U.S. patent application number 14/672804 was filed with the patent office on 2015-11-26 for hierarchical structures for superhydrophobic surfaces and methods of making.
The applicant listed for this patent is Ohio State Innovation Foundation. Invention is credited to Bharat Bhushan, Yong Chae Jung, Michael Nosonovsky.
Application Number | 20150336360 14/672804 |
Document ID | / |
Family ID | 41608653 |
Filed Date | 2015-11-26 |
United States Patent
Application |
20150336360 |
Kind Code |
A1 |
Bhushan; Bharat ; et
al. |
November 26, 2015 |
HIERARCHICAL STRUCTURES FOR SUPERHYDROPHOBIC SURFACES AND METHODS
OF MAKING
Abstract
Embodiments of a superhydrophobic structure comprise a substrate
and a hierarchical surface structure disposed on at least one
surface of the substrate, wherein the hierarchical surface
structure comprises a microstructure comprising a plurality of
microasperities disposed in a spaced geometric pattern on at least
one surface of the substrate. The fraction of the surface area of
the substrate covered by the microasperities is from between about
0.1 to about 1. The hierarchical structure comprises a
nanostructure comprising a plurality of nanoasperities disposed on
at least one surface of the microstructure.
Inventors: |
Bhushan; Bharat; (Powell,
OH) ; Jung; Yong Chae; (Columbus, OH) ;
Nosonovsky; Michael; (Kew Gardens, NY) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Ohio State Innovation Foundation |
Columbus |
OH |
US |
|
|
Family ID: |
41608653 |
Appl. No.: |
14/672804 |
Filed: |
March 30, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12234900 |
Sep 22, 2008 |
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14672804 |
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61085589 |
Aug 1, 2008 |
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Current U.S.
Class: |
428/195.1 ;
264/129 |
Current CPC
Class: |
B08B 17/065 20130101;
B32B 27/08 20130101; B08B 17/06 20130101; Y10T 428/24612 20150115;
B05D 5/08 20130101; B32B 27/06 20130101; Y10T 428/24355 20150115;
B32B 2307/73 20130101; B05D 7/02 20130101; B32B 27/283 20130101;
Y10T 428/24479 20150115; B05D 1/60 20130101; B32B 27/38 20130101;
Y10T 428/24802 20150115 |
International
Class: |
B32B 27/08 20060101
B32B027/08; B32B 27/28 20060101 B32B027/28; B32B 27/06 20060101
B32B027/06; B32B 27/38 20060101 B32B027/38 |
Claims
1. A superhydrophobic structure comprising a substrate and a
hierarchical surface structure disposed on at least one surface of
the substrate, the hierarchical surface structure comprising: a
microstructure comprising a plurality of microasperities disposed
in a spaced geometric pattern on at least one surface of the
substrate, wherein the fraction of the surface area of the
substrate covered by the microasperities is from between about 0.1
to about 1; and a nanostructure comprising a plurality of
nanoasperities disposed on at least one surface of the
microstructure.
2. The superhydrophobic structure of claim 1 wherein the plurality
of nanoasperities are arranged in a geometric pattern, a random
pattern, or combinations thereof.
3. The superhydrophobic structure of claim 1 wherein the
nanoasperities comprise tubules, platelets, or combinations
thereof.
4. The superhydrophobic structure of claim 1 wherein the plurality
of nanoasperities are disposed on the microasperities, on the
substrate in the spacing between adjacent microasperities, or
combinations thereof.
5. The superhydrophobic structure of claim 1 wherein the plurality
of microasperities comprise a height H of between about 1 to about
100 .mu.m, a diameter D of between about 1 to about 50 .mu.m, and a
pitch P of the microasperities is between 1 and 500 .mu.m, and the
plurality of nanoasperities comprise a height h of between about 1
to about 100 nm and a diameter d of between about 1 to about 300
nm.
6. The superhydrophobic structure of claim 1 wherein the
arrangement of microasperities on the microstructure defines the
following relationship ( {square root over
(2)}P-D).sup.2/R<H.
7. The superhydrophobic structure of claim 1 wherein the fraction
of the surface area of the substrate covered by the microasperities
is from between about 0.5 to about 1.
8. The superhydrophobic structure of claim 1 wherein the fraction
of the surface area of the substrate covered by the microasperities
is from between about 0.8 to about 1
9. The superhydrophobic structure of claim 1 wherein the
superhydrophobic structure comprises a contact angle of between
about 150.degree. to about 180.degree..
10. The superhydrophobic structure of claim 1 wherein the
superhydrophobic structure comprises a contact angle of between
about 165.degree. to about 180.degree..
11. The superhydrophobic structure of claim 1 wherein the
superhydrophobic structure comprises a contact angle hysteresis of
between about 0.degree. to about 10.degree..
12. The superhydrophobic structure of claim 1 wherein the
superhydrophobic structure defines a contact angle hysteresis of
between about 0.degree. to about 5.degree..
13. The superhydrophobic structure of claim 1 wherein the
microasperities comprise epoxy resin, silicon, or combinations
thereof.
14. The superhydrophobic structure of claim 1 wherein the
nanoasperities comprise tropaeolum wax, leymus wax,
n-hexatriacontane, or combinations thereof.
