WO2014035742A2 - Sprayable superhydrophobic coatings - Google Patents

Abstract

Methods of making superhydrophobic surfaces are described, as are the sprayable compositions used to prepare such surfaces and the surfaces themselves. The methods include simplified methods of spraying a composition to a surface without a need for chemical pre-treatment or post-coating passivation of the surface.

Description

SPRAYABLE SUPERHYDROPHOBIC COATINGS

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to U.S. Patent Application Ser. No. 61/694,816, filed August 30, 2012, which is incorporated by reference in its entirety herein for all purposes.

GOVERNMENT RIGHTS

[0002] The subject matter disclosed herein was made with government support under award/contract/grant number IIP-1 142300 awarded by the National Science Foundation/SBIR Phase I. The Government has certain rights in the herein disclosed subject matter.

TECHNICAL FIELD

[0003] The present invention relates to compositions capable of forming

superhydrophobic coatings on surfaces, substrates having superhydrophobic coatings formed of such a compositions, as well as to methods of producing of such superhydrophobic coatings. Preferably the coatings are optically transparent.

BACKGROUND

[0004] Surfaces with particular wetting characteristics - i.e., water repellent surfaces - are widely used and of great interest to various industries, such as the coating industry, building and construction industry, the aerospace industry, the automotive industry, the packaging industry, in fabrics, in medical devices, and in solar energy applications. For example, hydrophobic surfaces capable of self-cleaning are also of interest for use in exposed portions of photovoltaic cells, so as to allow the maximum amount of electromagnetic radiation to reach the photovoltaic cell. The use of surface modification techniques to impart such properties to surfaces of various substrates, natural or artificial, such as metal, glass, wood, ceramics, paper, polymers, fabrics, building materials, such as stone, concrete, marble, bricks, tiles, etc., to achieve the desired characteristics is a widely researched field.

[0005] The hydrophobicity of a material - i.e., its tendency to repel water - may be determined by the contact angle of a water droplet to the surface. In general, hydrophobicity is achieved by lowering the surface energy of the surface or material. Thus, non-hydrophobic materials may be rendered hydrophobic by applying a surface coating of low surface energy material. Chemically this may be done for example by incorporating apolar moieties, such as methyl or trifluoromethyl groups, onto the surface. Superhydrophobic surfaces are those surfaces that are extremely difficult to wet. Such surfaces are, in fact, defined as those which exhibit water contact angles larger than about 150° and theoretically up to 180°, and exhibiting a roll-off angle of a water droplet with size of 10 microliter (μΚ) or smaller (roll-off angle defined as the tilt angle when the liquid drop starts to move on a surface) of less than 10°. This latter effect is referred to as the "lotus effect," after that property exhibited by lotus leaves.

[0006] Many techniques of rendering surfaces superhydrophobic are described in the literature, including plasma polymerization or etching of apolar polymers like polypropylene or polytetrafluoroethylene, plasma enhanced chemical vapor deposition of methyl or fluorine containing silanes, solidification of molten polymers or waxes, sublimation material and paint or sprays containing hydrophobized microbeads or evaporation of volatile compounds. In many cases, superhydrophilicity is achieved by roughening the surface of a substrate. However, surface roughness and optical transparency are generally seen as antithetical to one another, owing to Mie scattering and Rayleigh scattering from the roughened surface. In some cases, in order to increase the roughness very often additional steps like mechanical treatment, chemical or plasma etching or anodic oxidation are necessary before or after the coating step. Beyond the problems of reduced transparency, these methods are often complicated and thus time- consuming procedures, and use expensive starting materials like fluorinated silanes, fluorinated polymers, and/or extreme reaction conditions which restrict the applicability to few resistant materials.

[0007] The present invention addresses at least some of these shortfalls. SUMMARY

[0008] The present invention provides compositions and methods of preparing superhydrophobic coatings, sufficiently durable for handling, using simple processing methodologies. The compositions may be spray coated onto substrates, which is of particular interest for large area application at a lower cost and applications directly by consumers. The present invention also obviates the need to use hazardous non-polar organic solvents, such as toluene and/or fluorinated solvents, thereby allowing the use of polymeric substrates that would otherwise be subject to whitening and crack formation through the use of such non-polar solvents. Instead, by eliminating the need for such hazardous solvents through use of more benign polar solvents, such as lower alcohols and even water based systems, the present invention provides for use of compositions having reduced environmental and worker safety and health impact.

[0009] Certain embodiments of the present invention provide compositions, each composition comprising coating compositions, each composition comprising a plurality of hydrophobic nanoparticles dispersed in a hydrophilic solvent comprising an amphiphilic silicone-containing dispersing resin. In certain independent embodiments, the compositions are aqueous-based dispersions, either with or without surfactant.

[0010] Other embodiments provide methods of preparing a superhydrophobic coating, each method comprising: (a) applying to a substrate a coating composition comprising a plurality of hydrophobic nanoparticles dispersed in a hydrophilic solvent comprising an amphiphilic silicone-containing dispersing resin; and (b) effecting removal of at least a portion of the hydrophilic solvent; and forming an adherent layer of an amphiphilic pre-cured coating layer on the substrate.

[0011] Still other embodiments provide superhydrophobic coatings, each coating comprising a plurality of hydrophobic nanoparticles embedded within a layer of cross-linked hydrophobic silicone-based polymer. In certain embodiments, the superhydrophobic coatings are prepared by the methods described herein.

[0012] The invention also provides for solar cells and energy storage and other devices comprising superhydrophobic coatings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] The present application is further understood when read in conjunction with the appended drawings. For the purpose of illustrating the subject matter, there are shown in the drawings exemplary embodiments of the subject matter; however, the presently disclosed subject matter is not limited to the specific methods, devices, and systems disclosed. In addition, the drawings are not necessarily drawn to scale. In the drawings, CA refers to static water contact angle and RA is the sliding or rolling-off angle.

[0014] FIG. 1 illustrates the chemical structures of silicone-based precursors before and after thermal curing. In the case of GR653L, as described in the Examples, R = R' = R" = methyl.

[0015] FIG. 2 is an AFM phase image of a glass substrate dip coated with 5 mg AEROSIL™ NP (NP = nanoparticles) per mL coating composition + lvol/vol% GR653L in isopropanol solution, after annealing 200°C for one hour. [0016] FIG. 3 is an SEM image of a glass substrate spray coated with 5 mg

AEROSIL™ NP per mL coating composition, without GR653L, in isopropanol solution, after annealing 200°C for one hour.

[0017] FIG. 4 is an SEM image of a glass substrate spray coated with 5 mg

AEROSIL™ NP per mL coating composition + 0.01vol/vol% GR653L in isopropanol solution, after annealing 200°C for one hour.

[0018] FIG. 5A and FIG. 5B are SEM images of a glass substrate spray coated with 5 mg AEROSIL™ NP per mL coating composition + 0. lvol/vol% GR653L in isopropanol solution, after annealing at 200°C for one hour. FIGs. 5A and 5B differ in their magnifications.

[0019] FIG. 6 is an SEM image of a glass substrate spray coated with 5 mg/mL AEROSIL™ NP + lvol/vol% GR653L in isopropanol solution, after annealing 200°C for one hour.

[0020] FIG. 7 shows a schematic of a water jet set-up, for example as described in Examples 3.4 and 6.22.

[0021] FIG. 8 shows data for the change of water contact angle (CA, Θ) and contact angle hysteresis (CAH = the difference between advancing water contact angle, Q_adv, and receding water contact angle, 6_rec) of glass substrates spray coated with AEROSIL™ NPs + GR653L in isopropanol of different concentrations of GR653L before and after water dropping tests. Nanoparticle (NP) concentrations, [NPs] = 5 mg nanoparticles per mL coating

composition.

[0022] FIG. 9 compares the UV-visible spectra obtained using various coatings on glass substrates - coatings include those from NPs only and NPs + GR653L (4 rounds of sprayed coatings; approximately 700 nm thick, estimated from AFM height images, based on depth of pin-hole)

[0023] FIG. 10 provides optical images of AEROSIL™ NPs suspension in

isopropanol/water mixtures with different volume fractions of each.

[0024] FIG. 11 shows water CA ad CAH data of glass substrates spray coated with AEROSIL™ NPs + GR653L in isopropanol/water mixtures, as a function of volume fraction of the water in the mixed solvent. Nanoparticle (NP) concentrations, [NPs] = 5 mg nanoparticles per mL coating composition; [GR653L] = 0.1 vol/vol%, after annealing at 200°C for 1 hr.

[0025] FIG. 12 provides optical images of water droplet on a GR653L/ AEROSIL™ composite film spray coated on a polycarbonate substrate. [NPs] = 5 mg nanoparticles per mL coating composition, [GR653L] = 0.1vol/vol% in isopropanol. The substrate was thermally annealed at 140°C for 1 hour. See Example 4.6. [0026] FIG. 13 shows the influences of processing parameters on wettability for the mixture with A- S1O₂ of 5 mg/mL and GR653L of 2mg/mL, (A) solvent (sprayed eight cycles), (B) spraying cycles (IP A) , as described in Example 5.2.

[0027] FIG. 14 provides SEM images of the superhydrophobic coatings sprayed from A5G2 ([A-S1O₂] = 5 mg/mL and [GR653L] = 2 mg/mL) for different cycles. (A) and (B), four cycles. (C) and (D), eight cycles. (E) and (F), twelve cycles. Arrows indicate the cracks on NPs surface caused by the shrinkage of the covering glass resin, as described in Example 5.2.

[0028] FIG. 15 provides SEM images of glass substrates spray coated with 5 mg/mL A-S1O₂ NPs only for six cycles. (A) Low magnification. (B) Higher magnification, as described in Example 5.2.

[0029] FIG. 16 compares the wettability by spray coating of the IPA solutions with different formulations for six to ten spraying cycles. (A) Effect of A-S1O₂ NP concentration ([GR653L] = 2 mg/mL). (B) Effect of GR653L concentration ([A-S1O₂] = 5 mg/mL) , as described in Example 5.2.

[0030] FIG. 17 compares the wettability of the prepared superhydrophobic coatings sprayed (eight to ten cycles) using the solutions of different concentrations of A-S1O₂ NPs and GR653L, while keeping [A-Si0₂]/[GR653L] = 2.5. The notion of different solutions referred to the concentration of the NPs or GR653L. For example, A5G2 means that [A-Si0₂] = 5 mg/mL and [GR653L] = 2 mg/mL, as described in Example 5.2.

[0031] FIG. 18 are AFM images of the coated glass substrates. (A) 3D image of the coating sprayed eight cycles from the solution of A2.5G1 in IPA; (B) 3D image of the coating sprayed four cycles from the solution of A5G2 in IPA.; (C) 3D image of coating sprayed eight cycles from the solution of A5G2 in IPA; (D) Corresponding 2D image of (C). Scan area, 5x5 microns, applicable to all images, as described in Example 5.2.

[0032] FIG. 19 are side-view SEM images of the coated glass showing the spray coated film thickness. (A) A2.5G1 in IPA sprayed eight cycles. (B) A5G2 in IPA sprayed eight cycles, as described in Example 5.2.

[0033] FIG. 20 are photographs of water droplets on A5G2 spray-coated (eight cycles) glass. (A) 1 and 5- microliter droplets on the surface, space between two lines in the background is 1 mm. (B) 1 and 10- microliter droplets. (C) Rolling off of the 10-microliter droplet from the substrate with a tilting angle less than 1°. (D) Rolling off of the 1- microliter droplet with a tilting angle of ca. 4°, as described in Example 5.2.

[0034] FIG. 21 compares (A) optical image of NPC suspension of A5G2 in IP A/water mixtures with different water volume fractions, and (B) the corresponding wettability of the coatings sprayed eight cycles on glass before and after the precursor IPA solution was diluted with water, as described in Example 6.2.1.

