.
A number of different pillar surfaces with inter-pillar spacing
varying between 10 .mu.m-40 .mu.m were fabricated, in order to vary
the fractional surface coverage, .PHI..sub.s. The pillar height and
cap width were held fixed at 7 .mu.m and 20 .mu.m,
respectively.
[0092] As the SiO.sub.2 nails were fabricated on flat Si wafers
(covered with a layer of SiO.sub.2), the contact angles can be
measured for the rough (with nails) and smooth (without nails)
surfaces on the same wafer. FIG. 14 shows that the advancing
contact angle for water on the SiO.sub.2 nails is about 143.degree.
(the inter-nail spacing is 40 .mu.m and the receding contact angle
on the surface is 134.degree.), in comparison the water contact
angle on the smooth SiO.sub.2 surface, on the same wafer, is about
10.degree.. The strength of the metastable Cassie state on the
SiO.sub.2 micronail surface is illustrated in FIG. 15, which shows
a series of pictures for a water droplet advancing and receding
from the pillar surface (the pillars have square caps, the
inter-pillar spacing is 10 .mu.m). It can be seen that the surface
resists both the advancing and receding of the water droplet.
Surfaces with higher inter-pillar spacing are not as stable.
[0093] Next, the capped SiO.sub.2 pillars were treated with vapor
phase tridecafluoro-1,1,2,2-tetrahydrooctyl-1-trichlorosilane, to
lower the substrate surface energy chemically. FIG. 16A shows the
advancing and receding contact angles for octane on the silanized
pillar surfaces as a function of .PHI..sub.s (the shape of the
pillar caps, square or circular, had no effect on the contact angle
and .PHI..sub.s was found to be the only important parameter). FIG.
16B shows a drop of octane on a silanized micropillar surface
(advancing contact angle about 163.degree., receding contact angle
about 145.degree.). These contact angles are the highest ever
reported for octane on any surface. Corresponding measurements of
the equilibrium contact angle for octane on a smooth SiO.sub.2
surface covered with the same silane coating give
.theta..about.55.degree.. Additional measurements show that octane
droplets on these model pillar surfaces exist in a metastable
state.
[0094] It can also be seen from the figure that the receding
contact angles for the surfaces decrease with increasing
.PHI..sub.s. This is due to the additional resistance offered to
the receding liquid, which is expected to be proportional to the
total number of pillars on the air-liquid-solid contact line, as
explained above. However, decreasing .PHI..sub.s also decreases the
breakthrough height (metastability strength). Thus, there is an
inverse relationship between contact angle hysteresis and the
stability of the composite interface which needs to be considered
while designing any superoleophobic surface. Electrospun fiber mats
can contain as little as 2 wt % POSS are strongly hydrophobic, even
though spin coated surfaces with the same fluorodecylPOSS/PMMA
composition remain hydrophilic. At higher concentrations of the
fluoroPOSS it is also possible to create highly oleophobic
substrates with low contact angle hysteresis; however, these
surfaces are metastable. The critical role of re-entrant surface
curvature in controlling the ability to generate Cassie surface
states is demonstrated by lithographically fabricating a model
surface of micronails covered with a fluorosilane chemical coating.
These model surfaces couple low surface energy with a re-entrant
surface geometry and lead to the first truly superoleophobic
surfaces.
[0095] The combination of surface chemistry and roughness on the
micron and nanoscale imbues enhanced repellency to many natural
surfaces, like the lotus leaf, when in contact with a high surface
tension liquid, such as water (surface tension .gamma..sub.lv=72.1
mN/m). This understanding has led to the creation of a number of
biomimetic superhydrophobic surfaces (water contact angles greater
than 150.degree., low hysteresis). However, researchers so far have
been unsuccessful in producing superoleophobic surfaces for liquids
with much lower surface tensions; for example, alkanes such as
decane (.gamma..sub.lv=23.8 mN/m) or octane (.gamma..sub.lv=21.6
mN/m).
[0096] FIG. 17A shows a drop of water (colored with methylene blue)
on the surface of a lotus leaf. As expected the water droplet beads
up and a very large contact angle is apparent. The leaf's surface
is textured with small 10 .mu.m-20 .mu.m protruding nubs, which are
further covered with nanometer size epicuticular wax crystalloids.
