The present description relates to superhydrophobic films having both microstructured and nanofeatured surfaces. The present description further relates to constructions utilizing such superhydrophobic films, and methods of producing such superhydrophobic films.
Hydrophobic films and coatings, and more particularly, superhydrophobic films and coatings have garnered considerable attention in recent years due to a number of attractive qualities. Highly hydrophobic surfaces have been recognized in nature, perhaps most prevalently on lotus leaves and also on cicada wings. Because of its hydrophobic properties, the lotus leaf is capable of self-cleaning by the washing away of dust particles and debris as water droplets roll off its surface. This ability to self-clean is desirable in a number of modern-day applications. However, it may be difficult to produce a self-cleaning superhydrophobic film that is capable of extended use in certain environments. The current description provides a superhydrophobic film that is highly durable and weatherable in variable conditions, for example, outdoors, and performs very effectively without serious performance concerns after abrasive exposure, even without a surface coating.
In one aspect, the present description relates to a superhydrophobic film. The superhydrophobic film has a surface that includes a plurality of microstructures. Each microstructure includes a plurality of nanofeatures, where both the microstructures and nanofeatures are made of a material that is a majority silicone polymer by weight. The film has a water contact angle of at least 150 degrees, and a sliding angle of less than 10 degrees.
In another aspect, the present description relates to a method of producing a superhydrophobic film. The method includes providing a film that is a majority by weight silicone polymer, such as poly(dimethylsiloxane) (PDMS), and has microstructures on its first surface. The method further includes applying a layer of metal oxide nanoparticles directly onto the microstructures. The metal oxide nanoparticles serve as an etch mask as the film is etched, and the etching results in nanofeatures formed on the microstructures on the film.
In a third aspect, the present description relates to a method of producing a superhydrophobic film. The method includes providing a first film comprising microstructures on a first surface of film. The method further includes applying a uniform layer of metal oxide nanoparticles directly onto the microstructures and etching the film, using the metal oxide nanoparticles as an etch mask. The etching results in nanofeatures formed on the microstructures of the first film. Next, a casting material is deposited onto the first film and a mold is formed with the casting material, where the mold is, at least in part, a negative of the microstructures and nanostructures of the first film. A silicone polymer is applied to the mold and cured to form a second film. The second film, when removed, exhibits a water contact angle of at least 150 degrees and a sliding angle of less than 10 degrees.
In a final aspect, the present description relates to a superhydrophobic film. The superhydrophobic film has a surface that includes a plurality of microstructures. Each microstructure includes a plurality of nanofeatures, where both the microstructures and nanofeatures are made of a material that is an elastomer. The film has a water contact angle of at least 150 degrees, and a sliding angle of less than 10 degrees.
a-c illustrate various shapes of microstructures according to the present description.
a-d illustrate a process for producing a superhydrophobic film.
a-e illustrate a process for producing a superhydrophobic film.
a-d provide illustrations of water droplets as related to measuring water contact angle, advancing angle, and receding angle.
a-c are different microstructure distributions for a superhydrophobic film.
Superhydrophobic films and surfaces are very desirable in a number of applications due to their ability to self-clean. Generally, a film may be considered “superhydrophobic” where the water contact angle is greater than 140 degrees. Superhydrophobic films may further be understood as generally nonwettable, as water beads off of the surface of the film upon contact. A further desirable quality for such films may be low contact angle hysteresis, that is, a small difference between the advancing and receding contact angles of the water droplet. A low contact angle hysteresis, or “sliding angle” allows for water beads to roll off of the surface of a film or other construction more easily. The combination of the ability to bead water that comes into contact with the surface of a structure and further roll the beaded water off of the surface is what makes the surface “self-cleaning.”
This ability to self-clean is desirable in a number of modern-day applications. For example, self-cleaning superhydrophobic surfaces may be useful on the sun-facing surfaces of solar (photovoltaic) cells, anti-icing applications, corrosion prevention, anti-condensation applications, wind blades, traffic signals, edge seals, anti-fouling applications, and drag reduction and/or noise reduction for automobiles, aircraft, boats and microfluidic devices, just to name a few. Such films may also have valuable anti-reflection properties. There have therefore been attempts to create superhydrophobic films either by microstructuring a film's surface in a manner resembling that of the lotus leaf, coating the film with a hydrophobic chemical coating, or a combination thereof. Unfortunately, a number of these attempts have resulted in films that are not sufficiently durable in outdoor or other harsh environments. This is especially unfortunate due to the difficult conditions to which such films are exposed in the exemplary applications noted. Those attempts at producing films that are durable in the harsh application environments may not display the highly superhydrophobic properties that are necessary for optimal self-cleaning performance. The present description therefore provides an improvement by offering a superhydrophobic film that is highly durable and weatherable in harsh conditions, for example, long-term use outdoors, and performs very effectively, even without a surface coating.