15. A method of making hierarchical structures comprising:
depositing a polymer mold onto a silicon surface comprising a
plurality of microasperities; removing the polymer mold after the
polymer mold has hardened; depositing a liquid epoxy resin into the
polymer mold; forming a microstructure with a plurality of
microasperities by separating the epoxy resin from the mold after
the epoxy resin has solidified; and forming a nanostructure by
depositing alkanes on the microstructure in the presence of solvent
vapor.
16. The method of claim 15 wherein the alkanes are
n-hexatriacontane, alkanes of tropaeolum wax, alkanes of leymus
wax, or combinations thereof.
17. The method of claim 15 wherein leymus wax is deposited in the
presence of a solvent vapor comprising chloroform.
18. The method of claim 15 wherein the tropaeolum wax is deposited
in the presence of a solvent vapor comprising ethanol.
19. The method of claim 1 wherein the microasperities comprise
epoxy resin, silicon, or combinations thereof.
20. The method of claim 1 wherein the nanoasperities comprise
tubules, platelets, or combinations thereof.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. Provisional
Application Ser. No. 61/085,589 filed Aug. 1, 2008.
BACKGROUND
[0002] The present invention relates generally to superhydrophobic
surfaces, and relates specifically to hierarchical structures
comprising microstructures and nanostructures.
SUMMARY
[0003] Advances in nanotechnology, including
micro/nanoelectromechanical systems (MEMS/NEMS), have stimulated
the development of new materials, for example, hydrophobic
materials. Embodiments of the present invention generally relate to
superhydrophobic hierarchical structures which comprise
microstructures and nanostructures, and methods of fabricating
these structures. The structure of the invention is modeled from
structures found in nature, such as Nelumbo nucifera (lotus). A
lotus leaf is superhydrophobic due to the intrinsic hierarchical
structure, built by convex cell papillae and randomly oriented
hydrophobic wax tubules, which have high contact angles with water
and show strong self-cleaning properties.
[0004] Hierarchical structures can provide non-adhesive and water
repellent properties similar to a lotus leaf. As used herein,
superhydrophobicity is the ability of a surface to have a very high
water contact angle, and low contact angle hysteresis. Hysteresis
is the difference between the advancing contact angle and the
receding contact angle. To achieve high static contact angle along
with low contact angle hysteresis, superhydrophobic surfaces should
form a composite interface with air pockets. Several factors can
destroy the composite interface. First, the capillary waves at the
liquid-air interface may destabilize the composite interface. The
effect of capillary waves is more pronounced for small asperities
with height comparable with wave amplitude. Second, nanodroplets
may condense and accumulate in the valleys between asperities and
destroy the composite interface. Third, even hydrophobic surfaces
are usually not chemically homogeneous and can have hydrophilic
spots.
[0005] To prevent destabilization of the composite interface, a
superhydrophobic structure comprising a substrate and a
hierarchical surface structure disposed on at least one surface of
the substrate may be utilized. The hierarchical structure prevents
destabilization of the composite interface and enlarges the
liquid-air interface thereby producing a high contact angle and a
low contact angle hysteresis. The microstructure of the
hierarchical structure resists capillary waves present at the
liquid-air interface, while nanostructures of the hierarchical
structure prevent nanodroplets from filling the valleys between
asperities.
[0006] The ability of a water drop to bounce off a surface
constitutes another benefit. This property is naturally related to
the first two properties, since the energy barriers separating
between the "sticky" and "non-sticky" states needed for bouncing
drops have the same origin as those needed for high contact angle
and low contact angle hysteresis. In some cases, droplets may
bounce off a superhydrophobic surface in an almost elastic manner.
The kinetic energy of the drop is stored in the surface deformation
during the impact. A deformed drop has a higher surface area and
thus higher surface free energy. Therefore, during the impact when
the drop is deformed, it can accommodate more kinetic energy.
[0007] Moreover, the hierarchical structures may be used in various
applications, including self cleaning windows, windshields,
exterior paints for buildings, navigation ships, utensils, roof
tiles, textiles and reduction of drag in fluid flow, e.g., in
micro/nanochannels. It can also benefit application such as
adhesive tape, fasteners, toys, wall climbing robots, space
(microgravity) applications, and MEMS assembly with high adhesive
properties. Additional applications include the reduction of the
capillary meniscus force by introducing roughness in the stable
Cassie regime, utilizing the possibilities of energy conversion and
microscale capillary engines provided by the reversible
superhydrophobicity, and creating superoleophobic surfaces for fuel
economy.
[0008] According to one embodiment of the present invention, a
superhydrophobic structure is provided. The superhydrophobic
structure comprises a substrate and a hierarchical surface
structure disposed on at least one surface of the substrate. The
hierarchical surface structure comprises a microstructure
comprising a plurality of microasperities disposed in a spaced
geometric pattern on at least one surface of the substrate, wherein
the fraction of the surface area of the substrate covered by the
microasperities is from between about 0.1 to about 1. The
hierarchical surface structure further comprises a nanostructure
comprising a plurality of nanoasperities disposed on at least one
surface of the microstructure.