FIG. 17E shows an SEM micrograph of the lotus leaf surface, the
scale bar is 5 .mu.m. However, when a droplet of hexadecane wets
the lotus leaf surface completely (because of its low surface
tension) and a contact angle of about 0.degree. can be observed
(FIG. 17B).
[0097] Here, we have developed a new class of fibers which are
resistant to both water and hexadecane. FIGS. 17C and 17D show
lotus leaves covered with these resistant fibers produced by
electrospinning a solution of PMMA and fluorodecyl POSS (44 wt %)
in Asahiflin AK-225 directly on top of the lotus leaf. Droplets of
hexadecane (colored with a red dye `oil red O`) now bead up on this
modified surface as is clearly visible. Apart from the oil
resistance of the fibers, this picture also shows our ability to
modify the oil repellent characteristics of surfaces with different
geometries/architectures. A reflective surface is visible
underneath the droplets in both pictures, indicating the presence
of microscopic pockets of air.
[0098] FIG. 18A shows the honeycomb-like structure of a
superhydrophobic polyelectrolyte multilayer film coated with silica
nanoparticles (see, e.g., L. ZHAI et al., "Stable superhydrophobic
coatings from polyelectrolyte multilayers," Nano Lett., Vol. 4
(2004) 1349-1353, which is incorporated by reference in its
entirety). FIG. 18B shows a droplet of water sitting on the
aforementioned surface, and FIG. 19 shows an optical image of a
glass slide coated with the superhydrophobic polyelectrolyte
multilayer surface submerged in a pool of water. FIG. 20A shows an
optical micrograph showing small water droplets sprayed on a
superhydrophobic surface with an array of hydrophilic domains
patterned using a 1% PAA water/2-propanol solution (see L. ZHAI,
supra, which is incorporated by reference in its entirety); FIG.
20B is an enlarged image from FIG. 20A.
[0099] To further elucidate the significance of re-entrant
curvature in the formation of a metastable composite interface, the
variation in the specific Gibbs free energy caused by the
propagation of the liquid-air interface on various rough surfaces
was calculated. These calculations are based on the formulation
described elsewhere (see, e.g., A. MARMUR, supra; and A. TUTEJA
supra; which are incorporated by reference in their entirety).
[0100] As an introductory example, the Gibbs free energy density
variation for water (FIG. 27A; .theta.=120.degree.) propagating on
a surface covered with sinusoidal wrinkles (see inset FIG. 27A) was
calculated. It can be seen from FIG. 27A that for water on the
hydrophobic surface, there are two local minima in the free energy
corresponding to the composite (penetration depth z.about.0.3) and
the fully wetted interface (penetration depth z=1.0). Further, the
composite interface was observed to have a much lower free energy
density as compared to the fully wetted state, and was therefore
the thermodynamically favored state. However, it was possible to
provide enough activation energy to force the droplet to transition
to the fully-wetted state. This is the idea used in the experiments
of Krupenkin et al. who use electrical current and voltage to
provide the activation energy required to reversibly transition
between the composite and fully-wetted states on the same surface
with water (see, for example, T. N. KRUPENKIN et al., "Reversible
wetting-dewetting transitions on electrically tunable
superhydrophobic nanostructured surfaces," Langmuir, Vol. 23
(2007), 9128-9133, which is incorporated by reference in its
entirety). Other calculations on this surface with sinusoidal
wrinkles show that when .theta.=.theta..sub.c=100.degree., the
fully-wetted interface has a lower free energy density as compared
to the composite interface and it becomes the thermodynamically
favored state.
[0101] FIG. 27B shows the calculations for Gibbs free energy
density for hexadecane (.theta.=80.degree.) propagating on the same
sinusoidal surface shown in FIG. 27A. In this case we only observe
a single global minimum (at z=1.0), corresponding to the
fully-wetted interface with q*=60.degree.; thus, this surface is
unable to support a composite interface.