In addition, an increasing number of applications require a superhydrophobic film construction that is transparent to visible or near-visible light. For example, a superhydrophobic film that is used as a front panel of a solar panel or as a protective film over a camera lens needs to be transparent in order to function effectively. The film described herein offers improvement in transparency compared to other superhydrophobic constructions in the art, along with the benefits of high superhydrophobic performance, and improved durability.
One embodiment of a superhydrophobic film construction according to the present description is illustrated in
In some embodiments, other silicone polymers besides PDMS may be useful, for example, silicones in which some of the silicon atoms have other groups that may be aryl, for example phenyl, alkyl, for example ethyl, propyl, butyl or octyl, fluororalkyl, for example 3,3,3-trifluoropropyl, or arylalkyl, for example 2-phenylpropyl. The silicone polymers may also contain reactive groups, such as vinyl, silicon-hydride (Si—H), silanol (Si—OH), acrylate, methacrylate, epoxy, isocyanate, anhydride, mercapto and chloroalkyl. These silicones may be thermoplastic or they may be cured, for example, by condensation cure, addition cure of vinyl and Si—H groups, or by free-radical cure of pendant acrylate groups. They may also be cross-linked with the use of peroxides. Such curing may be accomplished with the addition of heat or actinic radiation. Other useful polymers include polyurethanes, fluoropolymers including fluoroelastomers, polyacrylates and polymethacrylates. In another embodiment, polymers with a glass transition temperature of at least 25 degrees C. are useful. In at least some embodiments, the film may be an elastomer. An elastomer may be understood as a polymer with the property of viscoelasticity (or elasticity) generally having notably low Young's modulus and high yield strain compared with other materials. The term is often used interchangeably with the term rubber, although the latter is preferred when referring to cross-linked polymers.
In some embodiments, the nanofeatures and/or microstructures may also be composed of less than 1% of another material, for example indium tin oxide (ITO). The small amount of ITO on the microstructures 102 and nanofeatures 104 may be a remnant of an etching step used to create the nanofeatures, as discussed further below. Specifically, the small amount of ITO may be either an ITO nanoparticle or remnant of an ITO nanoparticle. The ITO nanoparticles used for etching the nanofeatures may generally have an appropriate diameter desired for surface area coverage during etching. For example, the nanoparticles may have an average diameter of between about 10 nm and about 300 nm, or more potentially an average diameter of between about 70 nm and about 100 nm. As further described below, the nanoparticles may be applied as an etch mask as part of a suitable coating suspension. In one embodiment, the liquid mixed with the ITO nanoparticles may be isopropanol.
Two of the most important measurements in determining just how superhydrophobic a film or coating is are that of water contact angle and sliding angle (or contact angle hysteresis). The water contact angle may be measured with a static contact angle measurement device, such as the Video Contact Angle System: DSA100 Drop Shape Analysis System from Kruess GmbH (Hamburg, Germany). In this particular system, a machine is equipped with a digital camera, automatic liquid dispensers, and sample stages allowing a hands-free contact angle measurement via automated placement of a drop of water (where the water drop has a size of approximately 5 μl). The drop shape is captured automatically and then analyzed via Drop Shape Analysis by a computer to determine the static, advancing, and receding water contact angle. Static water contact angle may be generally understood as the general “water contact angle” described and claimed herein.
The water contact angle may most simply be understood as the angle at which a liquid meets a solid surface. As shown in
The “sliding angle” or “contact angle hysteresis” is defined as the difference between the advancing and receding water contact angles. Advancing water contact angle and receding water contact angles relate not just to static conditions, but to dynamic conditions. With reference to
The metal oxide nanoparticle-masking followed by etching of silicone polymer (e.g. PDMS) microstructures results in a microstructured and nanofeatured surface of a common material that exhibits very high levels of hydrophobicity as well as durability. For example, in at least one embodiment, the film of the present description exhibits a water contact angle of at least 150 degrees. The film may further exhibit a sliding angle (or contact angle hysteresis) of less than 10 degrees. In some embodiments the film exhibits a water contact angle of at least 160 degrees, and in other embodiments the film exhibits a water contact angle of at least 170 degrees. Water contact angles of over 175 degrees may be achieved. The sliding angles may be less than 10 degrees, or less than 7.5 degrees or less than 5 degrees. The sliding angle may also be less than 2 degrees or less than 1 degree. In some embodiments, the water contact angle of a film according to the current description may be reduced by no more than 20 degrees, or less than 10 degrees, or less than 5 degrees, or potentially even less than 3 degrees when subjected a severe durability test such as falling sand.
