Functional coatings are very important in industry and the consumer marketplace today. There can be many reasons for coatings, ranging from performance for intended applications, to safety considerations, to total part costs (e.g., utilizing specialized materials for exposed surfaces rather than entire bulk objects).
Functional performance properties that are currently desired include anti-reflective, anti-icing, superhydrophobic, superhydrophilic, de-wetting, and self-cleaning properties. Anti-icing (or ice-repellent) coatings can have significant impact on improving safety in many infrastructure, transportation, and cooling systems. Among numerous problems caused by icing, many are due to striking of supercooled water droplets onto a solid surface. Such icing caused by supercooled water, also known as freezing rain, atmospheric icing, or impact ice, is notorious for glazing roadways, breaking tree limbs and power lines, and stalling airfoil of aircrafts.
When supercooled water impacts surfaces, icing may occur through a heterogeneous nucleation process at the contact between water and the particles exposed on the surfaces. Icing of supercooled water on surfaces is a complex phenomenon, and it may also depend on ice adhesion, hydrodynamic conditions, the structure of the water film on the surface, and the surface energy of the surface (how well the water wets it).
Melting-point-depression fluids are well-known as a single-use approach that must be applied either just before or after icing occurs. These fluids (e.g., ethylene or propylene glycol) naturally dissipate under typical conditions of intended use (e.g. aircraft wings, roads, and windshields). These fluids do not provide extended (e.g., longer than about one hour) deicing or anti-icing. Similarly, sprayed Teflon® or fluorocarbon particles affect wetting but are removed by wiping the surface. These materials are not durable.
Recent efforts for developing anti-icing or ice-phobic surfaces have been mostly devoted to utilize lotus leaf-inspired superhydrophobic surfaces. These surfaces fail in high humidity conditions, however, due to water condensation and frost formation, and even lead to increased ice adhesion due to a large surface area.
Superhydrophobicity, characterized by the high contact angle and small hysteresis of water droplets, on surfaces has been attributed to a layer of air pockets formed between water and a rough substrate. Many investigators have thus produced high contact angle surfaces through combinations of hydrophobic surface features combined with roughness or surface texture. One common method is to apply lithographic techniques to form regular features on a surface. This typically involves the creation of a series of pillars or posts that force the droplet to interact with a large area fraction of air-water interface. However, surface features such as these are not easily scalable due to the lithographic techniques used to fabricate them. Also, such surface features are susceptible to impact or abrasion during normal use.
Other investigators have produced coatings capable of freezing-point depression of water. This typically involves the use of small particles which are known to reduce freezing point. Many of these coatings can actually be removed by simply wiping the surface, or through other impacts. Others have introduced melting depressants (salts or glycols) that leech out of surfaces. Once the leeching is done, the coatings do not work as anti-icing surfaces.
Nanoparticle-polymer composite coatings can provide melting-point depression and enable anti-icing, but they do not generally resist wetting of water on the surface. When water is not repelled from the surface, ice layers can still form that are difficult to remove. Even when there is some surface roughness initially, following abrasion the nanoparticles will no longer be present and the coatings will not function effectively as anti-icing surfaces.
Generally, surfaces with anti-reflective, anti-icing, superhydrophobic, superhydrophilic, de-wetting, and self-cleaning properties rely on structural features (ordered or disordered) in the size range of a few nanometers to tens of micrometers. There are various approaches to achieve such surface structural features. Conventional approaches using light to create polymer surface patterns with anti-reflective, anti-icing, de-wetting, self-cleaning, and superhydrophobic/superhydrophilic properties require either a patterned exposure of light (patterning light intensity or polarization) or filler particles that provide the surface topology.
Reactive ion etching methods can be used to create nanopillars on a silicon wafer. A pattern is first formed by using patterned light and/or interference lithography. Pillars are then etched using a reactive ion beam. This method is very expensive and time consuming.
Self-assembly methods are known to form colloidal sphere crystals and inverse crystals. These are multi-step fabrication methods with expensive colloidal spheres as essential building blocks.
Nanoparticle-polymer composite coatings can be fabricated through co-assembly. Nanoparticles need to be pre-fabricated, and the technique is time-consuming and expensive. There is little control over surface structure.
Solvent-based coatings with filler particles are well-known in the coating industry. A surface texture that provides a matte finish is achieved when the solvent evaporates. Filler particles create the surface texture.
There is a need in the art for surface-textured coatings that can be conveniently produced without requiring patterned light during production or filler particles in the final coating product. Such coatings preferably utilize low-cost, lightweight, and environmentally benign materials that can be rapidly (minutes or hours, not days) produced over large areas. These surface-structured coatings should be able to be modified with various chemistries for use in anti-reflective, anti-icing, superhydrophobic, superhydrophilic, de-wetting, and self-cleaning applications.