[0102] Similar calculations can be performed for the propagation of
water (FIG. 27C; .theta.=120.degree.) and hexadecane (FIG. 27D;
.theta.=80.degree.) on the electrospun fibers of PMMA and 44.1 wt %
fluoroPOSS (these electrospun fibers were used to coat a lotus leaf
to render it superhydrophobic and oleophobic, as shown in FIGS. 17C
and 17D), shown schematically in FIG. 27B. For water propagating on
the electrospun surface, it can be seen that the composite
interface was extremely stable and was the thermodynamically
favored state, as was the case on the sinusoidal surface in FIG.
27A. For the case of the propagation of hexadecane, in contrast to
the sinusoidal surface, the presence of re-entrant curvature allows
for the formation of a metastable composite interface (near the
penetration depth z.about.0.6). It can also be seen that the
overall energy of the surface can be minimized substantially if the
surface transitions from the composite to the fully-wetted
interface, however, there was a significant energy barrier
preventing this transition. It was possible to provide the
activation energy necessary to induce this transition in a variety
of ways including dropping the liquid droplet from a height or
applying external pressure on the drop, leading to a fully-wetted
interface, as observed previously. See, for example, S.
HERMINGHAUS, supra; A. TUTEJA, supra; and A. LAFUMA, supra; each of
which is incorporated by reference in its entirety.
Estimation of Solid Surface Energy (g.sub.sv)
[0103] Previous work by Shibuichi et al. argued that for a
chemically homogeneous, smooth surface to exhibit
.theta.>90.degree. with any liquid, its solid surface energy
(.gamma..sub.sv) must be less than one-fourth the liquid surface
tension, (.gamma..sub.lv/4 (see, for example, K TSUJII, supra; and
S. SHIBUICHI, supra; each of which is incorporated by reference in
its entirety). Careful studies of monolayer films by W. A. ZISMAN,
supra; which is incorporated by reference in its entirety) show
that the contributions to the overall magnitude of surface energy
of a flat surface decreased in the order
--CH.sub.2>--CH.sub.3>--CF.sub.2>--CF.sub.2H>--CF.sub.3,
and based on this analysis, the lowest solid surface energy is
estimated to be about 6.7 mN/m (for a hexagonally closed packed
monolayer of --CF.sub.3 groups on a surface) (see, e.g., T.
NISHINO, supra, which is incorporated by reference in its
entirety). Taken in conjunction, these studies explain the absence
of non-wetting surfaces displaying equilibrium contact angles about
90.degree. with decane and octane, as a solid surface would need to
have a surface energy of about 5 mN/m to display
.theta.>90.degree. with these liquids (see, for example, A.
TUTEJA, supra; K. TSUJII, supra; S. SHIBUICHI, supra; and W. CHEN,
supra; each of which is incorporated by reference in its
entirety).
[0104] However, recently a few groups have reported extremely low
.gamma..sub.sv values; for example, Coulson (S. R. COULSON et al.,
"Ultralow surface energy plasma polymer films," Chem. Mater., Vol.
12 (2000) 2031; and S. R. COULSON, "Plasmachemical
functionalization of solid surfaces with low surface energy
perfluorocarbon chains," Langmuir, Vol. 16 (2000) 6287; each of
which is incorporated by reference in its entirety) report surface
energy values as low as 1.5 mN/m for coatings created by pulsed
plasma polymerization of 1H,1H,2H-perfluoro-1-dodecene.
[0105] Thus, the issue of the minimum surface energy seems to be a
bit controversial and unresolved in the literature. Measurement of
equilibrium contact angles only provides an indirect estimate of
the surface energy, and typically involves extrapolation or
assuming an additive decomposition of .gamma..sub.sv into
dispersive and H-bonding/polar contributions. The most accurate
determination of surface energies requires the measurement of the
work of adhesion, and this is infrequently done (see, e.g., M. J.
OWEN, supra, which is incorporated by reference in its
entirety).