In a number of superhydrophobic film constructions, the superhydrophobicity is caused by an application of a low-surface-energy coating application placed on the surface of a film. Another manner of creating superhydrophobicity on a surface is by creating surface features that may achieve high water contact angles and low sliding angles (or contact angle hysteresis). In order to further enhance hydrophobicity, often even structured films may utilize some sort of low-surface-energy coating. Combining the unique nature of the materials used in both the microstructures and nanofeatures of the present description, as well as the type of etch mask used to complement the properties of these materials, the film of the present description provides a structured surface that may be very superhydrophobic without any need for a low-surface-energy coating. However, it may be beneficial to include a low-surface-energy coating on top of the microstructured and nanofeatured surface of the current film. Therefore, in order to achieve even greater superhydrophobicity, perhaps such that water contact angles approach 180 degrees, a low-surface-energy coating 108 may optionally be applied over microstructures 102 and nanofeatures 104. However, as noted, the material properties and structural make-up of the films contemplated herein allow for great superhydrophobicity absent such a coating. The specific size and shape features of microstructures and nanofeatures according to the present description may be understood by reference to the nanofeatured microstructure of
Besides the very high superhydrophobic performance of the currently described films, other useful properties may be exhibited. For example, the microstructured and nanofeatured silicone polymer films described herein may exhibit very low reflectivity and therefore be highly transmissive. This is a highly beneficial property for applications where films are applied to solar cells, or any sort of window or light transmissive usage where the films are used for self-cleaning or anti-icing properties. The films described herein may reflect less than 5% of incident light, and may reflect less than 2% of incident light. In some application, only approximately 1% of light incident on the films is reflected.
Although
More generally, microstructures may be created that vary in one, two or three dimensions. A better understanding of this may be gained by reference to
Where the microstructures 102 of the current film are prisms, in one embodiment the prisms may have a peak angle θP (or angle between the two facets of the prism) of 90 degrees. As the prisms are isosceles triangles, the angle of intersection of the two facets with the plane of the film will then be an angle of 45 degrees. In other embodiments, the peak angle may be greater or less than 90 degrees. For example, the peak angle may be between 90 degrees and 100 degrees, or between 80 degrees and 90 degrees. In one embodiment, the peak angle may be between 70 degrees and 80 degrees. For example, the peak angle may be between about 74 degrees and 76 degrees, perhaps about 74 degrees. In this particular embodiment, the angles of the facets of the pyramid to the plane of the film θFAC will be 53 degrees. The specific angle chosen for the prism peak angle may allow for a better distribution of nanoparticles on the surface of the microstructures, as will be discussed further below.
Referring back to
A greater understanding of the structure of the microstructures and nanofeatures of a superhydrophobic film according to the present description may be gained by reference to the microstructure in
A number of embodiments as illustrated by the figures herein may include microstructures that are directly adjacent to one another, such that the base of a microstructure is directly in contact with the base of an adjacent microstructure. However, it should be understood that the microstructures may be further spaced apart, such that the facets of the microstructures are not in contact and are spaced apart by a segment of film surface that may, for example, be flat. This film surface that lies between the microstructures may also have nanofeatures on its surface. In fact, there may be such space between the microstructures that they have an average peak-to-peak distance of adjacent microstructures up to about 5 times the average height of the microstructures.
The nanofeatures 304 are formed into or onto the microstructure 302 and should cover a great deal of the surface of the microstructure. It should be noted that nanofeatures 304 are not drawn to scale in relation to microstructure 302. Nanofeatures 304 may generally have an average width 340 of between about 5 nm to about 250 nm. Nanofeatures 304 generally have an average height 330 of between about 10 nm and about 1000 nm, and potentially between about 100 nm and about 1000 nm As such, nanofeatures 304 may be understood as having high aspect ratios in a number of applications. In some embodiments, the nanofeatures exhibit an average aspect ratio of at least about 1 to 1, or at least about 2 to 1, or at least about 3 to 1, or at least about 4 to 1, or at least about 5 to 1, or at least about 6 to 1. In some embodiments, at least some of the nanofeatures may have the high aspect ratios discussed but be much larger in size. For example, the nanofeatures may be of a width that is on the order of one-fifth the width of the microstructure upon which it is positioned.