[0035] FIG. 22 provides SEM images of the spray coated glass surface using the A5G2 aqueous solutions with different water concentration in IP A/water mixtures, (A) and (B) 60 vol% water, (C) and (D) 70 vol% water, (E) and (F) 80 vol% water, as described in Example 6.2.1.

[0036] FIG. 23 provides SEM images of the NPCs sprayed from A5G2 for eight cycles after Scotch tape peeling. (A) Low magnification. (B) Higher magnification of (a), as described in Example 6.2.2.

[0037] FIG. 24 compares the transmittance changes of superhydrophobic coatings on glass, (A) the effect of spraying cycles (A5G2, IPA solutions), (B) changing A-Si02

concentration, (C) change with glass resin concentration, (D) the coatings with the same concentration ratio of A-S1O₂ to GR653L, as described in Example 6.2.3.

[0038] FIG. 25 compares the transparency of the superhydrophobic coatings sprayed eight cycles from aqueous solutions. (A) Coatings prepared from A5G2 with various water concentrations. (B) Comparison of the coatings using IP A/water mixture (90 vol% of water) as a solvent, as described in Example 6.2.3.

[0039] FIG. 26 compares the transmittance of the glass coated with A5G2 IPA solution (eight cycles) from visible to MR region. SEM images of the sample were shown in FIG 14C and 14D, while Table 7 and FIG. 20 provided the wettability.

[0040] FIG. 27 shows the wettability of glass substrates spray coated from A5G2 IPA solutions stored in air and refrigerator at different durations, as described in Example 6.2.4.

[0041] FIG. 28 illustrates the self-cleaning effect of coated glass substrates, where the glass is coated with A5G2 in IPA solution (left) in comparison to water spreading on untreated glass (right) as described in Example 6.2.5.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

[0042] The present invention may be understood more readily by reference to the following description taken in connection with the accompanying Figures and Examples, all of which form a part of this disclosure. It is to be understood that this invention is not limited to the specific products, methods, conditions or parameters described and / or shown herein, and that the terminology used herein is for the purpose of describing particular embodiments by way of example only and is not intended to be limiting of any claimed invention. Similarly, unless specifically otherwise stated, any description as to a possible mechanism or mode of action or reason for improvement is meant to be illustrative only, and the invention herein is not to be constrained by the correctness or incorrectness of any such suggested mechanism or mode of action or reason for improvement. Throughout this text, it is recognized that the descriptions refer both to the features and methods of making and using superhydrophobic coatings.

[0043] In the present disclosure the singular forms "a," "an," and "the" include the plural reference, and reference to a particular numerical value includes at least that particular value, unless the context clearly indicates otherwise. Thus, for example, a reference to "a material" is a reference to at least one of such materials and equivalents thereof known to those skilled in the art, and so forth.

[0044] When a value is expressed as an approximation by use of the descriptor "about," it will be understood that the particular value forms another embodiment. In general, use of the term "about" indicates approximations that can vary depending on the desired properties sought to be obtained by the disclosed subject matter and is to be interpreted in the specific context in which it is used, based on its function. The person skilled in the art will be able to interpret this as a matter of routine. In some cases, the number of significant figures used for a particular value may be one non-limiting method of determining the extent of the word "about." In other cases, the gradations used in a series of values may be used to determine the intended range available to the term "about" for each value. Where present, all ranges are inclusive and combinable. That is, references to values stated in ranges include every value within that range.

[0045] When a list is presented, unless stated otherwise, it is to be understood that each individual element of that list and every combination of that list is to be interpreted as a separate embodiment. For example, a list of embodiments presented as "A, B, or C" is to be interpreted as including the embodiments, "A," "B," "C," "A or B," "A or C," "B or C," or "A, B, or C."

[0046] It is to be appreciated that certain features of the invention which are, for clarity, described herein in the context of separate embodiments, may also be provided in combination in a single embodiment. That is, unless obviously incompatible or specifically excluded, each individual embodiment is deemed to be combinable with any other embodiment(s) and such a combination is considered to be another embodiment. Conversely, various features of the invention that are, for brevity, described in the context of a single embodiment, may also be provided separately or in any sub-combination. It is further noted that the claims may be drafted to exclude any optional element. As such, this statement is intended to serve as antecedent basis for use of such exclusive terminology as "solely," "only" and the like in connection with the recitation of claim elements, or use of a "negative" limitation. Finally, while an embodiment may be described as part of a series of steps or part of a more general structure, each said step may also be considered an independent embodiment in itself. [0047] Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Although any methods and materials similar or equivalent to those described herein can also be used in the practice or testing of the present invention, representative illustrative methods and materials are described herein. Additional information, which complements the teaching of the present invention, is described in PCT/US2012/03251 1, entitled "Design and Manufacture of Hydrophobic Surfaces," filed April 6, 2012, which is incorporated by reference in its entirety herein, for all purposes.

[0048] Certain embodiments of the present invention provide coating compositions, each composition comprising a plurality of hydrophobic nanoparticles dispersed in a hydrophilic solvent, said solvent further comprising an amphiphilic silicone-containing dispersing resin.

[0049] In present context, and as commonly understood, the term "hydrophobicity" connotes a property of a material, reflecting its water repellency. For present purposes, a hydrophobic material is defined as a material which, when provided as a flat surface, exhibits a water contact angle of at least 90°, and upwards to 180°. Similarly, "hydrophobic nanoparticles" are nanoparticles whose surfaces comprise a hydrophobic material. In some embodiments, at least some portion of these hydrophobic nanoparticles comprises hydrophobic surfaces; in some embodiments, "some portion" may refer to about 20 vol%, about 40 vol%, about 60 vol%, or about 80 vol% of the nanoparticles; in preferred embodiments, substantially all of the hydrophobic nanoparticles comprise or are completely covered by hydrophobic surfaces (e.g., at least 80 vol% of the nanoparticles). The nanoparticles may themselves be solid or hollow, or comprise a core-shell structure, whose inner content may be almost any material, provided the external surfaces are hydrophobic as defined herein. Representative, non-limiting materials include solid or hollow silicon-containing or polymeric nanoparticles, or core-shell nanoparticles comprising a metallic, non-metallic, ceramic, and/or semi-conductor core and a hydrophobic silicone-containing or polymer shell; e.g., coated with an alkyl silane or fluorosilane.

Hydrophobic silicone-containing materials may comprise a polysilicone, polysiloxane, polysilicate, such as fused silica, and/or a fluorosilane. Silicone-containing polymers (e.g., having a polysiloxane backbone) may have optionally fluorinated aliphatic or aromatic groups or fluorosilanes as side chain. Hydrophobic organic polymers include, but are not limited to, alkyds, epoxies, polyacrylates, polyalkenes, polyisocyanates, polyurethanes, and fluorinated and perfluorinated polymers. Non-limiting examples of partially fluorinated and perfluorinated polymers, include TEFLON™ PTFE, TEFLON™ FEP, and TEFLON™ PFA, which comprise mixtures of fully fluorinated polypropylene and polyethylene polymerized monomers, available from DuPont Polymer Products Department, Wilmington, DE. Fluorinated and perfluorinated polymers and copolymers are also available under the tradename CYTOP™, a trademark of Asahi Glass Co., Ltd. of Japan, available from Bellex International Corp., Wilmington, DE.

[0050] In specific embodiments, at least some of the plurality of hydrophobic nanoparticles in the coating compositions comprise or are completely covered by an alkane surface (such as exists in some commercially available fumed silica nanoparticles), a silica surface, or alkylsilane, phenylsilane, fluorosilane, or fluorinated polymer surface. In some embodiments, "some of the plurality" may refer to at least 20 vol%, at least 40 vol%, or at least 60 vol% of the nanoparticles; in preferred embodiments, substantially all of the hydrophobic nanoparticles comprise or are completely covered by an alkane surfaces (e.g., at least 80 vol% or at least 90 vol%). The hydrophobic nanoparticles in the coating compositions may also comprise a combination of particles and/or particle types, each having the same or different surface compositions or types.

[0051] In separate embodiments, the plurality of hydrophobic nanoparticles comprise hydrophobic nanoparticles having the same chemical compositions - e.g., mixtures of hollow, solid, and/or core-shell structures, each of different chemical compositions. In other

embodiments, the plurality of hydrophobic nanoparticles comprises nanoparticles having different chemical compositions. Coating compositions which contain the same type and structure of hydrophobic nanoparticles appear to be preferred.

[0052] In the broadest context, the term "nanoparticle" refers to a particle having at least one dimension in the nanoscale dimension (i.e., in a range of about 1 nm to about 1000 nm). In the present invention, however, certain embodiments contemplate a narrower particle size range. That is, in certain embodiments, at least some of the plurality of hydrophobic

nanoparticles has a mean cross-sectional dimension in a range of about 5 nm to about 300 nm. In other embodiments, substantially all of the hydrophobic nanoparticles in the composition have a mean cross-sectional dimension within this range. In separate embodiments, the coating composition (and the coating derived therefrom) comprises a plurality of particles having a mean cross-section dimension or diameter in a range independently bounded at the lower end of the range by 1 nm, 5 nm, 10 nm, 15 nm, 20 nm, or 25 nm, and at the upper end of the range by 1000 nm, 300 nm, 250 nm, 200 nm, 175 nm, 150 nm, 125 nm, or 100 nm. Exemplary embodiments within these ranges include those ranges of from 5 nm to 200 nm, from 5 nm to 50 nm, from 10 nm to 200 nm, or from 150 nm to 200 nm.

[0053] The nanoparticles may be of any shape. Non-limiting examples include needles, cubic, tetrahedral, octahedral, icosahedral, oblate spheroid, or substantially spherical. Non- needle-shaped particles are preferred. To the extent that a given particle or population of particles deviates from a purely spherical shape, such that each particle can be described as having a major and minor axis, the present invention includes embodiments wherein the ratio of the lengths of the major and minor axis of each particle can be about 2, less than 2, less than 1.5, less than 1.3, less than 1.2 or less, less than 1.1, or less than 1.05 or less than 1.02, for example, to 1. The term "substantially spherical" refers to a shape wherein the ratio of major / minor axis less than 1.1. Similarly, as used herein, where the particles are other than purely spherical, the term "mean diameter" or "mean cross-sectional dimension" refers to the arithmetic average of the lengths of the major and minor axes of the particles.

[0054] The particle sizes and distributions of the present invention may be

characterized separately in terms of their ability to transmit light, with minimal or no scattering, when incorporated in a finished coating as described herein. In such independent embodiments, the nanoparticles are of a size and distribution so as to transmit at least 50%, at least 75%, at least 80%, at least 85%, at least 90%, at least 98%, at least 99% or substantially all of incident light at the wavelength or wavelength range of interest. Such light may be within the ranges of infrared, visible, or ultraviolet light. As used herein, the term refers "visible light" to that portion of the electromagnetic spectrum that is visible to (or can be detected by) the human eye, typically in a range of wavelengths from about 390 to about 750 nm. Infrared light is that light having wavelengths higher than the high end of this range, and ultraviolet refers to light having wavelengths at wavelengths lower than the low end of this range. The ability to transmit light in the visible range is preferred.

[0055] Because light is scattered by particles that are large relative to the wavelength of the light, it is important for transparent coatings (and so coating compositions which are used to prepared such coatings), that not only the mean diameter, but also the distribution of particles be such as to minimize such an effect. Accordingly, in certain independent embodiments, the coating compositions contain particle aggregates having mean cross-section dimensions in the ranges described above for individual nanoparticles. That is, to the extent that a coating composition contains nanoparticle aggregates, these aggregates have mean cross-sectional dimensions in a range of about 5 nm to about 300 nm, so as not to scatter visible light. Further, in separate embodiments, any aggregates which exist within the coating compositions account for less than 20%, less than 10%, less than 5%, or less than 1 2% of the volume of the particles, as determined by light scattering experiments.