[0106] Indeed, Coulson et al. also report two different measures of
surface energy. They obtain values of .gamma..sub.sv=1.5 mN/m (on a
smooth glass substrate coated by pulsed plasma polymerization of
1H,1H,2H-perfluoro-1-dodecene) and 4.3 mN/m (on a smooth glass
substrate coated by pulsed plasma polymerization of
1H,1H,2H,2H-heptadecafluorodecyl acrylate) using the Zisman
analysis, or .gamma..sub.sv=8.3 mN/m and 10 mN/m using the
Owens-Wendt method for the same two surfaces. See S. R. COULSON,
Chem. Mater., supra; and S. R. COULSON, Langmuir, supra; each of
which is incorporated by reference in its entirety. It is therefore
unclear as to which method provides a more accurate value for
.gamma..sub.sv. An indication that the Zisman analysis might be
providing a .gamma..sub.sv value lower than the actual value for
their surface comes from the values of octane contact angles
obtained by Coulson et al. As mentioned above, if
.gamma..sub.sv<.gamma..sub.lv/4, the equilibrium contact angle
.theta. measured experimentally should be greater than 90.degree..
In contrast, Coulson et al. report values of advancing contact
angle, .theta..sub.adv=74.degree. and receding contact angle,
.theta..sub.rec=35.degree., respectively, on their coatings of
1H,1H,2H-perfluoro-1-dodecene when using octane
(.gamma..sub.lv=21.7 mN/m).
[0107] We have also computed the surface energy of the various spin
coated PMMA and fluoroPOSS surfaces (r.m.s roughness for all spin
coated surfaces was less than 4 nm) using the Zisman and the
Owens-Wendt methods. For a spin coated surface containing 44.4 wt %
POSS we obtain values of .gamma..sub.sv=-3 mN/m and
.gamma..sub.sv=7.8 mN/m (with the dispersive component of surface
energy, .gamma..sub.d=6.6 mN/m and the polar component,
.gamma..sub.p=1.2 mN/m) using the Zisman and the Owens-Wendt method
respectively. FIG. 38 shows the Zisman analysis for four different
spin coated PMMA and fluoroPOSS films, as well as, the data for the
Zisman analysis done by Coulson et al.
[0108] Although the negative value of the surface energy obtained
from the Zisman analysis of our surfaces were spurious (and arose
solely from the extrapolation process employed), however, these
calculations again point out the limitations of the various methods
that use measurements of equilibrium contact angles to compute
.gamma..sub.sv. It was clear from the data in FIG. 38 that, as was
expected, the surface energy of the PMMA and fluoroPOSS blends
decreases with increasing POSS concentration and for high
fluoroPOSS concentrations, the calculated interfacial energy
approached values consistent with those obtained by Coulson et
al.
Designing a Robust Composite Interface.
[0109] The presence of re-entrant texture is not a sufficient
condition for producing robust superhydrophobic or superoleophobic
surfaces as in many cases the activation energy required to
irreversibly transition from a composite interface to a fully
wetted interface can be extremely small. Further, even though a
Gibbs free energy approach can reliably predict the existence of a
composite interface, its ability to estimate the robustness of the
regime is limited as the analysis typically assumes a locally flat
liquid-vapor interface. See, e.g., A. TUTEJA, supra; and A. MARMUR,
supra; each of which is incorporated by reference in its entirety.
With actual droplets, possessing significant internal pressure or
under externally applied pressure, considerable sagging of the
liquid-vapor interface can occur and the actual failure of the
composite regime typically originates not from the activation
energy required to transition between the composite and
fully-wetted states, but from the sagging of the liquid-vapor
interface. Hence the robustness of a composite interface can be
significantly lower than the values obtained using Gibbs free
energy calculations.
[0110] To provide a relative measure of the pressure required to
cause the breakdown of a composite interface, we have developed the
robustness parameter, H*, which relates to the sagging of the
liquid-vapor interface as a result of pressure (Laplace pressure,
external pressure or gravity). H* compares the maximum pore depth
(h.sub.2 in FIG. 26A) with the sagging depth of the interface
(h.sub.1 in FIG. 21B).
[0111] Consider the idealized fiber mat surface shown schematically
in FIGS. 26A and 35A. Such a surface would fail if the liquid-vapor
interface touches the next layer of fibers and the liquid continues
to wet the solid substrate. The sagging depth of the liquid-air
interface (h.sub.1) in this case is given as
h.sub.1=.kappa..sup.-1[1-cos(sin.sup.-1(D.kappa.))] where .kappa.
is the curvature of the liquid-air interface. Generally,
.kappa.=pressure/2.gamma..sub.lv and it becomes the inverse of the
capillary length I.sub.cap= {square root over
(.gamma..sub.lv/.rho.g)} for liquid droplets on a surface in the
absence of any external pressure.
