Patent Application
20060216476
The invention relates to surfaces having low liquid wettability. More particularly, the invention relates to such surfaces, where the surface is characterized by having one or more of the following properties: low-friction properties, self-cleaning capability, and resistance to icing, fouling, and fogging. Even more particularly, the invention relates to articles having such surfaces.
The “liquid wettability” of a solid surface is determined by observing the nature of the interaction occurring between the surface and a drop of a given liquid disposed on the surface. A surface having a high wettability for the liquid tends to allow the drop to spread over a relatively wide area of the surface, thereby “wetting” the surface. In the extreme case, the liquid spreads into a film over the surface. On the other hand, where the surface has a low wettability for the liquid, the liquid tends to retain a well-formed, ball-shaped drop shape. In the extreme case, the liquid forms spherical drops on the surface that easily roll off of the surface at the slightest disturbance.
The extent to which a liquid is able to wet a solid surface plays a significant role in determining how the liquid and solid will interact with each other. A high degree of wetting results in relatively large areas of liquid-solid contact, and is desirable in applications where a considerable amount of interaction between the two surfaces is beneficial, such as, for example, adhesive and coating applications. By way of example, so-called “hydrophilic” materials have relatively high wettability in the presence of water, resulting in a high degree of “sheeting” of the water over the solid surface. Conversely, for applications requiring low solid-liquid interaction, the wettability is generally kept as low as possible in order to promote the formation of liquid drops having minimal contact area with the solid surface. “Hydrophobic” materials have relatively low water wettability; so-called “superhydrophobic” materials have even lower water wettability, resulting in surfaces that in some cases may seem to repel any water impinging on the surface due to the insignificant amount of interaction between water drops and the solid surface.
Articles having tailored surface properties are used in a broad range of applications in areas such as transportation, chemical processing, health care, and textiles. Many of these applications involve the use of articles having a surface with a relatively low liquid wettability to reduce the interaction between the article surface and various liquids. In particular, the wetting properties of a material can be tailored to produce surfaces having properties that include low-drag or low-friction, self-cleaning capability, and resistance to icing, fouling, and fogging.
Different methods of reducing drag and friction have been used in different applications. To reduce friction in a pipe, for example, pipes have been made macroscopically smoother. Macroscopic structures, such as ‘riblets,’ have been used to create flow patterns that offer reduced resistance to flow. Similarly, ‘compliant’ surfaces that change adaptively based on flow characteristics have been tried as well. Such macroscopic modifications have been able to produce at best a 5-10% reduction in drag.
Hydrophobic surfaces on articles have also been formed using hydrophobic materials, such as Teflon®, polymer gels and solutions, and the like. Such materials are typically deposited as a film on a substrate or are formed into the article itself. For example, polymeric solutions are applied to racing boats to reduce drag, and polymer gels are applied to the inner surfaces of oil pipelines. Surfaces comprising such materials generally reduce drag or friction by 5-10%. Such coatings are subject to rapid wear and are not thermally or chemically stable at higher temperatures.
Current approaches to the production of articles having minimal interaction with fluids have been focused on applications of limited scope, and have produced only limited success. Therefore, there remains a need across several industries for articles having a surface with low liquid wettability. Moreover, these industries also require methods for providing such a surface on an article.
The present invention meets these and other needs by providing an article having a surface with a low liquid wettability. The surface provides properties that may include, as non-limiting examples, one or more of low-drag or low-friction properties, self-cleaning capability, and resistance to icing, fouling, and fogging.
Accordingly, one aspect of the invention is to provide an article. The article comprises a surface disposed on a substrate, wherein the surface comprises
a. a material having a nominal liquid wettability sufficient to generate, with a reference liquid, a nominal contact angle in a range from about 60° to about 90°, the material comprising at least one material selected from the group consisting of a polymer and a ceramic; and
b. a texture comprising a plurality of features;
wherein the surface has an effective liquid wettability sufficient to generate, with the reference liquid, a contact angle in a range from about 120° to about 180°.