[0056] The compositions (and corresponding coatings) also may be characterized by distribution parameters beyond simple mean cross sectional dimensions. So as to maintain high transmittance in the visible light regions by minimizing the scattering by the particles, no more than about 10% by volume of the nanoparticles (or aggregates) should have mean particles dimensions greater than about 100 nm. In some cases, less than 2% by volume of the nanoparticles should have mean particle dimensions greater than 200 nm. For example, in certain independent embodiments, where the coating is intended to transmit visible light over a range of about 400 nm to about 800 nm, the nanoparticles are present in a distribution in which less than 10%, less than 8%, less than 6%, less than 4%, or less than 2% by volume of the hydrophobic nanoparticles have mean particle size dimensions greater than 100 nm. Similarly, in certain independent embodiments, where the coating is intended to transmit visible light over a range of about 400 nm to about 800 nm, the coating composition contains nanoparticles having about 2%, less than 2%, less than 1%, less than 0.5% or less than 0.1% by volume, or practically no nanoparticles having mean particle size dimensions about 200 nm or greater.

[0057] In other embodiments, the coating compositions may be characterized by the modality and the polydispersity about the mean(s). That is, in certain embodiments, the plurality of hydrophobic nanoparticles is characterized as having a monomodal particle size distribution (i.e., having a single Gaussian or Gaussian-like size distribution around a single mean).

Independently, such a monomodal distribution may exhibit a polydispersity (defined as the standard deviation in the particle diameter for a given mode divided by the mean particle diameter of that mode) of greater than 50%, about 50%, less than 50%, less than 25%, less than 10%, less than 5%, to about 1%. That is, a given monomodal particle size distribution may be broadly distributed or monodispersed (i.e., having a low degree of polydispersity). A broad particle size distribution may be preferred, for example, for compositions intended to be used in spray coating (e.g., broader distributions of differently sized nanoparticles may help forming network in spray coating, thereby preventing water contacting the underlying substrate), whereas a monodispersed particle size distribution may be preferred for a composition intended for spin coating (i.e., in spin coating, narrow particle size distribution may be preferred to form nearly closed packed structure to avoid exposure of the underlying substrate to water). The skilled artisan would understand and select the preferred distribution for his/her particular application.

[0058] In separate embodiments, the coating compositions may comprise hydrophobic nanoparticles having a bimodal, trimodal, or polymodal distributions particle size distribution (i.e., having a plurality of such Gaussian or Gaussian-like size distribution around multiple means), each mode having a polydispersity as described above. Each modal distribution may again comprise nanoparticles of the same or different chemical composition or coating surface. [0059] The invention also contemplates particular nanoparticle size loadings within the coating compositions. In certain embodiments, for example, the concentration of the

hydrophobic nanoparticles in the composition is in a range of about 0.5 to about 200 mg nanoparticles per mL of composition. Independent embodiments provide that the concentration of hydrophobic nanoparticles are in a range, in which the lower end of the range is about 0.5, about 1, about 2, about 3, about 4, about 5, about 10, about 25, about 50, or about 100 mg nanoparticles per mL of composition, and the upper end of the range is about 200, about 150, about 125, about 100, about 75, about 50, or 25 mg nanoparticles per mL of coating composition. Exemplary, non-limiting, embodiments, then, include those compositions in which the concentration of hydrophobic nanoparticles is in a range of about 0.5 to about 50 mg

nanoparticles per mL of coating composition or about 1 to about 25 mg nanoparticles per mL of coating composition.

[0060] Contrasting "hydrophobicity," as commonly understood, the term

"hydrophilicity" connotes a property of a material, reflecting its attraction to water; i.e., a "hydrophilic moiety" or "hydrophile" is molecule or other entity that is one that has a tendency to interact with or be dissolved by water and other polar substances. In certain embodiments of the present invention, a hydrophilic solvent refers to a solvent, generally a polar solvent, able to dissolve in water to form aqueous solutions holding at least 40%, at least 50%, at least 60%, at least 80%, or at least 90% by weight of the solvent, relative to the weight of the total aqueous solution, at ambient temperatures and pressures. In certain spin coating compositions, the solvent comprises mainly a high boiling (e.g., polyglycol) solvent for a uniform coating, so that the solvent does not evaporate too fast during spin coating.

[0061] In other embodiments, the solvents include C_1-4 alcohols or polyglycols. In sprayable compositions, the solvent is preferably volatile, so as to provide for quick drying of the applied pre-cured coating, thus leaving a uniform coating. Accordingly, in certain other embodiments, the solvent comprises at least one C_1-4 alcohol, cumulatively present in at least 50% by volume to about 98% by volume of the total solvent composition (i.e., more than 50 vol% comprises C_1-4 alcohols). Some amount high boiling point solvent may be useful in tuning the evaporation speed. Isopropanol is a preferred hydrophilic solvent in the present application, especially for such sprayable coating compositions. The solvent may also comprise added water, to the extent that the concentration does not compromise the ability of the amphiphilic dispersing resin from maintaining the hydrophobic nanoparticles dispersed in the composition. In separate embodiments, the solvent may also contain one or more higher boiling solvent, such as 2- butoxyethanol (1-3 %), which has been used in existing spray type cleaner, to tune the volatility of the coating, thus, the uniformity of the coating. Sprayable compositions may also contain hydrocarbons (e.g., propane and n-butane (1-3%)) or other propellants may also be used as a dispersant.

[0062] In other embodiments, the solvents used in the coating compositions are predominantly water; i.e, the solvent comprises water in a range of from about 50 vol% to about 98 vol% of the total solvent composition. In other independent embodiments, water may be present the solvent in a range having a lower value of about 60 vol%, 70 vol%, 80 vol%, or 90 vol% and having an upper value of about 98 vol%, 95 vol%, or 90 vol%.

[0063] As used in the present context, the term "amphiphilic" carries its generally accepted meaning of having both hydrophobic and hydrophilic moieties. In the present context, an "amphiphilic dispersing resin" is intended to connote an oligomeric or polymeric material capable of dispersing hydrophobic particles in a hydrophilic solvent, preferably water or a lower alcohol. An amphiphilic resin will contain both hydrophilic (e.g., hydroxyl or carboxylate groups) and hydrophobic moieties (e.g., alkyl and phenyl groups) either directly or via a linking chemical group (e.g., optionally fluorosubstituted alkyl, alkoxy, aromatic, or carboxylate moieties, including fatty acid or ester moieties) to oligomer or polymer backbone (preferably Si- O-containing backbone). The at least one hydrophilic moiety has an affinity for the hydrophilic solvent and the at least one hydrophobic moiety is compatible with the hydrophobic

nanoparticles. In the present context, the amphiphilic dispersing resin carries the additional requirement that when dehydrated, for example upon heating (or some chemical agent), the resulting material converts to a hydrophobic surface; e.g., comprising polyalkyl- or

polyarylsilsesquioxane frameworks. Such an amphiphilic resin may comprise a partially hydrated polyalkyl- or polyarylsilsesquioxane precursor; alternatively / additionally, it may comprise a partially polymerized alkyl or aryl silicone (e.g., partially polymerized methyl triethoxysilane or methyltrimethoxysilane; acid catalyzed), the alkyl or aryl groups conferring the hydrophobicity. In certain preferred embodiments, amphiphilic resins contain silicon - for example, are based on polysilicone, polysiloxane, and/or polysilicate chemistries.

[0064] In certain embodiments, the amphiphilic silicone-containing dispersing resin of the coating composition comprises an annealable glass resin, including optionally fluorine- substituted annealable resin. GR653L, available as a glass resin dispersed in a mixed alcohol solvent from Techneglas, Perrysville, Ohio, is an exemplary and preferred material in this context. Other resin materials available from Techneglas, such as GR100F, GR150F, GR630L, GR650F, GR651L, GR653L, GR653LPP, and GR654L, may also be used in this capacity, either as is or after partial acid-catalyzed polymerization, as may other materials of comparable or analogous chemistry.

[0065] In various embodiments, the concentration of the amphiphilic silicone- containing dispersing resin in the composition is in a range of about 0.001 to about 10% by volume, relative to the total volume of the composition. Additional independent embodiments include those compositions where the concentration of the amphiphilic silicone-containing dispersing resin is in a range having a lower boundary of about 0.001%, about 0.01%, about 0.1%, about 0.5%, about 1%, or about 2% by volume of the total composition, and having an upper boundary of about 10%, about 5%, about 4%, about 3%, or about 2% by volume of the total composition. Exemplary non-limiting embodiments include those ranges of about 0.1% to about 5%, about 0.55 to about 2%, or about 1% by volume, relative to the total volume of the composition.

[0066] In other embodiments, the concentration of the amphiphilic silicone-containing dispersing resin may be described on a weight percent basis, such as where the concentration of the amphiphilic silicone-containing dispersing resin is in the composition is in a range of about 0.001% to about 10% by weight, relative to the total volume of the composition. Additional independent embodiments include those compositions where the concentration of the

amphiphilic silicone-containing dispersing resin is in a range having a lower boundary of about 0.001%, about 0.01%, about 0.1%, about 0.5%, about 1%, or about 2% by weight of the total composition, and having an upper boundary of about 10%, about 5%, about 4%, about 3%, or about 2% by weight of the total composition. Exemplary non-limiting embodiments include those ranges of about 0.1% to about 5%, about 0.5% to about 2%, or about 1% by weight, relative to the total weight of the composition.

[0067] Without being bound by the correctness of any particular theory, it is believed that the ratio of the nanoparticle and amphiphilic silicone-containing dispersing resin within the composition is also important for the proper performance of the final coatings, and that it is generally preferred that the weight (or volume) of the nanoparticles is greater than the weight (or volume) of the resin (assuming a comparable density between the nanoparticles and resin).

Accordingly, independent embodiments provide that the ratio of the nanoparticles to resin is in a range of about 100: 1 wt/wt to about 1 : 1 wt/wt, or about 50: 1 to about 1 : 1, or about 20: 1 to about 1 : 1, or about 10: 1 to about 2: 1, or about 8: 1 wt/wt to about 2: 1 wt/wt nanoparticles/resin. It appears that, for the system involving AEROSIL™ NP nanoparticles and GR653L described below, a preferred embodiment provides a ratio of about 5: 1 wt/wt nanoparticles/resin. The skilled artisan would be able to optimize these ratios for a given nanoparticle / resin combination. [0068] To this point, the invention has been described in terms of a coating

composition, with some reference to its application to a surface and its ultimate formation into a superhydrophobic coating. It should be appreciated that each of these methods of application and superhydrophobic coating are also elements of the present invention.

[0069] That is, separate embodiments provide methods of preparing superhydrophobic coatings, each method comprising: (a) applying to a substrate a coating composition comprising a plurality of hydrophobic nanoparticles dispersed in a hydrophilic solvent comprising an amphiphilic silicone-containing dispersing resin; and (b) effecting removal of at least a portion of the hydrophilic solvent so as to form an adherent layer of an amphiphilic pre-cured coating layer on the substrate. During removal of at least a portion of the solvent, the amphiphilic resin may or may not experience some level of condensation (e.g., cross-linking with accompanying loss of pendant hydroxyls). In various embodiments, the coating composition of these methods may include any of the coating compositions described herein. The coating composition may be applied one or more times to a substrate, depending on the thickness of the final coating desired. Where multiple layers of the pre-cured coating are applied, it is preferred, but not required, that the removal of at least a portion of the hydrophilic solvent be effected before the application of a subsequent layer of the coating composition.