[0009] According to another embodiment of the present invention, a
method of making hierarchical structures comprising depositing a
polymer mold onto a silicon surface comprising a plurality of
microasperities, removing the polymer mold after the polymer mold
has hardened, depositing a liquid epoxy resin into the polymer
mold, forming a microstructure with a plurality of microasperities
by separating the epoxy resin from the mold after the epoxy resin
has solidified, and forming a nanostructure by depositing alkanes
on the microstructure in the presence of solvent vapor.
[0010] These and additional features and advantages provided by the
embodiments of the present invention will be more fully understood
in view of the following detailed description, the accompanying
drawings, and the appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] The following detailed description of specific embodiments
of the present invention can be best understood when read in
conjunction with the drawings enclosed herewith and where like
elements are identified by like reference numbers in the several
provided views.
[0012] FIG. 1 is a schematic view illustrating a hierarchical
structure comprising microasperities in the shape of circular
pillars with diameter D, height H, and pitch P, and pyramidal
shaped nanoasperities of height h and diameter d with rounded tops
according to one or more embodiments of the present invention;
[0013] FIG. 2A is a scanning electron micrograph (SEM) image
illustrating a plurality of microasperities according to one or
more embodiments of the present invention;
[0014] FIG. 2B is an SEM image providing an enlarged view of one of
the plurality of microasperities shown in FIG. 2A according to one
or more embodiments of the present invention;
[0015] FIG. 3A is an SEM image illustrating a hierarchical
structure comprising microasperities and nanoasperities (0.12
.mu.g/mm.sup.2 mass of n-hexatriacontane) disposed thereon
according to one or more embodiments of the present invention;
[0016] FIG. 3B is an SEM image providing an enlarged view of one of
hierarchical structures of FIG. 3A according to one or more
embodiments of the present invention.
[0017] FIG. 4A is an SEM image illustrating a hierarchical
structure comprising microasperities and nanoasperities (0.2
.mu.g/mm.sup.2 mass of n-hexatriacontane) disposed thereon
according to one or more embodiments of the present invention;
[0018] FIG. 4B is an SEM image providing an enlarged view of one of
hierarchical structures of FIG. 4A according to one or more
embodiments of the present invention.
[0019] FIG. 5A is an SEM image illustrating a hierarchical
structure comprising microasperities and nanoasperities (0.4
.mu.g/mm.sup.2 mass of n-hexatriacontane) disposed thereon
according to one or more embodiments of the present invention;
[0020] FIG. 5B is an SEM image providing an enlarged view of one of
hierarchical structures of FIG. 5A according to one or more
embodiments of the present invention;
[0021] FIGS. 6A and 6B are schematic views illustrating a
geometrical arrangement of microasperities on a substrate,
specifically highlighting the pitch P between adjacent
microasperities, according to one or more embodiments of the
present invention;
[0022] FIG. 7A is an SEM image illustrating a hierarchical
structure with a plurality of nanoasperities comprising tropaeolum
tubules, which were formed on the microstructure by deposition in
the presence of ethanol vapor according to one or more embodiments
of the present invention;
[0023] FIG. 7B is an SEM image provides an enlarged view of one of
the plurality of hierarchical structures shown in FIG. 7A according
to one or more embodiments of the present invention;
[0024] FIG. 8A, in contrast to FIG. 7A, is an SEM image
illustrating a hierarchical structure with a plurality of
nanoasperities comprising tropaeolum tubules, which were formed on
the microstructure by deposition without ethanol vapor according to
one or more embodiments of the present invention;
[0025] FIG. 8B is an SEM image provides an enlarged view of one of
the plurality of hierarchical structures shown in FIG. 8A according
to one or more embodiments of the present invention;
[0026] FIG. 9A is an SEM image illustrating a hierarchical
structure with a plurality of nanoasperities comprising leymus wax
tubules, which were formed on the microstructure by deposition with
chloroform according to one or more embodiments of the present
invention;
[0027] FIG. 9B is an SEM image provides an enlarged view of one of
the plurality of hierarchical structures shown in FIG. 9A according
to one or more embodiments of the present invention;
[0028] FIG. 10A through 10F are successive SEM photographs taken at
60 second intervals which illustrates the evaporation of a droplet
on a surface with only microasperities;
[0029] FIG. 11A through 11F, in contrast to FIGS. 10A through 10F,
are successive SEM photographs taken at 60 second intervals which
illustrates the evaporation of a droplet on a surface with a
hierarchical structure according to one or more embodiments of the
present invention;
[0030] FIG. 12 is an SEM image illustrating a dirt stain embedded
in a surface with only microasperities after a droplet of dirt
evaporates;
[0031] FIG. 13, in contrast to FIG. 12, is an SEM image
illustrating a dirt stain on top of a surface with a hierarchical
structure after a droplet of dirt evaporates according to one or
more embodiments of the present invention;
[0032] FIG. 14A is a graphical illustration comparing the
superhydrophobicity of a flat structure, a nanostructure, a
microstructure, and a hierarchical structure based on static
contact angle according to one or more embodiments of the present
invention;
[0033] FIG. 14B is a graphical illustration comparing the
superhydrophobicity of a flat structure, a nanostructures, a
microstructure, and a hierarchical structure based on contact angle
hysteresis according to one or more embodiments of the present
invention;
[0034] FIG. 14C is a graphical illustration comparing the
superhydrophobicity of a flat structure, a nanostructures, a
microstructure, and a hierarchical structure based on tilt angle
according to one or more embodiments of the present invention;
[0035] FIG. 14D is a graphical illustration comparing the
superhydrophobicity of a flat structure, a nanostructures, a
microstructure, and a hierarchical structure based on adhesive
force according to one or more embodiments of the present
invention;
[0036] FIG. 15 is a graphical illustration illustrating that the
relationship between the ratio P/D and the transition between
Cassie and Wenzel wetting regimes according to one or more
embodiments of the present invention;
[0037] FIGS. 16A-16F are successive photographs taken at 8 ms
intervals which illustrate the bouncing/and/or sticking of a
droplet with a radius of 1 mm and a velocity of 0.44 m/s according
to one or more embodiments of the present invention;
[0038] FIGS. 17A-17F, in contrast to FIGS. 16A-16F, are successive
photographs taken at 8 ms intervals which illustrate the
bouncing/and/or sticking of a droplet with a radius of 1mm and a
velocity of 0.88 m/s according to one or more embodiments of the
present invention; and
[0039] FIG. 18 is a graphical illustration illustrating how the
kinetic energy of a drop impacts the transition between Cassie and
Wenzel wetting regimes and the ability of a droplet to stick or
bounce on a surface according to one or more embodiments.