Still another aspect of the invention is to provide a method of making an article. The method comprises
a. providing a substrate and
b. forming a surface on the substrate, wherein the surface comprises a material having a nominal liquid wettability sufficient to generate, with a reference liquid, a nominal contact angle in a range from about 60° to about 90°, the material comprising at least one material selected from the group consisting of a polymer and a ceramic, and the surface further comprising a texture, wherein the texture comprises a plurality of features;
wherein the surface has an effective liquid wettability sufficient to generate, with the reference liquid, a contact angle in a range from about 120° to about 180°.
These and other aspects, advantages, and salient features of the present invention will become apparent from the following detailed description, accompanying drawings, and appended claims.
a is a photograph of a water droplet on a superhydrophobic polycarbonate surface;
b is a micrograph of the superhydrophobic polycarbonate surface shown in
c is a photograph of a water droplet on a nominally flat polycarbonate surface;
In the following description, like reference characters designate like or corresponding parts throughout the several views shown in the figures. It is also understood that terms such as “top,” “bottom,” “outward,” “inward,” and the like are words of convenience and are not to be construed as limiting terms. Furthermore, whenever a particular feature of the invention is said to comprise or consist of at least one of a number of elements of a group and combinations thereof, it is understood that the feature may comprise or consist of any of the elements of the group, either individually or in combination with any of the other elements of that group.
Referring to the drawings in general and to
Surface 120 comprises a material that has a moderately high wettability for a given reference liquid, yet surface 120 itself has a substantially lower wettability for the same reference liquid than that typically measured for the material. In particular, surface 120 comprises a material having a nominal liquid wettability sufficient to generate a nominal contact angle in a range from about 60° to about 90° with a given reference liquid. In particular embodiments, surface 120 consists essentially of the material. For the purposes of understanding the invention, a “nominal contact angle” 240 (
Surface 120 further comprises a texture 130 comprising a plurality of features 135. The present inventors have found that by providing a surface 120, comprising a material of moderately high nominal wettability, with a texture 130, the resulting textured surface can have remarkably lower wettability than that inherent to the material from which the surface is made. In particular, surface 120 has an effective wettability (that is, wettability of the textured surface) for the reference liquid sufficient to generate a contact angle in the range from about 120° to about 180°. Where the reference liquid is water, for example, the material would be considered to be hydrophilic based on its nominal contact angle, yet the surface 120, made of textured hydrophilic material, would be considered superhydrophobic due to its high contact angle.
An example of the behavior of a fluid drop on an article of the present invention is shown in
Surface 120 (
Substrate 110 may comprise at least one of a metal, an alloy, a plastic, a ceramic, or any combination thereof. Substrate 110 may take the form of a film, a sheet, or a bulk shape. Substrate 110 may represent article 100 in its final form, such as a finished part; a near-net shape; or a preform that will be later made into article 100.
Surface 120 may be an integral part of substrate 110. For example, surface 120 may be formed by replicating a texture directly onto substrate 110, or by embossing the texture onto substrate, or by any other such method known in the art of forming or imparting a predetermined surface texture onto a substrate surface. Alternatively, surface 120 may comprise a layer that is disposed or deposited onto substrate 110 by any number of techniques that are known in the art.
As described above, surface 120 is not a smooth surface, but instead has a texture 130 comprising a plurality of features 135. The plurality of features 135 may be of any shape, include at least one of depressions, protrusions, nanoporous solids, foamed structures, indentations, or the like. The features may include bumps, cones, rods, wires, channels, substantially spherical features, foamed structures, substantially cylindrical features, pyramidal features, prismatic structures, combinations thereof, and the like. In certain embodiments, as depicted in
The plurality of features 135 (
Another aspect of the invention is to provide a method for making surface 120 and article 100 described hereinabove. The method comprises providing a substrate 110 and forming a surface 120 on the substrate 110. Surface 120 comprises a material having a nominal liquid wettability sufficient to generate, with a reference liquid, a nominal contact angle in a range from about 60° to about 90°, and the material comprises at least one material selected from the group consisting of a polymer and a ceramic. Surface 120 further comprises a texture 130 comprising a plurality of features 135. As previously described, the resulting surface 120 has a significantly reduced wettability compared to the nominal wettability of the material: wettability levels sufficiently low to generate, with the reference liquid, a contact angle in a range from about 120° to about 180°.