[0070] The coating compositions and methods of applying them are flexible with respect to the nature of the substrates to which they can be applied. Such substrates may include, for example, polymers (e.g., polycarbonate), glass (e.g., silicates and borosilicates), semiconductors (e.g., silicon), metals, or any combination thereof. Most semi-conductor materials and metals contain oxide layers to which the hydrophilic portions of the amphiphilic resins are attracted and adhere. Similarly, polymers, especially polar polymers, provide attraction sites for the hydrophilic portions of the amphiphilic resins. No particular chemical pretreatment of the surfaces appears to be required, though the surfaces should be clean - i.e., free of chemical contaminants and particulate debris.

[0071] The coating compositions may be applied by any conventional method known in the art for applying coatings, including brush coating, (sol-gel) dip coating, drop-casting, spin coating, and spray coating, and the invention contemplates both compositions and methods of applying said compositions which have been adapted for each of these application methods. In particular, the present compositions are especially adaptable and suitable for spray coating, which allows the compositions to be applied effectively to large areas - e.g. 250 cm x 250 cm. In such cases, broad particle size distributions (i.e., higher polydispersity) and more volatile solvents are believed to be preferred. [0072] Once applied, the coating compositions provide a pre-cured coating layer which may be characterized either as hydrophilic or hydrophobic, depending on the balance of hydrophilicity / hydrophobicity in the amphiphilic resin. These pre-cured coating layers may then be converted to superhydrophobic coatings with the application of energy or chemical agent (though such application is not always required). That is, in various embodiments, the methods of preparing superhydrophobic coatings further comprise applying sufficient energy for a sufficient time to the pre-cured coating layer so as to convert the amphiphilic pre-cured coating layer to a cured superhydrophobic layer. The term "thermal curing" is intended to connote application of heat so as to raise the temperature of the coating layer to one higher than that used for drying (e.g., the latter being about 40°C to about 80°C). Using the coating compositions and methods described herein, said superhydrophobic layer comprise the plurality of hydrophobic nanoparticles embedded within a layer of cross-linked hydrophobic silicone-based polymer. In some embodiments, the layer of cross-linked hydrophobic silicone-based polymer is a glassy layer. These methods are particularly attractive because the preparation of the superhydrophobic surfaces can be prepared economically, over large areas, and without the need for chemical modifications to the surface before the application of the inventive coating compositions or after its cure.

[0073] As described herein, the energy may include thermal energy. The degree of heat applied to the coating can be as low as 140°C, allowing for the use of polymers (e.g., polycarbonate) as substrates. Higher curing temperatures (e.g., above 160°C, to about 500°C) may also be used on higher melting substrates (e.g., glass, silicon, metals). Since in many embodiments, the curing is a dehydration of the pre-cured coating layer, the heating may be done in either air or under inert atmosphere. In other embodiments, the energy may include radiant energy (e.g., UV light), depending on the groups pendant to the amphiphilic silicone-containing resin.

[0074] In certain applications, for example spray drying, better results are seen when diluter dispersions are applied in multiple layers. Accordingly, in certain independent embodiments, the coating compositions may be spray applied in 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12 coating cycles or more. In the Examples shown herein, good results were obtained for samples spray coated with three to ten layers. The preferable coating cycles depended on the solution compositions. The higher of the concentration of the solution, the fewer cycles of coating are needed to achieve good performance. In certain cases, when so applied, it is not necessary to apply any additional thermal annealing (i.e., above ambient temperatures) to achieve the desired adhesion and suphydrophobicity. [0075] Separate independent embodiments include those superhydrophobic coatings which result from the methods described herein. Once cured, the superhydrophobic coating may be characterized by its degree of superhydrophobicity (i.e., water contact angle) and/or its transparency. The present invention also provides separate embodiments wherein a

superhydrophobic layer comprise the plurality of hydrophobic nanoparticles (as described herein) embedded within a layer of cross-linked hydrophobic silicone-based polymer (as described herein, glassy or otherwise), which coatings are not necessarily, but may be, derived from the processes described herein. For example, a superhydrophobic coating exhibiting an architecture shown in any one of FIGs. 4, 5A-B, or 6 is considered within the scope of the present invention.

[0076] In independent embodiments, depending on the choice of nanoparticles and degree of curing, the superhydrophobic coating is characterized as transmitting at least about 50%, at least 60%, at least 70%, at least 80%, at least 90%, at least 95%, or at least 98% of incident light of the wavelengths in a range of 400 nm to 800 nm.

[0077] In other independent embodiments, the cured superhydrophobic coating is characterized as exhibiting a water contact angle of at least 150°, at least 155°, at least 160°, or at least 165°. In other independent embodiments, the cured superhydrophobic coating is characterized by a water contact angle hysteresis or roll-off angle of less than 10°, less than 5°, or less than 1°.

[0078] In still further independent embodiments, the superhydrophobic coating comprises nanoparticles conjoined by cured silicone-based polymer, wherein the ratio of the nanoparticles to silicone-based polymer is in a range of from 100: 1 wt/wt to 1 : 1 wt/wt, or from 50: 1 to 1 : 1, or from 20: 1 to 1 : 1, or from 10: 1 to 2: 1, or from 8: 1 wt/wt to 2: 1 wt/wt

nanoparticles/silicone-based polymer.

[0079] The superhydrophobic coatings of the present invention also are sufficiently robust as prepared to allow a user to handle them while maintaining their integrity. Certain embodiments provide that the superhydrophobic coatings can pass the so-called "Scotch tape test," as provided by ASTM D3359-09e2 ("Standard Test Methods for Measuring Adhesion by Tape Test"). In separate embodiments, less than 25%, less than 20%, less than 15%, less than 10%, or less than 5% of the superhydrophobic coating is removed by the application of this test. In other embodiments, the superhydrophobic coating retains its superhydrophobicity (water contact angle of at least 150°) after application of this test.

[0080] In still other embodiments, the superhydrophobic coating remains

superhydrophobic after a water drop test, in which ca. 1000 water droplets (each ca. 80 microliters) are dropped from about 1 foot above the sample and the coating still maintain the requisite high water contact angle. Furthermore, in ceratin embodiments, the coating remains superhydrophobic after high-pressure water jetting, for example, where 10 to 50 kPa water jects from 2 inches above the tilted sample (45°) for 1 to 10 minutes.

[0081] In addition to coating compositions, methods of applying said compositions, and the resulting superhydrophobic coatings, the present invention contemplates articles comprising these superhydrophobic coatings. In certain of these embodiments, the superhydrophobic surface surmounts and adheres to a polymer (e.g., polycarbonate), glass (e.g., silicate and borosilicate), semi-conductor (e.g., silicon), paper, concrete, metal, or any combination thereof. Preferred embodiments are those where the superhydrophobic surface surmounts and adheres to a polycarbonate, a silicate or borosilicate glass, (e.g., silicate and borosilicate), or an oxidized silicon wafer. Further embodiments include those where the superhydrophobic coating coats the surface of a solar cell or a mirror or transparent window glass. Additional embodiments include those energy storage devices comprising a solar cell comprising the superhydrophobic coating.

[0082] The following listing of embodiments in intended to complement, rather than displace or supersede, the previous descriptions.

[0083] Embodiment 1. A coating composition comprising: a plurality of hydrophobic nanoparticles dispersed in a hydrophilic solvent, the hydrophilic solvent comprising an amphiphilic silicone-containing dispersing resin.

[0084] Embodiment 2. The coating composition of Embodiment 1, wherein at least some of the plurality of hydrophobic nanoparticles comprise an alkane, an alkylsilane, a phenylsilane, a fluorosilane, a fluorinated polymer surface, or any combination thereof.

[0085] Embodiment 3. The coating composition of Embodiment 1 or 2, wherein at least some of the plurality of hydrophobic nanoparticles have a mean cross-sectional dimension in a range of 5 nm to 200 nm.

[0086] Embodiment 4. The coating composition of Embodiment 3, wherein less than 10% by volume of the hydrophobic nanoparticles have a cross-sectional dimension greater than about 100 nm.

[0087] Embodiment 5. The coating composition of any of the preceding

Embodiments, wherein less than 1% by volume of the hydrophobic nanoparticles have a cross- sectional dimension greater than about 200 nm

[0088] Embodiment 6. The coating composition of any of the preceding Embodiments, wherein at least some of the plurality of hydrophobic nanoparticles differs from one another in material composition. [0089] Embodiments 7. The coating composition of any of the preceding

Embodiments, wherein the plurality of hydrophobic nanoparticles is characterized as having a monomodal particle size distribution.

[0090] Embodiment 8. The coating composition of any of the preceding Embodiments, wherein the concentration of the hydrophobic nanoparticles in the composition is in a range of about 0.5 to about 50 mg of nanoparticles per mL of coating composition.

[0091] Embodiment 9. The coating composition of any of the preceding Embodiments, wherein the solvent comprises at least one C_1-4 alcohol, cumulatively present in a range of from about 50% to about 98% by volume of the total composition.

[0092] Embodiment 10. The coating composition of any one of Embodiments 1 to 8, wherein the solvent comprises water, present in a range of from about 50% to about 98% by volume of the total composition.

[0093] Embodiment 11. The coating composition of any of the preceding

Embodiments, wherein the amphiphilic silicone-containing dispersing resin comprises an annealable glass resin.

[0094] Embodiment 12. The coating composition of any of the preceding claims, wherein the concentration of the amphiphilic silicone-containing dispersing resin in the composition is in a range of about 0.01 to about 10 volume percent, relative to the volume of the composition.

[0095] Embodiment 13. The coating composition of any of the preceding

Embodiments, wherein the ratio of hydrophobic nanoparticles to amphiphilic silicone-containing dispersion resin is in a range of about 2: 1 to about 10: 1 wt/wt.

[0096] Embodiment 14. The coating composition of any of the preceding

Embodiments, wherein the composition is adapted for spray coating.

[0097] Embodiment 15. A method of preparing a superhydrophobic coating, said method comprising:

(a) applying, to a substrate, a coating composition comprising a plurality of hydrophobic nanoparticles dispersed in a hydrophilic solvent comprising an amphiphilic silicone-containing dispersing resin; and

(b) effecting removal of at least a portion of the hydrophilic solvent so as to form an adherent layer of an amphiphilic pre-cured coating layer on the substrate.

[0098] Embodiment 16. The method of Embodiments 15, wherein the coating composition comprises the coating composition of any one of Embodiments 1 to 14. [0099] Embodiment 17. The method of Embodiment 15 or 16, wherein the substrate comprises a polymer, glass, semi-conductor, metal, or any combination thereof.

[0100] Embodiment 18. The method of any one of Embodiments 15 to 17, wherein the coating composition, adapted for spray coating, is applied by spray coating at least one coating layer to the substrate, wherein the concentration of the hydrophobic nanoparticles in the composition is in a range of from about 1.5 mg to about 50 mg of nanoparticles per mL of coating composition.

[0101] Embodiment 19. The method of any one of Embodiments 15 to 18, wherein the coating composition, having solvent comprising water present in a range of from about 50% to about 98% by volume of the total solvent composition, adapted for spray coating, is applied by spray coating at least two coating layers to the substrate.

[0102] Embodiments 20. The method of any one of Embodiments 15 to 19, wherein the coating composition, having solvent comprising water present in a range of from about 80% to about 98% by volume of the total solvent composition, adapted for spray coating, is applied by spray coating between six and ten coating layers to the substrate, preferably about 8 coating layers.

[0103] Embodiment 21. The method of any one of Embodiments 15 to 20 further comprising applying sufficient energy for a sufficient time to the pre-cured coating layer so as to convert the amphiphilic pre-cured coating layer to a cured superhydrophobic layer, said superhydrophobic layer comprising the plurality of hydrophobic nanoparticles embedded within a layer of cross-linked hydrophobic silicone-based polymer.

[0104] Embodiment 22. The method of Embodiment 21, wherein the energy is thermal energy.