[0040] The embodiments set forth in the drawings are illustrative
in nature and not intended to be limiting of the invention defined
by the claims. Moreover, individual features of the drawings and
invention will be more fully apparent and understood in view of the
detailed description.
DETAILED DESCRIPTION
[0041] Referring generally to the embodiment of FIG. 1, a
hierarchical surface 1 may comprise a microstructure 10 comprising
a plurality of microasperities 12 disposed in a geometric pattern
on at least one surface of a substrate 100, and a nanostructure 20
disposed on at least one surface of the microstructure 10. The
microasperities 12 should be high enough so that a droplet does not
touch the valleys between adjacent microasperities 12. In the
embodiment of FIG. 1, the microasperities 12 may comprise a height
H of between about 1 to about 100 .mu.m and a diameter D of between
about 1 to about 50 .mu.m, wherein the fraction of the surface area
of the substrate 100 covered by the microasperities 12 may range
from between about 0.1 to about 1. In further embodiments, the
fraction of the surface area of the substrate covered by the
microasperities is from between about 0.5 to about 1, or from about
0.8 to about 1.0.
[0042] Further as shown in FIGS. 1, 6A, and 6B, the pitch P between
adjacent microasperities may range from between about 1 and about
500 .mu.m. The SEM image of FIG. 2A, which only illustrates
microstructures, provides an example of a geometric arrangement of
microasperities 12 on a substrate. Although the present disclosure
emphasizes geometrically arranged microasperities 12, it is
contemplated that alternative arrangements may be desirable, e.g.,
random patterns of microasperities 12. Moreover, although the
microasperities illustrated in FIGS. 3B through 5B are cylindrical
with a rounded top, numerous other shapes of microasperities are
contemplated herein. For additional embodiments of asperity shapes,
U.S. Publication No. U.S. 20060078724 entitled Hydrophobic Surface
with Geometric Roughness Pattern is incorporated by reference
herein in its entirety.
[0043] Referring again to FIG. 1, the nanostructure 20 of the
hierarchical surface 1 may comprise a plurality of nanoasperities
22 disposed on at least one surface of the microstructure 10. As
shown, the plurality of nanoasperities 22 may be disposed on the
microasperities 12, on the substrate 100 in the spacing between
adjacent microasperities 12, or combinations thereof. Furthermore,
the nanoasperities 22 may comprise a height h of between about 1 to
about 100 nm and a diameter d of between about 1 to about 300 nm.
In further embodiments, the plurality of nanoasperities 22 may be
arranged randomly as shown in FIGS. 3B, 4B, and 5B, whereas the
schematic illustration of FIG. 1 shows the nanoasperities 22
arranged in a geometric pattern. Combinations of geometric and
random patterns of nanoasperities 22 are contemplated herein. Like
the microasperities 12, the nanoasperities 22 may comprise various
asperity shapes as disclosed in U.S. Publication No. U.S.
20060078724 incorporated by reference herein in its entirety. For
example, in the embodiment of FIG. 1, the nanoasperities 12 may
comprise pyramidal pillars with rounded tops.
[0044] As shown generally in the figures, various arrangements are
contemplated for the hierarchical structures and the nanoasperities
and microasperities thereon. As described above and as shown in
FIGS. 3A-5B, the microasperities 12 may comprise circular pillars.
In a specific embodiment, the circular pillar microstructure may
achieve a composite interface defined by the following relation (
{square root over (2)}P-D).sup.2/R<H. In accordance with the
relation and as illustrated in FIG. 1, a droplet with a radius R on
the order of 1 mm or larger, a microasperity height H on the order
of 30 .mu.m, a microasperity diameter D on the order of 15 .mu.m, a
pitch P between microasperities on the order of 130 .mu.m may be
optimum.