[0112] The system transitions from a composite interface to a fully
wetted interface when the sagging height (h.sub.1) becomes equal to
the original clearance between the liquid-vapor interface and the
next level of fibers (pore depth), h.sub.2=R(1-cos .theta.)
(neglecting any shift in contact angle due to sagging). When
D=1/.kappa..apprxeq.I.sub.cap (which is true for most micro or nano
scale textures), sin(D.kappa.) D.kappa.. Thus,
h.sub.1.apprxeq..kappa..sup.-1(1-cos(D.kappa.)).apprxeq..kappa.D.sup.2/2.
[0113] Therefore, the ratio,
H*=h.sub.2/h.sub.1.apprxeq.2(1-cos .theta.)RI.sub.capD.sup.2
(4)
[0114] The robustness parameter for the micro-nail geometry (FIG.
26B) can be similarly calculated to be: H*=2((1-cos
.theta.)R+H)I.sub.cap/D.sup.2.
[0115] Thus, a rough structure possessing a high pore depth
(h.sub.2) will have an extremely high value of H*. However, even if
the composite interface on a surface is expected to be extremely
resistant to failure with its high pore depth, it can still readily
fail due to a shift in the local contact angle as a result of the
sagging liquid-vapor interface. Initially, on any rough surface
(for example consider FIG. 35C), the liquid-vapor interface makes
an angle .psi. with the solid substrate (re-entrant region in this
case). As the applied pressure increases, the liquid-vapor
interface becomes more and more severely curved or distorted. This
leads to an increase in the contact angle between the liquid-vapor
interface and the solid substrate, until eventually the local
contact angle becomes equal to the equilibrium contact angle for
the liquid (as shown schematically in FIG. 35C). Any additional
pressure will make the interface move and penetrate into the solid
structure. Thus, the composite interface transitions to the
fully-wetted interface when the sagging angle
.delta..theta.=.theta.-.psi. (thus any liquid with .theta.<.psi.
will fail immediately). Considering a liquid drop with a radius
equal to the capillary length of the liquid, as in the definition
of H*, simple trigonometry shows that
.delta. .theta. = sin - 1 ( D R ) = sin - 1 ( D I cap ) .apprxeq. D
I cap ##EQU00001##
by assuming D<<I.sub.cap (as done for the derivation of H*).
Therefore,
T * = .theta. - .psi. .delta. .theta. = .theta. - .psi. sin - 1 ( D
I cap ) .apprxeq. .theta. - .psi. D I cap = I cap .theta. - .psi. D
. ##EQU00002##
[0116] Note that for both the electrospun and the micro-nail
surfaces, re-entrant curvature leads to .psi.=0.degree., which
maximizes the value of (.theta.-.psi.) for any liquid. Geometries
with .psi.<0.degree. (for example a spade geometry) can lead to
even higher values of T*. Given a fixed value of .psi., T* can be
maximized by increasing the value of the equilibrium contact angle
(.theta.), which can be accomplished by lowering the surface energy
of the structure. This is the reason why various low surface energy
molecules are applied as coatings on various re-entrant geometries,
thereby simultaneously increasing the values of both the design
parameters H* and T*.
[0117] The design parameter T* can be considered to be a robustness
angle, while H* is a robustness height. A composite interface can
therefore transition irreversibly to a fully-wetted interface by
either of the two mechanisms discussed above, and it is expected
that the robustness of any composite interface will be proportional
to the minimum between the values of the two robustness
parameters.
[0118] A third design parameter (D* or the spacing ratio) relates
the surface texture parameters to the obtained apparent contact
angles with any liquid. The apparent contact angles for a composite
interface are determined by .PHI..sub.s, as defined through the
Cassie relation. For any given equilibrium contact angle .theta.,
the fraction .PHI..sub.s on the electrospun fiber surface (see FIG.
14) is controlled by the variable D*=(R+D)/R. Cassie and Baxter
showed in their work that .PHI..sub.s=(.pi.R/(R+D))(1-.theta./180).