In a different aspect, the present description relates to a method of producing a superhydrophobic film. One particular embodiment of such a method is illustrated in
The next step in the method of producing a superhydrophobic film involves applying a layer of metal oxide nanoparticles 412 directly onto the microstructures 402, as shown in
The next step after applying the uniform layer of metal oxide nanoparticles to the film is illustrated in
In addition to the beneficial nature of combining metal oxide nanoparticles with a PDMS surface for purposes of distributing or dispersing the particles uniformly over the surface of the microstructures, indium tin oxide nanoparticles exhibit other desirable properties for etching. For example, metal oxide nanoparticles such as indium tin oxide nanoparticles generally etch at a substantially slower rate than the silicone polymer material used for the film (e.g. PDMS). As such, the mask remains in place while etchant moves deep into the microstructured surface. For example, the etched nanofeatures may have a height of between about 10 nm to about 1000 nm and potentially between about 100 nm to about 1000 nm. This large etch rate ratio also allows for nanofeatures with high aspect ratios, such as 4 to 1, 5 to 1, 6 to 1 or greater as discussed with respect to the description of the article in
As a final optional step, a low surface energy coating 408 may be applied to the microstructures 402 and nanofeatures 404 of film 410 as shown in
The films of the current description may also be made by some sort of replication method as illustrated in
The process described in
The “falling sand” test performed as specified above generally will create a great deal of abrasion on the surface of a film, especially a film that is microstructured and/or nanofeatured. As such, it is to be expected that most superhydrophobic film constructions in the art that had to go through the test would see serious degradation to the structures on the film's surface. This would necessarily result in lower hydrophobicity (i.e. lower water contact angles and high sliding angles). It has been discovered in accordance with this description that utilizing a silicone polymer, and in at least some embodiments, specifically utilizing a polymer that contains PDMS, and potentially as much as 95% PDMS, as the material for both the microstructures and nanofeatures on the film allow the films to weather such exposure without suffering drastically in performance
Testing the water contact angle and sliding angle of the superhydrophobic film after the falling sand test is a highly valuable metric of the durability of such a film. The film of the current description may, after exposure to the falling sand test still exhibit a water contact angle of greater than 145 degrees, or 150 degrees, or potentially even 160 degrees. The sliding angle after the falling sand test may be less than 10 degrees or less than 5 degrees.
In order to understand the importance of this performance after exposure to such high levels of abrasion, it is helpful to show the difference in performance between the film of the current description and a hydrophobic film of the prior art after exposure to the falling sand test. One suitable prior art film is described in U.S. Patent Publication No. 2008/0090010 (Zhang et al.). This film has a coating that includes a composition with both microparticles and nanoparticles applied on the microparticles. The comparative film also included microparticles and nanoparticles (though this should not be understood as a claim that the comparative film falls directly within the scope of Zhang et al.'s description). The comparative film includes 4.5 micrometer silicon dioxide microparticles coated with 190 nanometer silicon dioxide nanoparticles. Measurements of a PDMS film according to the current and then the prior art particle film were both taken prior to exposure to the falling sand test. Next, each of the films was exposed to the falling sand test as described above and the water contact angle and roll-off angle measurements were once again taken. The results of the test are provided in Table 1 below.
The “roll-off angle” is a comparable measurement to sliding angle. A tilt angle (the angle of the liquid-solid interfacial line) for a water drop on the above sample was conducted. The sample was placed on the Automated Tilting Base and adjusted for leveling with a bubble level. Then 5 uL DI water was delivered using a 10 uL syringe (Hamilton). The tilting base was then turned-on manually and off when the water droplet rolls off. The tilt angle was recorded and the tilt base back to 0° for next measurement. As clearly shown in the table, the PDMS film lost very little water contact angle performance (only 4 degrees), and had a roll-off angle that remained below 1 degree after the falling sand test. By comparison, the prior art film had an initially high water contact angle of 151 degrees that was reduced by 70 degrees to 81 degrees after the falling sand test. The roll-off angle of the comparative film drastically increased from less than 1 degree to greater than 60 degrees. The results provide a dramatic illustration of the durability of the film of the present description while maintaining a high and acceptable superhydrophobic performance. By contrast, the film of the prior art is rendered non-hydrophobic by exposure to the test.
Although the superhydrophobic film construction and methods of producing such a film have been described herein with respect to several embodiments, those of skill in the art will recognize that modifications may be made in form and detail without departing from the spirit and scope of the film and method disclosure.
The present application relates generally to the following co-filed and commonly assigned U.S. patent applications: “Superhydrophobic Films”, Attorney Docket No. 66911US002, and “Superhydrophobic Films”, Attorney Docket No. 66994US002, each of which is incorporated herein by reference in its entirety.
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/US11/57048 | 10/20/2011 | WO | 00 | 7/22/2013 |
Number | Date | Country | |
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61407806 | Oct 2010 | US |