[0045] Referring again to FIG. 1, the nanoasperities 22 may pin the
liquid-air interface and thus prevent liquid from filling the
valleys between microasperities 12. The nanoasperities 22 may also
support nanodroplets, which may condense in the valleys between
microasperities 12, especially larger microasperities 12.
Therefore, nanoasperities 22 should have a small pitch to handle
nanodroplets having a size less than about 100 nm to about 1 nm
radius. The nanoasperities 22 should be high enough so that a
droplet does not touch the valleys between adjacent nanoasperities
22. For example, the nanoasperities 22 may comprise a height h of
about 5 to about 50 nm, or about 10 to about 20 nm. Additionally,
the nanoasperities 22 may comprise a diameter d of up to about 100
nm, or between about 50 to about 100 nm.
[0046] Various materials are contemplated for use in the
microasperities and nanoasperities of the hierarchical structure.
The microasperities may include suitable inorganic or organic
materials operable suitable to support a droplet. For example and
not by way of limitation, the microasperities may comprise epoxy
resin, a silicon based resin, or combinations thereof. As stated
above, the nanoasperities are fabricated with the goal of mimicking
the structure of a lotus leaf. Consequently, the nanoasperities may
include hydrophobic compositions, for example, and not by way of
limitation, hydrophobic alkanes. The hydrophobic alkanes may
include tropaeolum wax (Tropaeolum majus), leymus wax (Leymus
arenarius), n-hexatriacontane, or combinations thereof. Referring
to the embodiments illustrated in the SEM images of FIGS. 3B, 4B,
and 5B, nanoasperities may comprise three-dimensional platelets of
n-hexatriacontane with three different nanoasperity surfaces having
0.12 .mu.g/mm.sup.2, 0.2 .mu.g/mm.sup.2, and 0.4 .mu.g/mm.sup.2
masses, respectively. Platelets are flat crystals grown
perpendicular to the surface of the microasperities. In one
embodiment, the platelet thickness may vary between 50 and 100 nm,
whereas their length varies between 500 and 1000 nm. Various other
nanostructures may be produced by changing the crystal density of
the three-dimensional alkane crystals.
[0047] As detailed above, the performance of the superhydrophobic
hierarchical structure may be quantified through the static contact
angle and contact angle hysteresis metrics. The hierarchical
surface structure is operable to achieve a static contact angle
with a liquid of between about 150.degree. to about 180.degree.,
and a contact angle hysteresis of between about 0.degree. to about
10.degree.. The contact angle hysteresis is the difference between
the advancing contact angle and receding contact angle. In specific
embodiments, the superhydrophobic structure may comprise a static
contact angle of between about 165.degree. to about 180.degree.,
and a contact angle hysteresis of between about 0.degree. to about
5.degree..
[0048] Various methods of fabricating hierarchical structures are
contemplated herein. One such method is the production of
microstructures using surface structure replication and the
subsequent production of nanoasperities via the self-assembly of
hydrophobic alkanes. A number of superhydrophobic surfaces have
been fabricated with hierarchical structures using molding,
electrodeposition, nanolithography, colloidal systems and
photolithography. Molding is low cost and reliable way of surface
structure replication and can provide a precision on the order of
10 nm. Self-assembly of the nanostructures may be achieved via
various methods familiar to one of ordinary skill in the art, for
example, thermal deposition and/or evaporation processes.
[0049] In one embodiment, a method of making hierarchical
structures comprises the steps of depositing a polymer mold onto a
silicon surface comprising a plurality of microasperities, removing
the polymer mold after the polymer mold has hardened, depositing a
resin, for example, a liquid epoxy resin into the polymer mold, and
forming a microstructure with a plurality of microasperities by
separating the epoxy resin from the mold after the epoxy resin has
solidified.
[0050] The method further includes the steps of forming
nanoasperities by depositing alkanes such as n-hexatriacontane or
alkanes of plant waxes (e.g. leymus and tropaeolum) on the
microstructure optionally in the presence of solvent vapors such as
ethanol and chloroform. The following examples are experimental
examples in accordance with embodiments of the present
invention
EXAMPLES
Example 1
[0051] A two-step molding process was used to fabricate the
microstructure on a substrate surface, in which at first a negative
mold is generated and then a positive mold. As a master template, a
Si surface with pillars of 14 pm diameter and 30 pm height with 23
pm pitch, fabricated by photolithography was used. A
polyvinylsiloxane dental wax (e.g. President Light Body.RTM. Gel
manufactured by Coltene Whaledent) was applied via a dispenser on
the surface and immediately pressed down with the cap of a Petri
dish or with a glass plate. After complete hardening of the molding
mass (at room temperature for approximately 5 minutes), the silicon
master surface and the mold (negative) were separated. After a
relaxation time of 30 minutes for the molding material, the
negative replicas were filled up with a liquid epoxy resin (e.g.,
Epoxydharz L.RTM. manufactured by Conrad Electronics) with hardener
(e.g., Harter S, Nr 236365 manufactured by Conrad Electronics). The
liquid epoxy resin was added near the edge of the negative replica
to prevent trapped air. Specimens were immediately transferred to a
vacuum chamber at 750 mTorr (100 Pa) pressure for 10 seconds to
remove trapped air and to increase the resin infiltration through
the structures. After hardening at room temperature (24 h at
22.degree. C., or 3 h at 50.degree. C.), the positives replica were
separated from the negative replica. The second step can be
repeated to generate a number of replicas.