Higher values of D* lower .PHI..sub.s and consequently increase the
apparent contact angle .theta.*, in accordance with the Cassie
equation.
[0119] To achieve both extremely high apparent contact angles and a
robust composite interface, the design parameters D*, H*, and T*
are preferably simultaneously minimized. In the case of the
electrospun fibers, the three design parameters are inherently
coupled. Increasing the spacing between the fibers (D) leads to
higher D* values, however, this also leads to lower values of both
T* and H* corresponding to more severe sagging of the liquid-air
interface. This, in turn, allows for easier liquid penetration
through the structure. For the micro-nail geometry, on the other
hand, the spacing ratio takes the new form
D * = 1 / f s = ( W + D W ) 2 . ##EQU00003##
As the nail spacing (W) and height (H) can be varied independently
(see FIG. 26B), the spacing ratio (D*) and the robustness parameter
(H*) were easily decoupled to attain both high apparent contact
angles and a highly robust composite interface on the micro-nail
surface, at the same time.
[0120] These design parameters therefore provide a mechanism for
designing surfaces that are able to support super-repellency, with
both high apparent contact angles and a robust composite interface.
Further, they also provide a tool to rank-order various
super-hydrophobic or oleophobic surfaces discussed in the
literature. FIG. 28 shows a plot of the robustness parameter (H*)
as a function of the spacing ratio (D*) for octane on various
natural and artificial surfaces discussed in the literature. More
details for each surface, including the values of the apparent
contact angles with water and octane, as well as their
corresponding design parameters are listed in Table I.
TABLE-US-00001 TABLE I The values of the apparent contact angles
(.theta.*) with water and octane, as well as the corresponding
values for the design parameter, H*, for various natural and
artificial surfaces discussed in the literature. Water Octane
Structure .theta.* H* .theta. - .psi..sup.a .theta.* H* .theta. -
.psi..sup.a Vertical pillars.sup.39 ~160.degree. ~70 30.degree.
0.degree. ~50 ~30.degree. Fractal structure.sup.17 b ~165.degree.
740-3800 75.degree. 0.degree. 600-2500 0.degree. Cassie's wire
~150.degree. 3.4-34 105.degree. N.A..sup.d 0.5-8 45.degree.
gratings.sup.30 Electrospun fiber ~165.degree. ~210 120.degree.
~140.degree. .sup. ~50 60.degree. surface.sup.15 Lotus leaf.sup.c
~155.degree. ~180 ~15.degree. 0.degree. ~0 N.A..sup.d
Micro-hoodoos.sup.15 ~165.degree. 95-1500 120.degree.
140-165.degree. 64-1000 60.degree. Nano-nails.sup.19 ~150.degree.
150-150000 120.degree. 130-150.degree. 100-10000 60.degree.
.sup.aAny liquid for which .theta. - .psi. .ltoreq. 0.degree. will
immediately yield a fully-wetted interface. .sup.b Re-entrant angle
.psi. is hard to measure on randomly shaped textures. On these
fractal-like structures, .psi. is expected to be about 45.degree.
as octane penetrates into the surface texture. .sup.cGeometry of
the lotus leaf has been estimated through the inspection of various
published SEM images and is possibly prone to error. .sup.dNot
available. .sup.e Vertical pillars, B. HE, supra., Fractal
structure, K. TSUJII, supra., Cassie's wire gratings, A. B. D.
CASSIE et al., "Wettability of porous surfaces," Trans. Faraday
Soc., Vol. 40 (1944) 546-551; electrospun fiber surface and
micro-hoodoos, A. TUTEJA, supra, nano-nails, A. AHUJA et al.,
"Nanonails: a simple geometrical approach to superlyophobic
surfaces," Langmuir, Vol. 24 (2008) 9-14 each of which is
incorporated by reference in its entirety.
Preparation of Tunably Wettable Surfaces
[0121] Many natural and commercial surfaces such as woven and
non-woven fabrics, feathers, plant leaves, spheres, cylinders etc.
already have intrinsic re-entrant geometries and these surfaces can
be rendered oleophobic through various simple surface treatments.