In one embodiment, shown schematically in
In another embodiment, the plurality of nanoparticles 540 and polymer solution 550 are blended together and applied to substrate 510 in a single step using commonly used coating techniques, such as spray casting, screen printing, roll casting, drop casting, dip coating, and the like. The nanoparticles comprise from about 0.001 volume percent to about 50 volume percent of the solution. The resulting coating may then be heat treated as previously described to ensure good cohesive and adhesive strength. Additional or alternative textures may then be provided by etching processes, as previously described hereinabove.
Alternatively, the plurality of nanoparticles 540 is deposited on substrate 510 to achieve substantially complete coverage. A monomer, such as a silazane monomer, is then infiltrated into the gaps between nanoparticles 540. The monomers are then crosslinked by the use of catalysts, heat, UV radiation, thermal crosslinking, solvents, and the like, to provide a robust polymeric coating. An additional or alternative texture may then be provided by etching processes, as previously described hereinabove.
In another embodiment, a plurality of nanoparticles 640 are assembled in a block copolymer matrix 600, as shown in
The methods of providing texture 130 that have been previously described above rely on the formation of at least one coating or layer on substrate 110. In other embodiments, texture 130 is provided to or formed directly on a surface of substrate 110. Texture 130 may be formed directly onto the surface 120 of substrate 110 by any one of replication, embossing, molding or extrusion. In one embodiment, a replicating means such as, but not limited to, a mold or a die, is provided with a template corresponding to the texture to be provided to a surface. A template of the replicating means is brought into contact with the surface, thereby imparting texture 130 to the surface.
The formation of texture 130 on a polycarbonate surface 120 is illustrated in
In one embodiment, article 100 comprises a film or sheet that is extruded through at least one die having a face having a texture corresponding to the desired texture for article 100. Texture 130 is transferred from the die face to article 100 during extrusion. In another embodiment, texture 130 is either compression molded or injection molded onto surface 120 of article 100 to produce a textured, low-wettability surface 120 on article 100. A mold having at least one face comprising texture is used to impart corresponding texture 130 to surface 120 of article 100 during the molding process.
In another embodiment, a plurality of nanoparticles is combined or mixed with a ceramic precursor such as, but not limited to, polysilazane precursors to form either a slurry or suspension of nanoparticles in the ceramic precursor. The slurry (or suspension) is then applied to a surface of an article by means that are well known in the art, such as spraying, spin coating, painting, dipping, and the like. The coated article is then heated to convert the ceramic precursor into a ceramic, such as, but not limited to an oxide, carbide, nitride, silicide, or combinations thereof, thereby forming a textured, low-wettability surface comprising the plurality of nanoparticles embedded in a ceramic coating. Such heating may involve calcining the ceramic precursor or heating under a reactive atmosphere. The resultant surface may be used in applications such as icing resistant coatings for aircraft turbines, where stability at high temperature is desired.
Additionally, surface 120 may be formed by vapor-based deposition techniques such as, but not limited to, PVD, LPCVD, CVD, PECVD, and combinations thereof.
Moreover, in some embodiments, the forming step is accomplished by chemically forming the features 130 onto the substrate 110. In certain embodiments, this is accomplished by manipulating the surface chemistry of the material to form one or more discrete chemical phases on the surface, for example via such well-known techniques as molecular self-assembly, crystallization, or other processes known to induce a phase separation on the surface. In other embodiments, chemical etching may be used by applying an etchant to the substrate. The etchant may comprise an acid, a base, a solvent, or other agent with suitable properties to react with the substrate to form features on the surface.
Where the reference liquid (i.e., the liquid for which the surface of the article shows low wettability) is water, the superhydrophobic nature of surface 120 makes it suitable for a number of applications that require resistance to fogging, soiling, contamination, and icing. Article 100 having surface 120 may also be used in applications in which a surface having low-drag, self-cleaning, and heat transfer properties are desirable.