[0052] The nanostructure was created by self assembly of the
Tropaeolum and Leymus waxes, which were deposited by thermal
evaporation. These waxes are provided by Botanical Garden of the
University of Bonn. The specimens of smooth surfaces and
microstructure replicas were placed in a vacuum chamber at 30 mTorr
(4 kPa pressure), 2 cm above a hot plate loaded with 500, 1000,
1500 and 2000 .mu.g waxes. The waxes were evaporated by heating it
up to 120.degree. C. Evaporation from the point source to the
substrate occurs over a hemispherical region. In order to estimate
the amount of sublimated mass, the surface area of the half sphere
is first calculated using the formula 2.pi.r.sup.2, whereby the
radius (r) represents the distance between the specimen to be
covered and the heating metal with the substance to be evaporated.
Next, the amount of sublimated mass per surface area can be
calculated by an amount of alkane loaded on a hot plate divided by
surface area. After coating, the specimens were placed in a glass
crystallization chamber with ethanol or chloroform solution to
increase molecule mobility for recrystallization and then placed in
the oven at 500 C for 3 days. The chamber should be opened to
prevent the condensation of water inside. After that, the specimens
were placed in a desiccator at room temperature for 4 days for
crystallization of the alkanes.
Example 2
[0053] For nanostructures of Tropaeolum wax, two different
experimental conditions, after storage at 50.degree. C. with and
without ethanol vapor, were used to identify optimized structures.
FIGS. 7A and 7B is an SEM micrograph illustrating a hierarchical
structure fabricated with 0.8 .mu.g/mm.sup.2 of Tropaeolum wax
after storage at 50.degree. C. with ethanol vapor, whereas FIGS. 8A
and 8B illustrate a hierarchical structure fabricated with 0.8
.mu.g/mm.sup.2 of Tropaeolum wax after storage at 50.degree. C.
without ethanol vapor. When comparing the SEM images of FIG. 7B to
8B, the presence of ethanol vapor yields an increase in tubules on
flat and microstructure surfaces after deposition of tropaeolum
wax. The formation of tubules requires mobility of wax molecules on
the surface, which is provided at least in part by the solvent
(e.g., the ethanol vapor).
Example 3
[0054] Unlike Tropaeolum wax as described in Example 2, crystal
growth of Leymus wax was not found on the surface after storage at
50.degree. C. with ethanol vapor. However, chloroform solution
yielded increased molecule mobility, and thus increased mobility of
Leymus wax. FIGS. 9A and 9B are SEM images illustrating the
hierarchical structure fabricated with a mass of 0.8 .mu.g/mm.sup.2
of Leymus wax after storage at 50.degree. C. with chloroform vapor.
An increase in tubule length was found after deposition of higher
wax mass. The higher mass of deposited wax led to an increase in
the amount of tubules with a more up right orientation. The tubular
crystals were randomly orientated on the surface and embedded into
an amorphous wax layer, and may be two to five times longer than
the tubules of Tropaeolum wax. For example, the leymus wax tubules
may comprise a tubular diameter of may vary between about 200 to
about 300 nm, and a length between about 1500 to about 4000 nm.
Example 4
[0055] To study the effect of structure on superhydrophobicity, the
following metrics (static contact angle, contact angle hysteresis,
and tilt angle, and adhesive forces) were used to evaluate four
structures as illustrated in the graphs of FIGS. 14A-14D. The first
structure i.e., the flat refers to a flat surface coated with a
film of Tropaeolum wax. The second structure i.e., the
nanostructure refers to a flat surface with a nanostructure of 0.8
.mu.g/mm.sup.2 mass Tropaeolum wax tubules. The microstructure and
hierarchical structures are the third and fourth structures,
wherein the hierarchical structure includes a nanostructure of 0.8
.mu.g/mm.sup.2 mass Tropaeolum wax tubules. As shown in FIG. 14A,
the static contact angle for the flat structure with Tropaeolum wax
film was 112.degree., but increased to 164.degree. when Tropaeolum
wax formed a nanostructure of tubules on it. For the
microstructure, the static contact angle was 154.degree., but
increases to 171.degree. for the hierarchical surface
structure.
[0056] Referring to FIGS. 14B and 14C, the contact angle hysteresis
and tilt angle for flat, microstructure and nanostructure surfaces
show similar trends. The flat surface showed a contact angle
hysteresis angle of 61.degree. and a tilt angle of 86.degree.. The
microstructure surface shows a reduction of contact angle
hysteresis and tilt angles, but a water droplet still needs a tilt
angle of 31.degree. before sliding. Due to the addition of tubules
formed on the flat and microstructure surfaces, the nanostructure
and hierarchical structure surfaces, respectively, have static
contact angles of 164.degree. and 171.degree., respectively, and
low hysteresis of 5.degree. and 3.degree., respectively, thereby
fulfilling the criteria for superhydrophobic and self-cleaning
surfaces.