These treatment are described in further detail below:
[0122] Chemical Vapor Deposition (CVD):
[0123] CVD is a chemical process used to coat a substrate with
uniformly deposited high-purity, high-performance solid material.
In a typical CVD process, the substrate is exposed to one or more
volatile precursors, which react and/or decompose on the substrate
surface to deposit the desired coating. Micro-nail structures
become oleophobic after a CVD process using various fluoro-silanes
as reactive, volatile precursors (see, for example, FIGS. 11-13B
and 14-16B). CVD can produces a conformal coating on various
surfaces irrespective of their geometry, and therefore is a useful
coating process for re-entrant surfaces.
[0124] Chemical Solution Deposition (CSD):
[0125] CSD uses a liquid precursor, usually dissolved in an organic
solvent, which reacts and thereby adheres conformably to any
surface. This is a relatively inexpensive, simple process that is
able to produce uniform and conformal thin coatings. Unlike CVD,
which is carried out in a highly controlled environment (such as in
a vacuum chamber), CSD allows for producing a coating with less
rigorous/stringent environmental conditions.
[0126] Dip Coating:
[0127] Dip coating refers to the immersing of a substrate into a
tank containing the coating material, removing the coated substrate
from the tank, and allowing it to drain. The coated substrate can
then be dried, for example, by convection or baking. Dip coating
can be, generally, separated into three stages (see FIG. 29):
[0128] Immersion: the substrate is immersed in the solution of the
coating material at a constant speed. Preferably the immersion is
judder free--in other words, the substrate is lowered into the
solution in a smooth motion. [0129] Dwell time: the substrate
remains fully immersed and motionless to allow for the coating
material to apply itself to the substrate. [0130] Withdrawal: the
substrate is withdrawn, again avoiding judders. Coating thickness
can be influenced by the withdrawal speed: the faster the substrate
is withdrawn from the tank, the thicker the coating.
[0131] We have dip-coated various naturally occurring and synthetic
surfaces that inherently possess re-entrant curvature, to make them
superoleophobic. A few examples are shown in FIGS. 30A-32, where
both duck feathers (FIG. 30A, uncoated; FIG. 30B, coated) and a
commercial polyester fabric (FIG. 30C) were coated with FluoroPOSS.
It is seen that the coating is transparent and maintains the
inherent texture of both the fabric and the feather. The feather
and the fabric can also be coated with mixtures of FluoroPOSS and
various commercially available polymers (like poly
methylmethacrylate or TECNOFLON from Solvay-Solexis, etc.), to
obtain similar results. Dip coating with a polymer-fluoroPOSS
mixture also prevents the formation of fluoroPOSS crystals on the
fabric or feather surface (see FIGS. 31B and 31C), while
maintaining the transparency of the coating and its performance.
FIG. 32 shows droplets of water (.gamma..sub.lv=72.1 mN/m),
methylene iodide (.gamma..sub.lv=50.8 mN/m), hexadecane
(.gamma..sub.lv=27.5 mN/m) and methanol (.gamma..sub.lv=22.7 mN/m)
on the polyester fabric's surface, after dip-coating with a
solution of TECNOFLON and fluorodecyl POSS.
[0132] Mechanical durability of the dip-coated fabrics (obtained by
dip-coating with pure fluoroPOSS and fluoroPOSS-polymer mixtures)
was tested by stretching the fabric multiple times and mechanically
rubbing the fabric surface by hand. All of these experiments did
not damage the coating (this was confirmed by imaging the
microstructure of the fabric using a scanning electron microscope)
or reduce performance (as determined by measuring the contact
angles with various liquids, before and after testing).
[0133] One application of the dip-coated fabrics is separation of
liquids having different surface tensions. Stretching of the fabric
changes the pore size within the fabric (leading to a change in the
value of the design parameters, H* and T*, for different liquids).
This then allows for some liquids to wet the fabric and permeate
through it, while other liquids remain unable to wet the surface.
