Surface cleaning and repair of, for example, buildings, vehicles, and energy collection devices, are time-consuming and costly, and a surface with an inherent repellency of water, oil, and dirt can be a significant advantage. Surface wetting is governed by surface-energy parameters between the surface and the contacting liquid or solid surface. Where the sum of the free surface energies between materials components is very low, adhesion between these materials is weak. Hence, it is generally beneficial to lower the free surface energy of an edifice in order to ignore its cleaning and repair. Non-stick materials, such as perfluorinated hydrocarbons, for example, Teflon®, have very low surface energies such that few materials adhere. The wetting of these low surface energy materials is reflected in the contact area that is observed between the surface of the low surface energy solid and a wetting material. The interactions between these materials generally result from van der Waals forces.
Nature diminishes the interaction of a surface of a solid and water without resorting to materials with surface energies as low as Teflon®. This is achieved by reducing the amount of the surface that contacts the water. For example, lotus leaves, cabbage leaves, and various fruits are covered by small wax bumps that reduce the van der Waals contact area presented to a water droplet that forms due to its high surface tension, which significantly reduces the adhesion of the droplets to the surface. These superhydrophobic textured surfaces display water contact angles that are in excess of 150° and display low sliding angles, which is the critical angle from horizontal of the inclined surface where a water droplet of a defined mass rolls off the inclined surface. This “Lotus effect” provides a self-cleaning surface, as contact water droplets adhere to dust particles and, to a much lesser degree, to some oils that are poorly adhered to the surface, which allows the “dirt” to be carried away as the water droplet rolls off the surface.
Recently, products have been introduced to the market for environmental coatings and other surfaces based on the “lotus effect” that display superhydrophobicity, where water readily rolls off with particulates that have soiled the surface when exposed to water. StoCoat® Lotusan® is a one-part coating that is brushed, rolled, or sprayed onto a surface and Rust Oleum® Neverwet® is available for two-part spray coating. Both of these products have durability and performance issues and are not oil repellent.
Most oils are not readily removed from such hydrophobic surfaces, as the enlarged surface area increases the effective van der Waals interface and the Lotus-effect surface does not repel oils that cannot interact more favorably with water than the textured surface. Oil repellent surfaces are an engineering challenge because the surface tensions of oily liquids are usually in the range of 20-30 mN/m. Hence, the essential criterion, for having a surface with oleophobicity, is to maintain oil drops in a Cassie-Baxter (CB) state, one where vapor pockets are trapped underneath the liquid. The CB state is dependent on the surface's structure and the surface energy of the material. If the structure and surface area are insufficient, the meta-stable energetic state is transformed into Wenzel state. The geometric features that allow this state have re-entrant structures, such as mushroom heads, micro-hoodoos, or horizontally aligned cylindrical rods. A re-entrant structure implies that a line drawn vertically, from the base solid surface through the geometric feature, must proceed through more than one solid interface of that feature.
Although oleophobic surfaces have been produced, there remains a need for superhydrophobic and oleophobic surfaces that can be produced easily on substrates and display a durability that is not presently available.