[0105] Embodiment 23. A superhydrophobic coating prepared using the method of any one of Embodiment 15 to 22.

[0106] Embodiment 24. A superhydrophobic coating comprising a plurality of hydrophobic nanoparticles embedded within a layer of cross-linked hydrophobic silicone-based polymer.

[0107] Embodiment 25. The superhydrophobic coating of Embodiments 23 or 24, wherein the superhydrophobic coating is characterized as transmitting at least 50% of incident light of the wavelengths in a range of about 400 nm to about 800 nm.

[0108] Embodiment 26. The superhydrophobic coating of Embodiment 25 wherein the cured superhydrophobic coating is characterized as transmitting at least 95% of incident light of the wavelengths in a range of about 400 nm to about 800 nm. [0109] Embodiment 27. The superhydrophobic coating of any one of Embodiments 23 to 26, wherein the cured superhydrophobic coating is characterized as exhibiting a water contact angle of at least 150°.

[0110] Embodiment 28. The superhydrophobic coating of any one of Embodiments 23 to 27, comprising hydrophobic nanoparticles conjoined by cured silicone-based polymer, wherein the ratio of the nanoparticles to cured silicone-based polymer is in a range of about 100: 1 wt/wt to about 1 : 1.

[0111] Embodiment 29. The superhydrophobic coating of any one of Embodiments 23 to 28, characterized by an integrity such that it passes the Adhesion Tape Test of ASTM D3359 - 09e2 Standard Test Methods for Measuring Adhesion by Tape Test

[0112] Embodiment 30. The superhydrophobic coating of any one of Embodiments 23 to 29, wherein the superhydrophobic coating surmounts and adheres to an oxidized silicon wafer.

[0113] Embodiment 31. A solar energy cell comprising a superhydrophobic coating according to any one of Embodiments 23 to 30.

[0114] Embodiment 32. An energy storage device comprising a solar energy cell according to Embodiment 31.

[0115] EXAMPLES

[0116] The following Examples are provided to illustrate some of the concepts described within this disclosure. While each Example is considered to provide specific individual embodiments of composition, methods of preparation and use, none of the Examples should be considered to limit the more general embodiments described herein.

[0117] In the following examples, efforts have been made to ensure accuracy with respect to numbers used (e.g. amounts, temperature, etc.) but some experimental error and deviation should be accounted for. Unless indicated otherwise, temperature is in degrees C, pressure is at or near atmospheric.

[0118] Example 1: Materials

[0119] All chemicals were used as received. Hydrophobic fumed silica nanoparticles (NPs) treated with DDS (dimethyldichlorosilane), AEROSIL™ R972 NPs ("NPs" =

nanoparticles; mean particle size 16 nm; "A-SiCV), were provided by Evonik Industries (Piscataway, NJ). Glass resins (GR-653L, GR-653LPP, GR650F) were provided by Techneglas Technical Products (Perrysburg, OH). Isopropanol (IP A), formic acid (88 wt%), and glass slides (l"x 3") was purchased from Fisher Scientific. Polycarbonate was purchased from US Plastic Corp. (Lima, OH). Note here that concentration of the glass resin refers to the resin itself, corrected for its original content (in solvent) as supplied from Techneglas. References such as "5 mg/mL" and "vol/vol%" or "wt/wt%" are given with respect to the volume of the total coating composition.

[0120] The GR653L and GR653LPP are glass resin liquid with ca. 30 %

polymethylsiloxane dissolved in n-butyl alcohol (25-45%), ethyl or methyl alcohol (10-30%), formic acid (0.6%). GR653LLP is a primerless hardcoat with added adhesion promoter. In the liquid, it also has methyl amyl ketone (0.8%), and monomethyl ether 1-2%). The GR653L and GR653LPP resins were used as supplied, so that the concentrations the used in mixing were the same as the concentrations of the commercial resin solutions. The GR650F resin is available in flake form, and was converted to useable dispersions by preparing them according to manufacturer's instructions. That is, flake GR650F was dissolved in IPA to prepare 30 wt.% solution, using formic acid as a catalyst. Formic acid was first dissolved in IPA (formic acid: IPA = 2: 1 vol/vol), and then mixed with GR650F solution (30 wt% in IPA) at a concentration of 0.2-0.5 wt/wt. The concentration of GR650F for mixing was 30 wt% IPA solution. The NPs concentration referred to the final content in the mixed solutions.

[0121] Example 2: Preparation of superhydrophobic coatings

[0122] Example 2.1: First Generation Method (single coatings from alcohol-based solutions/dispersions): In some applications, AEROSIL™ R972 NPs (to provide 5 mg nanoparticles per mL coating composition) and GR653L glass resin (0.1 vol/vol%) were dispersed by sonicating the precursors in IPA. The mixture was sonicated for 15 min before use (using a Branson 2210 Ultrasonic Cleaner, 47 kHz frequency, 100 W output power, Branson Ultrasonics Corp., Danbury, CT). The well-mixed suspension was sprayed on a substrate using an airbrush under compressed nitrogen (5 psig working pressure). The solution was sprayed in the line-to-line fashion over a given surface until the surface was fully covered. For comparison purposes, solutions with NPs only and glass resin only were also spray-coated on the glass. After spraying, the sample was annealed in oven for 1 hour. Typical annealing temperatures were ca. 70°C for compositions derived from GR653L, 85°C for those derived from GR653LPP and 90°C for those derived from GR650F.

[0123] Example 2.2: Second Generation (optimized) Method (multiple coatings from alcohol-based solutions/dispersions): AEROSIL™ R972 NPs and GR653L glass resin were individually dispersed by sonicating the precursors (as above) for 15 min. in separate equal volumes of isopropanol. The two dispersed mixtures were then combined into a single volume and combined mixture was sonicated (described as above) for an additional 15 min before use. In the mixture, the concentration was 0.5 to 50 mg/mL for A-S1O2 and 0.2 to 20 mg/mL for GR653L. The well-mixed suspension was sprayed on a substrate using an airbrush under compressed nitrogen (10-40 psig working pressure). The solution was sprayed in the line-to-line fashion over a given surface until the surface was fully covered. A single application such as this is considered one cycle of spray coating. Typically, spray coating of three cycles or more resulted in superhydrophobic surfaces. Also, holding the airbrush nozzle closer to the substrate typically resulted in a stronger adherent coating to the substrate, which could be attributed to higher impact energy of the solutions toward the substrate. For example, the initial velocity of the solution jetting from the nozzle was ca. 40 m/s at the transporting pressure of 20 psig, and it decreased to zero at the distance of ca. 30-40 cm because of air resistance. The ratio of the impulse forces of the sprayed NPCs on the surface was about 1.2 to 1.5 if the spraying distance was 5 cm and 10 cm, respectively, because the force is directly proportional to the particles velocity according to the impulse-momentum theorem. However, for a uniform coating, it is desired to have the sprayed solution coming out of the sprayer's nozzle nearly dry before arriving at the substrates. A typical distance of 5-15 cm was used in the experiments depending on the formulation of the solution, choice of solvent, transporting-gas pressure, and nozzle diameter. It was preferred that the conditions were chosen (e.g., concentration and spray distance) such that the spray was nearly dry (i.e., solvent- free) before arriving at the substrate. The spray was applied so as to avoid puddles or aggregates of the solutions on the sprayed area. Whereas in initial experiments is was necessary to thermally anneal the samples to get good performance, subsequently tuned experiment, this anneal was found not to be necessary. For comparison purposes, solutions with NPs only and glass resin only were also spray-coated on the glass.

[0124] Example 3: General Characterization Methods

[0125] Example 3.1: Deionized (D I) water contact angle (CA) measurement: The

CA was measured by Rame-Hart standard automated goniometer (Model 290). The static CA (6 ) was measured from a 5 uL deionized ("DI") water droplet. Advancing and receding CA (βαάν, dree) were measured by adding and removing water from the substrate, respectively. All water contact angle values were averaged over three measurements on different areas of each sample. The difference between the advancing CA and receding CA was referred to the contact angle hysteresis (CAH).

[0126] Example 3.2: Surface topography by atomic force microcopy (AFM) and scanning electron microscopy (SEM): The surface topography of the samples was imaged by Dimension 3100 Atomic Force Microscopy (Digital Instruments), with a S1₃N4 cantilever in tapping mode. The root mean square (RMS) roughness values were calculated from 5 μιη x 5 μιη images using nanoscope VII software.

[0127] Scanning electron microscopy (SEM) images were obtained using JEOL 7500F scanning electron microscope with accelerating voltage of 15 kV. Prior to SEM imaging, a thin gold layer was sputter-coated on the samples.

[0128] Example 3.3: UV-Vis spectroscopy: The transmittance of the composite coating on glass substrates was measured using a Varian UV-Vis-NIR Cary 5000

spectrophotometer. Bare glass was used as a reference.

[0129] Example 3.4: Water dropping and water jetting tests: Water dropping test was conducted to investigate the durability of the coating. Water was dropped from 1 foot above from the sample and hit the substrate perpendicularly continuously for 15 min. Contact angles were measured before and after water dropping to evaluate the durability of the coating. The samples prepared by second and third generation solutions were jetted under high-pressure water to examine wear resistance to water flow. For applications involving self-cleaning substrates, resistance to impingement of rain water is critical. An illustration of the water jet setup is shown in FIG. 7. Samples were exposed to the water jet at different kinetic energy levels by varying the pressure of the water ejected from the tape nozzle. The samples were placed 5 cm below the pipe and the runoff plate was tilted at 45°. The exposure time was 1 min at each pressure. After each test, the CA and RA of the samples were measured as described previously.

[0130] Example 4: Results for Coatings Derived from First Generation Alcohol-Based Solutions / Dispersions

[0131] Example 4.1: Quality of the Dip-Coated Resin-Only Surfaces: As depicted in FIG. 1, GR653L glass resin is a partially polymerized methyl silicone precursor obtained from methyltriethoxysilane (or methyltrimethoxysilane) catalyzed by acid. The resin had 30 wt% of methyl silsesquioxane (MSQ) in butanol/methanol. The GR653L has unreacted hydroxyl groups and ethoxy (or methoxy) groups, which makes the precursor hydrophilic and miscible with C_1-4 alcohols; the presence of methyl groups makes the GR653L miscible with hydrophobic fumed silica nanoparticles. Upon thermal annealing above 120°C, the resin was cured through polycondensation reactions and the hydroxyl and ethoxy (or methoxy) groups disappear, forming Si-O-Si cross-linked networks (see e.g., FIG. 1). The cured MSQ film became hydrophobic due to the methyl groups in the cross-linked network. Table 1. Water contact angles (CAs) and contact angle hysteresis (CAH) of dip coated GR653L on Si wafer with and without thermal annealing.

Annealed 6'adv (°) dree (°) CAH (°)

Yes 101.2±1.2 101.8±0.1 99.9±0.9 1.9

No 83.4±0.6 89.5±2.4 81.2±3.0 8.3

[0132] As seen in Table 1, the GR653L dip coated on Si wafer and dried at ambient condition had a static water CA of 83.4±0.6 ⁰ and CAH of 8.3 °. After annealing at 200°C for 1 h, majority of hydroxy 1 groups are consumed, leaving methyl groups exposed toward the air. The static water CA increased dramatically to 101.2+1.2 °, confirming the hydrophobic nature of the cured GR653L. Meanwhile, the CAH decreased to 1.9 °, indicating a quite uniform surface coverage of the hydrophobic methyl, CH₃, groups. Due to the extremely low CAH, the water droplet on the cured GR653L surface was highly mobile.

[0133] Example 4.2: Water Repellency of AEROSIL™ NPs / Glass Resin

Nanocomposite Coatings. The precursor compositions of the superhydrophobic coatings were prepared by first mixing a desired amount of AEROSIL™ NPs with GR653L in isopropanol (IP A). The suspension was sonicated for at least 15 min to ensure uniform dispersion. The solution was applied to a substrate by either dipping or spray coating, followed by thermal annealing at 200°C for 1 hour.