Generally, liquids with lower surface tensions begin to wet the
surface first as the pore size increases. Wetting liquids are able
to pass through the fabric. This is illustrated in FIGS. 33-34B,
where at a particular pore size, methanol (having the lowest
surface tension .gamma..sub.lv=22.7 mN/m) is able to pass through
the fabric, while the other liquids are unable to wet the fabric
surface, and remain on top. Stretching the fabric further (or
changing the pore size) allows for hexadecane=27.5 mN/m) to also
pass through the fabric, while the other liquids still remain on
the fabric surface. By changing the pore size of the fabric as well
as the surface energy of the dip-coating material (as guided by the
design parameters, H* and T*), it is possible to separate various
liquids, even though they may only have a very slight difference in
surface tensions.
[0134] Controlling Contact Angle Hysteresis.
[0135] Although apparent contact angles on any surface are governed
by fraction of solid in contact with a liquid (.PHI..sub.s), the
amount of contact angle hysteresis (i.e., the difference between
the advancing and receding contact angles) can vary significantly
depending on the details of each individual surface texture. Hence
a surface that supports a robust composite interface can also be
tailored to enhance or reduce contact angle hysteresis. Low
hysteresis results in very small roll off angles, corresponding to
easy movement of the liquid droplets on the surface. On the other
hand, high hysteresis implies that a significant amount of energy
needs to be expended in moving the liquid droplet (see, e.g., W.
CHEN, supra, which is incorporated by reference in its entirety).
This in turn can be used to adhere the liquid droplet at a
particular spot on the surface.
[0136] To achieve both these aims, we have fabricated two kinds of
micro-nail structures, with different surface textures, as shown in
FIGS. 36A and 36B. Both samples are Archimedean spirals with n=0
(FIG. 36A, results in concentric circles) or n=1 (FIG. 36B).
Further, both samples are made of the same material (silicon
dioxide) and have the same value of .PHI..sub.s (area fraction of
the solid surface). However, the local distortion of the three
phase (solid-liquid-vapor) contact line during advancing and
receding of any liquid is expected to be markedly different for the
two samples (see, for example, D. ONER et al., "Ultrahydrophobic
surfaces. Effects of topography length scales on wettability,"
Langmuir, Vol. 16 (2000) 7777-7782, which is incorporated by
reference in its entirety). These differences can cause a
significant variation in the obtained contact angles on the two
surfaces.
[0137] The texture shown in FIG. 36A was expected to exhibit
maximum hysteresis, because of the marked difference in the local
conditions experienced by the contact line while advancing as
compared to the local conditions while receding. These variations
led to .theta.*.sub.adv.about.180.degree., while
.theta.*.sub.rec.about..theta., (where .theta. is the equilibrium
contact angle, as given by the Young's equation). Due to the high
hysteresis, it is very difficult for any liquid to roll or slide
off the surface. In effect, any liquid on Sample A remains adhered
at the spot at which it was placed initially.
[0138] The texture shown in FIG. 36B was expected to lead to
minimum hysteresis, allowing for easy movement of liquid drops on
the surface, because the local conditions experienced by the three
phase contact line as it advances or recedes are similar. Thus, two
surfaces fabricated with same material, same .PHI..sub.s and very
similar geometry can lead to extremely different behavior of liquid
droplets placed on them.
[0139] Another structure (FIG. 37A) fabricated was a striped
micro-nail surface, which shows different hysteresis depending on
the direction of advancing and receding, as shown in FIGS. 37A and
37B.
[0140] All three designs discussed above are expected to be useful
for different applications. Concentric circles can enhance contact
angle hysteresis. Such samples can be used to position and confine
liquid drops at preferred locations, with the preferred shape.
Surface texture-directed liquid immobilization can be useful for
cell culturing, localizing liquid droplets on quartz crystal
microbalances, or in chemical or biological sensors.
[0141] A spiral texture (as in FIG. 36B) can reduce contact angle
hysteresis, allowing for easier liquid mobility while maintain
superior liquid repellency. Such surfaces can be useful for most
applications that require superoleophobic surfaces.
[0142] A texture of parallel lines, or stripes, leads to
anisotropic hysteresis. Such surfaces can be useful in developing
structures with directional wettability. These surfaces also allow
for easy control over the path that any liquid follows on these
surfaces, which could be very useful in controlling the movement of
small volumes of liquid, for example in micro-fluidic channels.
[0143] Each reference cited herein is incorporated by reference in
its entirety. Other embodiments are within the scope of the
following claims.
* * * * *