In one embodiment, the surface 120 primarily provides article with an increased resistance to “icing:” the formation and accretion of ice through deposition and freezing of supercooled water droplets on a surface. In this embodiment, article 100 is an airfoil 1000, such as, but not limited to, aircraft wings, propellers, low pressure compressor and fan components of gas turbine engines, wind turbine blades, and helicopter blades—articles that are particularly susceptible to icing under certain conditions. A schematic representation of a cross-section of an airfoil having a low-drag icing-resistant surface is shown in
Due to the high contact angle on a superhydrophobic surface, the water on surface 120 forms small droplets and rolls off instead of sheeting, and the droplets carry away dirt particles along with them, thus leaving a clean dry surface. In situations such as fogging of a surface, small droplets formed by processes such as condensation deposit on the surface and reflect the light, thus making the surface “fog.” Because of the high contact angle 140 between the condensed droplets and surface 102, the droplets do not adhere to surface 120, but instead roll off. Accordingly, article 100 is resistant to such fogging. In one embodiment, article 100 is either transparent or translucent. Examples of article 100 in which surface 120 serves primarily as a self-cleaning, anti-fogging surface include lighting products, automotive products (such as headlamps, windows, and mirrors) building components (such as glass panes, windows, and mirrors), lenses, video displays and screens, and the like.
In one embodiment, the low-drag properties of surface 120 can be adapted to facilitate the transport of fluids, such as crude oil, water, and the like, through long pipelines. The friction within a pipeline typically leads to significant pressure drop. To overcome such a pressure drop, greater power is required to pump the fluids. Accordingly, article 100 may be a pipe, conduit, or the like, for conducting fluids and gases, having surface 120 is disposed within the pipe along the path of the fluid or gas; i.e., inside the pipe. Surface 120 with texture 130 will reduce the friction between fluid and the wall of pipe. Consequently, the power required to pump the fluid through the pipe will be significantly reduced.
Reduction of hydrodynamic drag has always been a priority for marine vessels. Increased drag not only increases the fuel consumption of the ship, but also is harmful to the environment due to larger amounts of emissions. Fouling of watercraft hulls by marine organisms is a prime source of increased drag. In yet another embodiment, the low-drag and self-cleaning properties of surface 120 can be adapted to reduce fouling and friction between water and a hydrodynamic body. Significant amounts of energy are required to overcome friction due to water. Such bodies experience significant flow friction from the water. Thus, article 100 may include various watercraft, ranging from ocean-going vessels and submarines to small sailboats and canoes. While hulls and other surfaces are shaped in such a way to reduce friction, additional reductions in skin friction may be obtained by providing the hull with surface 120 in accordance with embodiments of the present invention. Moreover, the low-wettability surface prevents marine organisms from adhering to the surface of a watercraft.
In another embodiment, the low-drag and self-cleaning properties of surface 120 can be adapted to fabrics for use in garments, furniture, hospital equipment, and the like. For example, article 100 may include clothing used in sports such as swimming, track and field, and bicycling. In another example, article 100 includes fabric upholstery and bedding having surface 120, thus utilizing the self-cleaning capability of surface 120. In such applications, at least one superhydrophobic film or coating in accordance with embodiments of the present invention may be applied to a surface of the article.
The self-cleaning properties of surface 120 are also useful in other applications. Accordingly, surface 120 may be incorporated into various household appliances, such as refrigerators, dishwashers, ovens, ranges and the like.
Surface 120 may be used in heat transfer applications, such as, but not limited to, heat exchangers, cooling towers, and other thermal-management systems, that rely on a phase change (e.g., boiling). Air bubbles on the texture of surface 120 nucleate at a higher rate than on a nominally a flat surface, facilitating heat transfer through the phase change and bubble formation and migration.
While typical embodiments have been set forth for the purpose of illustration, the foregoing description should not be deemed to be a limitation on the scope of the invention. Accordingly, various modifications, adaptations, and alternatives may occur to one skilled in the art without departing from the spirit and scope of the present invention.