[0057] Referring to FIG. 14D, the adhesive force, which was
measured using a 15 .mu.m radius borosilicate tip in an atomic
force microscope (AFM), also showed similar trends as the wetting
properties. As shown, the adhesion force of the hierarchical
surface structure was lower than either that of microstructured and
nanostructured surfaces, because the contact between the tip and
surface was lower as a result of contact area being reduced in both
levels of structuring.
[0058] In order to identify propensity of air pocket formation for
the four structures, roughness factor (R.sub.f) and fractional
liquid-air interface (f.sub.LA) are needed. Superhydrophobicity is
usually caused by high surface roughness. The roughness is
characterized by the non-dimensional Wenzel roughness factor,
R.sub.f, which is equal to the ratio of surface area to its flat
projection. The R.sub.f for the nanostructure, (R.sub.f).sub.nano,
was calculated using the AFM map. The calculated results were
reproducible within .+-.5%. The R.sub.f for the microstructure was
calculated for the geometry of flat-top, cylindrical pillars of
diameter D, height H, and pitch P distributed in a regular square
array. For this case, roughness factor for the microstructure was
calculated using the following equation,
( R f ) micro = ( 1 + .pi. DH P 2 ) . ##EQU00001##
The roughness factor for the hierarchical structure is the sum of
the roughness values for microstructure and nanostructure
(R.sub.f).sub.micro and (R.sub.f).sub.nano.
[0059] For calculation of (f.sub.LA) we make the following
assumptions. For the microstructure, we consider that a droplet in
size much larger than the pitch P contacts only the flat-top of the
pillars in the composite interface, and the cavities are filled
with air. For the nanostructure, only the higher crystals are
assumed to come in contact with a water droplet. For
microstructure, the fractional flat geometric area of the
solid-liquid and liquid-air interfaces (f.sub.LA).sub.micro was
calculated with the following equation,
( f LA ) micro = ( 1 + .pi. D 2 4 P 2 ) . ##EQU00002##
The fractional geometrical area of the top surface for the
nanostructure was calculated from an SEM micrograph with top view
(0.degree. tilt angle). The fractional geometrical area of the top
surface with Tropaeolum wax was found to be 0.14, leading to
f.sub.LA of 0.86. For the hierarchical structure, the fractional
flat geometrical area of the liquid-air interface is defined by
( f LA ) hierarchical = ( 1 + .pi. D 2 4 P 2 ) [ 1 - ( f LA ) nano
] . ##EQU00003##
The values calculated for the various structures are summarized in
Table 1 below.
TABLE-US-00001 TABLE 1 Contact Contact angle R.sub.f f.sub.LA angle
(.degree.) hysteresis (.degree.) Flat 112 61 Nanostructure 11 0.86
164 5 Microstructure 3.5 0.71 154 27 Hierarchical 14.5 0.96 171
3
[0060] The roughness factor (R.sub.f) and fractional liquid-air
interface (f.sub.LA) of the hierarchical structure are higher than
those of the nanostructures and microstructures, thus demonstrating
that the air pocket formation in hierarchical structured surfaces
occurs at both levels of structuring, which decreases the
solid-liquid contact and thereby contact angle hysteresis and tilt
angle.
Example 5
[0061] To further verify the effect of hierarchical structure on
propensity of air pocket formation, evaporation experiments with a
droplet on microstructure and hierarchical structure were
conducted. The hierarchical structure included a nanostructure
fabricated with 0.8 .mu.g/mm.sup.2 mass of Tropaeolum wax with
ethanol vapor at 50.degree. C. FIGS. 10A through 10F are successive
SEM images taken 60 seconds apart of a droplet evaporating on a
microstructure, whereas FIGS. 11A through 11F are successive SEM
images taken 60 seconds apart of a droplet evaporating on a
hierarchical structure. For the microstructure surface as shown in
FIGS. 10A through 10F, the light passes below the droplet and air
pockets can be seen, thus the droplet is in the Cassie-Baxter
wetting regime. When the radius of the droplet decreases to 425
.mu.m, the air pockets are not visible anymore and the droplet is
in Wenzel wetting regime. This transition results from an
impalement of the droplet in the patterned surface, characterized
by smaller contact angle. For the hierarchical structure as shown
in FIGS. 11A through 11F, the air pocket was clearly visible at the
bottom area of the droplet throughout and the droplet was in a
hydrophobic state until the droplet evaporated completely.
Consequently, a hierarchical structure with nanostructures prevents
the droplets from filling the gaps between the pillars.
[0062] FIGS. 12 and 13 provide a further contrast of microstructure
surfaces (FIG. 12) and hierarchical surfaces (FIG. 13). When a
droplet of 1 mm radius was placed on a microstructure surface and
then evaporated, the dust particles became embedded into the
microstructure surface in the spacing between microasperities as
shown in FIG. 12. When the droplet was placed on a hierarchical
structure, the dust particles do not become embedded in the
hierarchical structure due to the droplet sinking into the spacing
between nanoasperities.