[0134] AFM phase and SEM images of the resulting surfaces are shown in FIG. 2 through FIG. 6.

[0135] FIG. 2 shows the AFM phase image of the surface of a substrate dip coated with 5 mg AEROSIL™ NPs per mL coating composition + 1 vol/vol% GR653L in isopropanol solution and annealed. The average feature size was estimated to be ca. 40 nm with glass resin wet on top and in-between the nanoparticles. Nanoparticles were found present only in the areas covered by glass resin, suggesting that glass resin worked as the wetting layer or binder between the nanoparticles and the substrate (here, glass).

[0136] Because the glass resin used here was partially polymerized methyl silicone, it contained both hydroxyl groups (hydrophilic) and methyl groups (hydrophobic). Considering its amphiphilic structure, the glass resin was able to work as a surfactant to stabilize hydrophobic AEROSIL™ NPs in hydrophilic isopropanol solvent media. Indeed, it was observed that the NPs suspension with glass resin was more stable than that without glass resin. The NP suspension remained stable in isopropanol for more than one week when glass resin was added into the solution. In contrast, the hydrophobic NPs phase separated from isopropanol solution after one day.

[0137] The wetting properties on sprayed coated films comprising nanoparticles and GR653L with various amount of GR653L was studied to optimize the coating composition. The static and dynamic water CA values were summarized in Table 2 and Table 3. In Table 2, the concentrations of NP and GR653L were varied, whereas in Table 3, the NP concentration in isopropanol was held constant at 5 mg nanoparticles per mL coating composition.

Table 2. Wetting behavior of glass substrates spray-coated with AEROSIL™ NPs +

GR653L in isopropanol of different concentrations of GR653L.

[NP] [GR653L] CAH [NP]/[GR653L] (mg/mL) 6'adv (°) dree (°)

(v/v%) o (mg/(mL-v/v%)

0.625 0.0125 146.9 ± 3.3 149.1 ± 8.2 145.3 ± 5.0 3.8 50

1.25 0.025 155.9 ± 3.7 156.2 ± 3.7 152.1 ± 5.4 4.1 50

2.5 0.05 154.8 ± 0.9 157.8 ± 1.0 151.2 ± 3.2 6.6 50

5 0 162.1±2.1 162.3±0.4 156.6±4.5 5.7 CO

5 0.01 163.0±1.1 162.2±0.2 161.9±1.2 6.3 500

5 0.05 163.7±4.1 165.2±4.7 161.0±4.3 4.2 100

5 0.1 165.2±0.4 162.6±1.8 159.6±3.5 3 50

5 0.5 160.0±1.4 160.7±0.8 156.7±1.5 3.9 10

5 1 146.4±1.5 150.3±0.8 133.4±1.0 16.9 5

50 1 156.7±0.1 157.2±5.8 151.6±8.7 5.6 50

50 10 126.7±2.1 133.6±5.1 100.6±8.4 33 5 Table 3. Wetting behavior of glass substrates spray-coated with AEROSIL™ NPs + GR653L in isopropanol of different

concentrations of GR653L, followed by annealing at 200°C for 1 hour. The concentration of nanoparticles, [NPs], was kept

constant at 5 mg/mL in isopropanol.

[GR653L]

6'adv (°) dree (°) CAH (°)

(vol/vol%)

0 162.1±2.1 162.3±0.4 156.6±4.5 5.7

0.01 163.0±1.1 162.2±0.2 161.9±1.2 6.3

0.05 163.7±4.1 165.2±4.7 161.0±4.3 4.2

0.1 165.2±0.4 162.6±1.8 159.6±3.5 3

0.5 160.0±1.4 160.7±0.8 156.7±1.5 3.9

1 146.4±1.5 150.3±0.8 133.4±1.0 16.9

[0138] Superhydrophobicity (with static and advancing water CA's above 160 ° and CAH < 6 °) was observed for samples with [GR653L] < 1 vol/vol%. At higher concentration of GR653L, it is possible that the glass resin has covered the nanoparticles, thus, decreasing surface roughness and CA. As of CAH, a typical envelop curve was observed as roughness increased and reaching the minimal CAH of 3° at 0.1 vol/vol% [GR653L]. Further increase of [GR653L] led to increase of CAH.

[0139] Corresponding AFM and SEM phase images were collected to understand the wetting behavior of the coating with different amount of glass resin (see FIG. 3 - FIG. 6 for SEM images). AFM phase images (not shown) indicated that the spray coated hydrophobic nanoparticles (i.e., without glass resin) only did not completely cover the underlying substrate. Since the nanoparticles were hydrophobic, they did not wet well on a hydrophilic glass substrate, leaving nanoparticle aggregates and non-wetted islands. The exposed hydrophilic substrate would trap water, thus, decreasing water CA and increasing CAH. Due to aggregation of nanoparticles on substrate, surface roughness was increased.

[0140] When a small amount of glass resin was added (0.01 vol/vol% GR653L, FIG. 4), the surface feature size increased to ca. 100 nm due to aggregation of individual particles.

However, some regions of the substrate were still exposed.

[0141] Further increase of the glass resin concentration (0.1 vol/vol% GR653L, FIG. 5A) led to increase of nanoparticle aggregates due to preferred wetting between glass resin and NPs. However, the substrate appeared completely covered with glass resin. These results suggested that cured silicone-based polymer served as a buffer layer between nanoparticles and glass substrate; it enhanced the coverage of hydrophobic coating on an intrinsic hydrophilic substrate. FIG. 5A-B shows the quality of the surface coating.

[0142] At the highest glass resin concentration (lvol/vol% GR653L, FIG. 6), the largest feature size (ca. 200 nm) was observed. Meanwhile, the surface roughness decreased. As a result, the water CA decreased. The substrate was completely covered by the nanoparticle / GR653L nanocomposite film.

[0143] The roughness index (r) is defined as the ratio between the actual surface area and projected surface area. Roughness values were calculated from AFM images using

Gwyddion 2.25 AFM software, after which the theoretical Wenzel contact angles (#^w) from Wenzel equation, cos#^w = rcos#o were estimated, where £¾ is the equilibrium contact angle on a flat solid, or Young's contact angle, defined by °°^S ^^sv ^ ^ where y_sv, y_si, and Yi_v are the interfacial tension of the solid- vapor, the solid-liquid, and the liquid-vapor, respectively. In the present experiments, the cured resin exhibited a θο = 101.2°. The concentration of glass resin, 0_st, r and 6^W of the above four samples are summarized in Table 4. The decreased r value confirmed that the surface smoothed when more glass resin was used. It is clear that all measured 0_st's are much larger than the theoretical 6^W s, suggesting that the nanocomposite films were in Cassie-Baxter non-wetting state.

[0144] Example 4.3: Durability of AEROSIL™ NPs / GR653L Nanocomposite Coatings: The durability of the coating was tested by water dropping experiments. Tap water droplets were dropped at a rate of about 1 drop per second onto the samples from a height of about 1 foot for 15 minutes. The CA and CAH were measured and compared to the original one (see FIG. 8). The smaller the change of CA and CAH indicated a more durable coating. The 9st (referred as CA in Figures) dropped dramatically from 162.1 ⁰ to 24.2 ⁰ on substrates coated with the hydrophobic NPs only (see leftmost points, FIG. 8) since there was no chemical bonding between NPs and NP with the substrate due to no pre- and post-surface treatment. The NP coating was very fragile. When glass resin was added, a small change of water CA, ca. 10 ⁰ decrease was observed after water dropping test with 6st remained 145 °~150 °. The CAH also increased slightly (ca. 5 to 10 °).

[0145] Example 4.4: Optical Characterization of the AEROSIL™ NPs / GR653L Nanocomposite Coatings: The transparency of spray coated superhydrophobic coating was characterized by UV-Vis spectroscopy, across the wavelength range 400 to 800 nm (see FIG. 9). Compared to the glass substrate, the glass coated with AEROSIL™ NP only has higher transparency. The Rayleigh scattering due to small particle size reduces the reflection at the interface, thus, increasing the transparency. When GR653L was added, slightly decrease on transparency was observed, presumably due to increase of nanotexture size, which was still much smaller than the wavelength of visible light (400 nm ~ 800 nm). Nevertheless, the overall transmittance, compared to the uncoated glass, is equal or higher than glass substrates, given to be 100%.

[0146] Example 4.5: Superhydrophobic Coatings from First Generation Aqueous Mixtures: Additional experiments were conducted to assess the compatibility of dispersing AEROSIL™ NPs and GR653L in aqueous solvents. First, 5 mg/mL AEROSIL™ 972 was dispersed in isopropanol and sonicated for 5 min. Then different amount of water was added to test the stability of the suspension. The suspension was found homogeneous and appeared clear until water constituted more than 50 vol% of the solvent mixture (see FIG. 10). When sufficient water was added such that the solvent comprised 75 vol% water, the appearance of solid particles in the suspension was observed, suggesting the phase separation of the hydrophobic NPs from the water.

[0147] Having determined that AEROSIL™ NPs could be dispersed in

isopropanol/water mixture, various ratios of NPs and GR653L were dispersed in

isopropanol/water solution, with water being 25, 50 and 75 vol% of the entire mixture, and these dispersions used to prepare superhydrophobic coatings. The mixed suspensions were spray coated on a glass substrate and the water CA and CAH were measured (see FIG. 11). Compared to the results from pure isopropanol solvent (see Tables 2 and 3), addition of water did not seem to affect the water CA, which remained at ca. 160 °. However, the CAH increased slightly to about 8 ⁰ when water volume fraction was 75 vol%. [0148] Example 4.6: Superhydrophobic Coatings on a Polycarbonate Substrate:

Since the superhydrophobic coating was prepared from a mixture in alcohol or mixed solvent of isopropanol and water, the coating solution could be used polymeric substrates, such as polycarbonate. To demonstrate this, the NP/GR653L mixture in isopropanol was spray coated onto a polycarbonate substrate, followed by thermal annealing at 140°C for lh. The annealing temperature was slight below the T_g of PC (ca. 145 °C) to avoid softening of the substrate. A higher annealing temperature, 200°C, was used for glass substrate. The optical image of a beaded up water droplet on the coated PC was shown in FIG. 12. No visible damage of the substrate was observed.

[0149] Example 5: Results for Coatings Derived from Second Generation Alcohol-Based Solutions / Dispersions

[0150] Example 5.1: Quality of the Dip-Coated Resin-Only Surfaces

[0151] In separate experiments the hydrophobic coatings were directly obtained after spray coating the glass resin on glass without heat treatments (see Table 5). Here, low- concentration glass resin solution (0.2-20 mg of as-received glass resin per mL) were sprayed instead of dipping the substrate in the as-received glass resin solution, as describe previously in Example 4.1. Dip coating usually caused a thick film of glass resin, thus requiring additional heat treatment. However, at a low resin concentration (0.2-20 mg/mL), small resin droplets were formed due to good solubility in IPA, and partially cured at ambient environment during their transportation from the spray nozzle to the substrate. During spraying, the curing process was accelerated under high pressure at a low resin concentration. As seen in Table 5 spraying of two cycles was enough to make the surface hydrophobic. Further spraying did not improve hydrophobicity. The CAH (contact angle hysteresis) of air-dried coatings was more than 10° and the droplet mobility was not very good: sliding angle was greater than 57° even when the water droplet size was increased to 20 microliters. Sliding angle, a, sometimes also referred to as roll- off angle, is defined as the tilt angle when the liquid drop starts to move on a surface. It suggested that that the film was only partially cured at the outer surface without thermal annealing.

Table 5. Hydrophobicity of sprayed glass resins on glass before annealing (4 mg/mL in IPA).

Glass resin 6st (°) 9adv (°) 9rec (°) CAH (°) a (°) ^b GR653L 91.6±3.3 94.2±0.4 79.6±0.5 14.6 61±2

GR653LPP 91.2±1.1 93.3±0.8 81.2±1.3 12.1 60±3

GR650F 92.1±1.2 92.5±1.0 80.1±1 12.4 57±4

GR650F ^a 91.4±1.2 92.2±1.2 79.0±0.6 13.2 58±3 a: The coating was prepared by spraying the solution six cycles on glass but the others are twice, b: 20 //L water droplet.

[0152] Example 5.2: Optimizing Alcohol Type and Loadings

[0153] The processing parameters, including solvent, spraying cycles and formulation, were all shown to have significant influence on wetting properties of the sprayed coatings. After testing four kinds of polar alcohols, including methanol, ethanol, isopropanol, and n-butanol, nanocomposite coatings using isopropanol ([A-Si0₂] = 5 mg/mL, [GR653L] = 2 mg/mL) were found to provide the best water repellency (See FIG. 13A and 13B). High CA of more than 150° could be achieved on the coated glass after spraying three cycles or more, although a's were higher than 20° until spraying coating cycle was increased to eight and higher. High static CA, 6st (168°) and low a (less than 10°) were obtained on the coated glass. Further increase of spraying cycles above ten did not improve water repellency. SEM images revealed that NPCs did not completely cover the underlying surface when spraying cycle was less than six (FIG 14A and 14D) due to pinholes in the film. Since NPCs were coated on the substrate layer by layer, increasing spraying cycles to eight to ten would gradually fill in the original uncovered regions (FIG. 14E) while nano-pores were formed, which was beneficial to improve surface roughness and therefore resulted in excellent water repellency (6st = 168.8±1.1°, a = 8±1°, FIG. 13B). However, further spraying to twelve cycles decreased the roughness and therefore increased adhesion to droplets (a = 17±2°, FIG. 13B, FIG. 14F). In comparison, there was no observation of aggregates and bonding between NPs from NPs only coatings since the hydrophobic NPs did not wet well with each other (FIG. 15). While not intending to be bound by the correctness or incorrectness of any particular theory, it is believed that glass resin played a significant role here. It may be postulated, for example, that the glass resin used here contained both hydroxyl and methyl groups because it is partially polymerized methyl silicone. If so, the ability to wet and bind the hydrophobic silica nanoparticles with each other and with the substrates would allow it to act as a binder to improve mechanical robustness, as indicated by arrows in FIG. 14E and FIG. 14F, in comparison to the substrate coated with NPs only (FIG. 15B). Further, considering the amphiphilic nature of the glass resin in IP A, it is possible that the glass resin acted as a surfactant to stabilize NPs in the solution. Indeed, the NPs suspension with glass resin remained effective for more than one month, more stable than pure NP solutions. For the latter, the hydrophobic NPs phase separated from the IPA solution after one day. Even further, the glass resin enhanced the coverage of hydrophobic coating on a hydrophilic substrate such as glass, where the glass resin acted like a chemical modifier to the substrates. As described in Example 2.1 and Example 4.1, when NPs and glass resin were dispersed together in IPA, followed by spray coating on the substrates at 5 psig working pressure, the glass resin could not wet the NPs very well, resulting in phase separation and incompletely coverage on the surface (see FIG. 2, FIG. 4, FIG. 5 and FIG. 6). Further thermal annealing was needed for curing the glass resin to obtain hydrophobicity. However, different from the procedure in the first generation mixture, here, we prepared the second generation suspension by individually dispersing NPs and glass resin in IPA and then combining them into a single volume with the volume ratio of 1 : 1 to make sure uniform distribution of NPs and sufficient coverage of glass resin on the surface of NPs. Under high working pressure (10-40 psig), the resin was partially cured when sprayed onto the samples, further leading to superhydrophobicity without thermal annealing.

[0154] In an attempt to optimize the composition of the superhydrophobic coatings, the concentrations NPs and GR653L were varied, as described in in FIG. 16 and FIG. 17). As seen in FIG. 16, superhydrophobicity with static CA above 160° and RA less than 10° was observed from samples with [A-Si0₂] = 5 mg/mL (referred as A5) and [GR653L] <10 mg/mL. At a low concentration of A-S1O2 (e.g., less than 4 mg/mL), while keeping [GR653L] = 2 mg/mL (referred as G2), the NPs were not sufficient to cover the surface. (Note: as used herein, the convention of AxGy refers to a composition having x mg/mL of A-S1O2 and y mg/mL GR653L). It was also possible that there was too much glass resin, which embedded the NPs, thus, decreasing surface roughness and increasing RA. At a high concentration of A-S1O2 NPs (e.g. 10 mg/mL), the effect of the glass resin embedding NPs was minimized. In FIG. 5, the ratio of A-S1O2 NPs and GR653L was kept constant at 2.5. Under certain working pressure, spraying distance from the nozzle to the samples, and moving velocity of the spray gun, the spraying cycles of the samples depended on the concentration. To [A-Si0₂] < 5 mg/mL, at least eight cycles were required for achieving superhydrophobicity, while they were five cycles and three cycles for 5 mg/mL < [A- Si0₂] < 8 mg/mL and [A-Si0₂] > 10 mg/mL. The coatings from A0.5G0.2 (i.e., IPA solution containing 0.5 mg/mL A-S1O2 and 0.2mg/mL GR653L) and A1G0.4 were not superhydrophobic due to the inadequate spraying cycles and NPCs coverage. Further increase of spraying cycles could also make the surface superhydrophobic if the spray solution concentration would be kept low. Furthermore superhydrophobicity was achieved for other concentrations when the ratio was kept in a range of 0.625 and 1 (see Table 6). Typically, the concentration ratio of A-S1O₂ NP/GR653L was kept at 0.6-5 to create a superhydrophobic surface by adjusting number of spraying cycles.

[0155] To investigate the surface topography and roughness of the spray coated films to wettability, AFM images were taken of coatings derived from A2.5G1 and A5G2 solutions, which could produce superhydrophobic coatings. As seen in FIG. 18A, 18C, and 18D, the surface was uniformly covered by NPCs and its aggregates with some nanopores, whereas the surface sprayed for four cycles still had some exposed region (see the middle part of FIG. 18B), in agreement with the observation from the SEM images (FIG. 14). Side-view SEM images of coated glass surface (FIG. 19) indicated that the thickness of the coatings by A2.5G1 and A5G2 were about 500 nm to about 1 micron and about 1 to about 1.5 micron, respectively.

[0156] In addition to GR653L, GR653LPP and GR650F were also added into A-Si0₂ NP solutions to prepare superhydrophobic coatings. When [A-Si0₂] was kept 5 mg/mL, 6st and a were 156.8±0.9° and 8±2°, respectively from [GR653LPP] = 2 mg/mL, and 159.4±3.7° and 18±2°, respectively, from [GR650F] = 1.25 mg/mL.

[0157] An ideal superhydrophobic surface should repel water droplet of any size. However, small satellite-like droplets formed when splashed onto the substrate often adhered to the coating strongly because of the incomplete coverage of NPCs and the large van der Waals interactions between the coating and tiny droplets. Here, films spray coated eight cycles from A5G2 in IPA were shown to have high water repellency even with water droplet size as small as 1 microliter (Table 7 and FIG. 20). All the droplets shown in FIG. 20A and FIG. 20B were highly spherical. The 10-microliter droplet (ca. 1.8 mm in diameter) could roll off the surface at a titling angle of less than 1° (FIG. 20C), and the 1- /L droplet rolled off at a RA of 4° (FIG. 20D).

[0158] Example 6: Third Generation Method (from aqueous-based solutions / dispersions):

[0159] Example 6.1: Preparation of Samples: The precursor for preparing an optimized aqueous solution for spray coating was prepared using the same procedure as that for isopropanol solution in Example 2.2 except that the concentrations of NPs and glass resin were higher (10-50 mg/mL for NPs and 4-20 mg/mL for GR653L). Water was added into the concentrated precursor to make a diluted suspension, which was then sonicated (as described above) for 10-30 min to prevent aggregation. Unlike the continuous spraying of the isopropanol solution, a 10 to 60 second time interval was provided after each cycle of spraying the isopropanol/water mixture to make sure the coating was dry before the next spray cycle, to accommodate the higher boiling point of the water. In some cases, gas flows over the sprayed area (0.5-10 m/s) and substrate heating (40-80 °C) were used to accelerate the evaporation of water droplets in the coatings, but there is still no need for thermal annealing after the spraying process. The distance between the surface and the airbrush depended on the specific water concentration, but was typically held at a distance of 10 to 15 cm. Surprising, using this method, it was not necessary to anneal the coatings to obtain the desirable superhydrophobic properties

[0160] Example 6.2: Results for Coatings Derived from Third Generation

Aqueous-Based Solutions / Dispersions

[0161] Example 6.2.1: Wettability of the coatings sprayed from the aqueous solutions

[0162] While the IPA solutions showed high water repellency, the high volatility raised concern of safety and cost in storage and transportation, leading to investigations of formulations of aqueous solutions from A-S1O2 NPs and GR653L with water content upwards of 90 vol% and higher. The aqueous solutions were prepared from IPA solution of different concentrations while keeping the final concentration of [A-Si0₂] = 5 mg/mL and [GR653L] = 2 mg/mL in IP A/water mixed solvent. For example, to make aqueous A5G2 solution with 50 vol% water, the same volume of water was added into A10G4 in pure IPA. Similarly, to obtain A5G2 in IP A/water mixtures with 60 vol%, 70 vol%, 80 vol% and 90 vol% water, the IPA-based precursors of A12.5G5, A16.7G6.7, A25G10 and A50G20 were diluted by water with volume ratio to precursor of 3 :2, 7:3, 4: 1 and 9: 1, respectively. The aqueous suspensions were found homogeneous but hazy due to phase separation of hydrophobic NPCs from the water, leading to formation of small aggregates (see FIG. 21A). Nevertheless, the suspension remained stable over one week. The mixed aqueous solutions were sprayed onto glass and wettability was investigated. As seen in FIG. 21B, it was clear that excellent water repellency was achieved from the coatings spray coated from the aqueous solutions, somewhat better than the results obtained from the IPA precursor solution. Especially when the water concentration was increased to above 60%, extremely low a of 2±1° was observed. Importantly, it was shown that the IP A/water solutions containing 90 vo/% of water were also possible to produce

superhydrophobicity, thus these coating solutions could be shipped anywhere by any method without special labeling or handling. Compared to the results from pure IPA solutions (see FIG. 17 and Table 8), 90 vol% aqueous suspensions did not seem to affect water CA and a only when the concentrations of both A-Si02 NP and GR653L became very low (e.g. A1G0.4), leading to incomplete coverage of NPCs on the substrates. SEM images (see FIG. 22) showed that the nanocoating was composed of aggregates of NPCs and nano-pores, similar to those seen from coatings sprayed from the IPA based solutions (see FIG. 14). It did not appear that the water content had any obvious influence on coating morphology. In Example 4.5, various ratios of NPs and GR653L were dispersed in IP A/water solution, with water volume being 25, 50 and 75 vol% of the entire mixture, and these dispersions were used to prepare superhydrophobic coatings. However, in third generation aqueous solutions, we prepared precursor solutions in IPA with high concentration of NPs and glass resin to ensure the wetting between glass resin and NPs. A dilution was conducted by adding water into the IPA-based precursor until desired volume. Although separation of the nanocomposites of NPs and glass resin occurred in the diluted IP A/water suspension, superhydrophobic coatings could still be obtained unless the aggregates were too large so as to clog the nozzle. Table 8. Water CA and RA data of glass substrates coated with various aqueous solutions with 90 vol% of water

Formulation (mg/mL) IPA-based solution Aqueous solution with 90 vol.% water

A-Si0₂ NP GR653L 9st (°) a (°) 6!sf (°) a (°)

2.5 1 167.2±3.3 2±1 170.0±3.4 2±1

4 1.6 168.3±2.1 2±1 168.4±0.4 2±1

2 2 170.6±2.3 2±1 172.1±0.7 2±1

2 3.2 163.8±2.4 25.3 174.0±1.6 2±1

[0163] Example 6.2.2: Mechanical robustness of the coatings derived from Second and Third Generation solutions

[0164] To investigate the durability of the NPCs coatings, Scotch tape peeling tests were done on the prepared surfaces and the water contact angles compared before and after peeling. The results were summarized in Table 9 and FIG. 23. The Ost's of the substrates coated with A5G2 and A5G4 (sprayed eight to ten cycles) remained > 150° after the Scotch tape peeling, although a decreased to 31° and higher. The SEM images indicated that some NPCs and its aggregates were removed. Since the remaining NPCs and the glass resin were hydrophobic, high CA was still achieved, however, pin-hole formation after tape peeling led to high RAs. Nevertheless, the results showed that improved mechanical robustness of the coatings when fine- tune the formulation and spraying the solution over eight cycles. Similarly, samples were spray coated with various solutions under working pressure of 20 psig for three to ten cycles, and these coatings were test for their wetting behavior before and after water jetting, as described in

Example 3.4. The data in Table 10 and Table 11 showed that most of these coatings were robust to water jetting; even the pressure was as high as 50 kPa. The coatings derived from third generation solutions were better than those derived from IPA-based solution.