Example 6
[0063] To demonstrate the energy transition due to an impacting
droplet on a hierarchical surface, additional experiments were
conducted. The two series of patterned Si surfaces, covered with a
monolayer of hydrophobic tetrahydroperfluorodecyltrichlorosilane
(contact angle with a nominally flat surface,
.theta..sub.0=109.degree., advancing and receding contact angle
.theta..sub.adv0=116.degree. and .theta..sub.rec0=82.degree.),
formed by flat-top cylindrical pillars. Series 1 had pillars with
the diameter D=5 .mu.m, height H=10 .mu.m, and pitch values P (7,
7.5, 10, 12.5, 25, 37.5, 45, 60, and 75) .mu.m, while series 2 had
D=14 .mu.m, H=30 .mu.m, P=(21, 23, 26, 35, 70, 105, 126, 168, and
210) .mu.m. The two series were designed in this manner to isolate
the effect of the pitch, pitch-to-height, and pitch-to-diameter
ratios. The contact angle and contact angle hysteresis of
millimeter-sized water drops upon the samples were measured.
Referring to the graph on FIG. 15, it was found that for small P,
the drops were in the Cassie state sitting on top of the pillars,
whereas with increasing P the transition to the Wenzel state
occurred. The transition to the Wenzel state occurred when the drop
radius decreased below a certain critical value. The drop radius,
R, at the Cassie-Wenzel transition was found to be proportional to
P/D. Consequently, the energy barrier .DELTA.E, which for
cylindrical flat-top pillars, has the following equation
.DELTA. E = A 0 .pi. HD P 2 ( .gamma. LV cos .theta. 0 )
##EQU00004##
is proportional to the RD/P or RH/P. Since the area under the drop
A.sub.0=.pi.(Rsin.theta.).sup.2, taking sin.sup.2.theta.=0.1,
cos.theta..sub.0=cos109.degree.=-0.33, .gamma..sub.LV=0.072
J/m.sup.2 and the observed value RD/P=50 .mu.m yields
.DELTA.E=1.2.times.10.sup.-10 J. This quantity can be associated
with the vibrational energy of the drop due to surface waves,
etc.
Example 7
[0064] The impact of water drops with 5 .mu.L volume (about 1 mm
radius) was also conducted upon hierarchical surfaces. Drops
impacting the surface with low velocity bounce off the surface as
shown in FIGS. 16A-16F, whereas those having high impact velocity
stuck to the surface as shown in FIGS. 17A-17F. Sticking was
associated with being in the Wenzel state with a large solid-liquid
contact area, while the drops that bounced off the surface were in
the Cassie state with an air pocket under them. Thus, the energy
barrier of the Cassie-Wenzel transition can be estimated as the
kinetic energy of the drops. The graph of FIG. 18 shows the
dependence of the kinetic energy corresponding to the transition,
E.sub.kin, on .DELTA.E/(A.sub.0cos.theta..sub.0). It is observed
that the dependence is close to linear, however, the series of
smaller pillars has larger energies of transition. The value of
A.sub.0 is in the range 0.11 mm.sup.2<A.sub.0<0.18 mm.sup.2
for Series 1 and 0.05 mm.sup.2<A.sub.0<0.11 mm.sup.2 for
Series 2, which is of the same order as the actual area under the
drop.
[0065] These results show that the energy barrier for the
Cassie-Wenzel transition is proportional to the area under the
drop. For drops sitting on the surface or evaporating, the
transition takes place when the size of the barrier decreases to
the value of the vibrational energy, approximately
E.sub.vib=10.sup.-10 J, which was estimated from the energy
barrier. This may happen because the size of the drop is decreased,
or because the pitch between the pillars that cover the surface is
increased. A different way to overcome the barrier is to hit the
surface by a drop with a certain kinetic energy.
[0066] It is further noted that terms like "preferably,"
"generally", "commonly," and "typically" are not utilized herein to
limit the scope of the claimed invention or to imply that certain
features are critical, essential, or even important to the
structure or function of the claimed invention. Rather, these terms
are merely intended to highlight alternative or additional features
that may or may not be utilized in a particular embodiment of the
present invention.
[0067] For the purposes of describing and defining the present
invention it is additionally noted that the term "substantially" is
utilized herein to represent the inherent degree of uncertainty
that may be attributed to any quantitative comparison, value,
measurement, or other representation. The term "substantially" is
also utilized herein to represent the degree by which a
quantitative representation may vary from a stated reference
without resulting in a change in the basic function of the subject
matter at issue.
[0068] Having described the invention in detail and by reference to
specific embodiments thereof, it will be apparent that
modifications and variations are possible without departing from
the scope of the invention defined in the appended claims. More
specifically, although some aspects of the present invention are
identified herein as preferred or particularly advantageous, it is
contemplated that the present invention is not necessarily limited
to these preferred aspects of the invention.
[0069] All documents cited in the Detailed Description are, in
relevant part, incorporated herein by reference; the citation of
any document is not to be construed as an admission that it is
prior art with respect to the present invention. To the extent that
any meaning or definition of a term in this written document
conflicts with any meaning or definition of the term in a document
incorporated by reference, the meaning or definition assigned to
the term in this written document shall govern.
[0070] While particular embodiments of the present invention have
been illustrated and described, it would be obvious to those
skilled in the art that various other changes and modifications can
be made without departing from the spirit and scope of the
invention. It is therefore intended to cover in the appended claims
all such changes and modifications that are within the scope of
this invention.
* * * * *