Table 9. Water contact angles and sliding angles on glass substrates prepared from mixture of A- S1O2 NP and GR653L of different concentrations before and after Scotch tape peeling. Formulation (mg/mL) As-sprayed coatings on glass Coatings after peeling

A-Si0₂ NP GR653L 9st (°) a (°) 9st (°) a (°)

5 0 162.8±4.9 1 1±3 Easily moved when pressing tape

0 2 91.6±3.3 >90 89.8±0.3 >90

5 2 168.5±3.8 5±1 159.4±2.9 31±2

5 4 171.2±2.8 6±1 155.4±2.8 71±3

10 2 162.4±3.1 21±5 Easily moved when pressing tape

Table 11. Water contact angles and sliding angles on glass substrates prepared from aqueous mixture of A-S1O2 NP and GR653L of different concentrations before and after water jetting test. Coatings after Coatings after Coatings after

Formulation As-sprayed

jetting with 10 jetting with 25 jetting with 50 (mg/mL) coatings on glass

kPa water kPa water kPa water

A- Si0₂ GR653L 9st (°) a (°) 9st (°) a (°) 6st (°) a (°) 6!sf (°) a (°) NP

2 0.8 172.6±2.7 2±1 168.5±1.8 5±1 169.2±1.3 5±2 165.9±0.2 5±3

2 2 172.1±0.7 2±1 167.1±1.1 2±1 166.5±0.9 5±2 166.4±3.4 5±1

2 3.2 174.0±1.6 2±1 170.3±1.3 2±1 166.9±1.4 2±1 166.2±1.2 3±1

[0165] Example 6.2.3: Optical properties of the coatings derived from Second and Third Generation solutions

[0166] The transmittance of the prepared samples was characterized by UV-vis spectroscopy in the range of 400-800 nm. Transparency decreased with the number of spraying cycles due to increased film thickness (FIG. 14 and FIG. 24A). When A-Si0₂ NP concentration was increased, slightly decrease on transparency was observed, presumably due to increase of NPCs thickness (FIG. 24B) at the same spraying cycle. Gradually adding GR653L into the solution also tended to reduce optical transparency (FIG. 9C), which was probably due to the increase of nanotexure size since glass resin acted as binders of NPs. High transparency, high water CA and low RA could be obtained by keeping the suspension concentration low and the concentration ratio of NP/GR653L=2.5 (FIG. 17 and FIG. 24D). The lowest concentrations for A-S1O2 NPs and GR653L for spray coating of eight to ten cycles were [A-Si0₂] = 1.5 mg/mL and [GR653L] = 0.6 mg/mL. When water was added to the IPA solution up to 90 vol%, the sprayed coating showed similarly high transmittance as that from IPA solutions (FIG. 25). Moreover, the transmittance of the coated sample in the wavelength of near IR region (NIR, > 1000 nm) was slightly higher than that of bare glass (FIG. 26), indicating anti-reflective property of the nanoparticle coating. Such coating may be of interest for coating on solar panels.

[0167] Example 6.2.4: Stability of IP A-based nanocomposite solutions

[0168] As depicted above, the glass coated with A5G2 displayed high CA, extremely low RA, high tranparency and good mechanical robustness. Therefore, the following experiments were conducted based on this coating formulation. As seen in FIG. 27, the film spray coated from the solution on shelf for 7 weeks remained superhydrophobic, and the solution stored in the refrigerator seemed to offfer higher CA's and lower a's compared to those stored in air. This could be explained by the lower curing rate of glass resin in the refrigerator.

[0169] Example 6.2.5: Coatings derived from IPA and aqueous-based dispersions on various substrates

[0170] The alcohol and aqueous NPC formulae were applied to a wide range of solid substrates, including aluminum foil, cotton, paper, polycarbonate (PC), and polyethylene terephthalate (PET), all of which showed superhydrophobicity with high transparency. Water droplets were spherical on all substrates and could be easily rolling off at a very low titling angle (<10°). Further, the resulting superhydrophobic surface presented excellent self-cleaning property, which removed the particles by enrolling water droplet (FIG. 28).

[0171] It is to be understood that while the invention has been described in conjunction with the preferred specific embodiments thereof, that the foregoing description and the examples that follow are intended to illustrate and not limit the scope of the invention. It will be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the scope of the invention, and further that other aspects, advantages and modifications will be apparent to those skilled in the art to which the invention pertains. In addition to the embodiments described herein, the present invention contemplates and claims those inventions resulting from the combination of features of the invention cited herein and those of the cited prior art references which complement the features of the present invention. Similarly, it will be appreciated that any described material, feature, or article may be used in combination with any other material, feature, or article, and such combinations are considered within the scope of this invention.

[0172] The disclosures of each patent, patent application, and publication cited or described in this document are hereby incorporated herein by reference, each in its entirety, for all purposes.

Claims

What is Claimed:

1. A coating composition comprising a plurality of hydrophobic nanoparticles dispersed in a hydrophilic solvent, the hydrophilic solvent comprising an amphiphilic silicone-containing dispersing resin.

2. The coating composition of claim 1, wherein at least some of the plurality of hydrophobic nanoparticles comprise an alkane, an alkylsilane, a phenylsilane, a fluorosilane, or a fluorinated polymer surface, or a combination thereof.

3. The coating composition of claim 1, wherein at least some of the plurality of hydrophobic nanoparticles have a mean cross-sectional dimension in a range of about 5 nm to about 200 nm.

4. The coating composition of claim 3, wherein less than 10% by volume of the

hydrophobic nanoparticles have a cross-sectional dimension greater than 100 nm.

5. The coating composition of claim 1, wherein less than 1% by volume of the hydrophobic nanoparticles have a cross-sectional dimension greater than 200 nm.

6. The coating composition of claim 1, wherein at least some of the plurality of hydrophobic nanoparticles differs from one another in material composition.

7. The coating composition of claim 1, wherein the plurality of hydrophobic nanoparticles is characterized as having a monomodal particle size distribution.

8. The coating composition of claim 1, wherein the concentration of the hydrophobic nanoparticles in the composition is in a range of about 0.5 to about 50 mg of nanoparticles per mL of coating composition.

9. The coating composition of claim 1, wherein the solvent comprises at least one Ci_₄ alcohol, cumulatively present in a range of from about 50% to about 98% by volume of the total composition.

10. The coating composition of claim 1, wherein the solvent comprises water, present in a range of from about 50% to about 98% by volume of the total composition.

1 1. The coating composition of claim 1, wherein the amphiphilic silicone-containing dispersing resin comprises an annealable glass resin.

12. The coating composition of claim 1, wherein the concentration of the amphiphilic silicone-containing dispersing resin in the composition is in a range of about 0.01 to about 10 volume percent, relative to the volume of the composition.

13. The coating composition of claim 1, wherein the ratio of hydrophobic nanoparticles to amphiphilic silicone-containing dispersion resin is in a range of about 2: 1 to about 10: 1 wt/wt.

14. The coating composition of claim 1, wherein the composition is adapted for spray coating.

15. A method of preparing a superhydrophobic coating, said method comprising:

(a) applying, to a substrate, a coating composition comprising a plurality of hydrophobic nanoparticles dispersed in a hydrophilic solvent comprising an amphiphilic silicone-containing dispersing resin; and

(b) effecting removal of at least a portion of the hydrophilic solvent so as to form an adherent layer of an amphiphilic pre-cured coating layer on the substrate.

16. The method of claim 15, wherein the coating composition comprises the coating composition of claim 1.

17. The method of claim 15, wherein the substrate comprises a polymer, a glass, a semiconductor, a metal, or a combination thereof.

18. The method of claim 15, wherein the coating composition, adapted for spray coating, is applied by spray coating at least one coating layer to the substrate, wherein the concentration of the hydrophobic nanoparticles in the composition is in a range of from 1.5 mg to 50 mg of nanoparticles per mL of coating composition.

19. The method of claim 15, wherein the coating composition, having solvent comprising water present in a range of from 50% to 98% by volume of the total solvent composition, adapted for spray coating, is applied by spray coating at least two coating layers to the substrate.

20. The method of any one of claim 19, wherein the coating composition, having solvent comprising water present in a range of from 80% to 98% by volume of the total solvent composition, adapted for spray coating, is applied by spray coating between six and ten coating layers to the substrate, preferably about 8 coating layers.

21. The method of claim 15, further comprising applying sufficient energy for a sufficient time to the pre-cured coating layer so as to convert the amphiphilic pre-cured coating layer to a cured superhydrophobic layer, said superhydrophobic layer comprising the plurality of hydrophobic nanoparticles embedded within a layer of cross-linked hydrophobic silicone-based polymer.

22. The method of claim 21, wherein the energy is thermal energy.

23. A superhydrophobic coating prepared using the method of claim 15.

24. A superhydrophobic coating comprising a plurality of hydrophobic nanoparticles embedded within a layer of cross-linked hydrophobic silicone-based polymer.

25. The superhydrophobic coating of claim 23, wherein the superhydrophobic coating is characterized as transmitting at least 50% of incident light of the wavelengths in a range of about 400 nm to about 800 nm.

26. The superhydrophobic coating of claim 25 wherein the cured superhydrophobic coating is characterized as transmitting at least 95% of incident light of the wavelengths in a range of about 400 nm to about 800 nm.

27. The superhydrophobic coating of claim 23, wherein the cured superhydrophobic coating is characterized as exhibiting a water contact angle of at least 150°.

28. The superhydrophobic coating of claim 23, comprising hydrophobic nanoparticles conjoined by cured silicone-based polymer, wherein the ratio of the nanoparticles to cured silicone-based polymer is in a range of about 100: 1 wt/wt to about 1 : 1.

29. The superhydrophobic coating of claim 23, characterized by an integrity such that it passes the Adhesion Tape Test of ASTM D3359 - 09e2 Standard Test Methods for Measuring Adhesion by Tape Test

30. The superhydrophobic coating of claim 23, wherein the superhydrophobic coating surmounts and adheres to an oxidized silicon wafer.

31. A solar energy cell comprising a superhydrophobic coating according to claim 23.

32. An energy storage device comprising a solar energy cell according to claim 31.