Today, various ways of producing superhydrophobic surfaces are being developed given the diverse uses and applications that such surfaces have. Superhydrophobic are in high demand today given a unique set of characteristics that makes them more durable, efficient, and economic. Superhydrophobic surfaces have exceedingly high water repellence, and because of this they have the characteristic ability to self-clean. Self-cleaning is perhaps the most attractive characteristic that superhydrophobic surfaces possess. All surfaces that are in constant contact with the exterior are constantly stained, dirtied, and wetted. Therefore, any structure using superhydrophobic exterior surfaces would surely benefit from the self-cleaning attribute that such surfaces possess. Designers and builders begin to recognize this; society as a whole does too. It is because of this that today the demand for superhydrophobic surfaces in all kinds of material has increased astronomically in recent years.
Superhydrophobic and self-cleaning surfaces are of interest for various applications including self-cleaning windows, windshields, exterior paints for buildings and navigation of ships, utensils, roof tiles, textiles, solar panels and applications requiring antifouling and a reduction in drag in fluid flow, e.g. in micro/nanochannels (Bhushan, Jung, and Koch 1,632).
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It is equally important to note that research has been undertaken in order to develop superhydrophobic paints. Paint technology has been modified so that once applied on a given surface, the paint dries and forms a highly durable and rough layer that covers the surface, permeating it and giving it superhydrophobicity. These kinds of paints manage to diminish adhesion and increase water repellence given their special composition. Superhydrophobic paints “contain particles, solvent, and a polymeric binder. If the particle loading is high and binder content is low as the solvent evaporates, particles or aggregates distort the binder to generate a roughened surface” (Shirtcliffe, McHale, and Newton 1,204).
Superhydrophobic surfaces are characterized by increased water repellence, increased roughness, and decreased adhesion. Due to this, they have the distinct characteristic of self-cleaning. Based on this, superhydrophobicity can be defined as the “extreme water repellence of highly textured surfaces” (Shirtcliffe, McHale, and Newton 1,203). Hydrophobicity is a characteristic shared by all surfaces, regardless of the material that they are made of or the paint they are coated with. Hydrophobicity will vary depending on the surface material’s characteristics. There are cases in which the surface’s roughness, or topography, “modifies the surface interaction with a liquid to enhance the apparent hydrophobicity over that of a smooth surface of the same material” (Shirtcliffe, McHale, and Newton 1,203). In general, when this occurs hydrophobicity becomes superhydrophobicity and any water or liquid that comes in direct contact with the surface will be truncated, forming spherical shapes that will slide through the surface (taking with them any dirt clinging to the surface).
Another important consideration to be made when talking about superhydrophobic surfaces is the contact angle. The water hits the surface at a specific angle, and this angle determines whether or not the liquid hitting the surface slides off it. Therefore, in order to guarantee that a surface is superhydrophobic, it is important that the contact angle is just right. Based on the conducted research it has been possible to determine that as the surface roughens and becomes more superhydrophobic, the contact angle increases as well as the interaction surface.
As a water repellent surface is roughened, the contact angle between water drops and the surface increases, but they adhere more strongly, because the roughness increases the interaction area under the drop. Eventually, the roughness becomes so great that bridging over the roughness, leaving gas pockets under the liquid, is lower in energy than wetting the whole surface.
Usually the contact angle for flat surfaces revolves around 120 degrees. This is the exact contact angle that polytetrafluoroethylene (PTFE) presents; other flat surfaces have contact angles slightly higher. Research has uncovered rough surfaces with contact angles as high as 180 degrees, but as the angle measure increases it becomes harder to make precise measurements. As well, it is important to keep in mind that as the contact angle increases, so does the surface’s adhesion. However, despite of the increased adhesion liquids can easily slide off such surfaces because of the increased contact angle hysteresis. In these cases in which the contact angle and the contact angle hysteresis is exceedingly high, the contact area decreases and the liquid slides off the surface. In other words, the liquid would rapidly slide off the surface “if it is slightly tilted, carrying particulate contamination away and leaving no residue”.
Finally, it is equally important to discuss the two superhydrophobic states that are most commonly found: Wenzel’s state and Cassie’s state. First, Wenzel’s state is the state in which “the water droplets pin the surface in a wet-contact mode, and as a result, high CA hysteresis is observed”. Second, Cassie’s state is that in which “the water droplets adopt a non-wet-contact mode on solid surfaces and can roll off easily owing to their low adhesive force” (ref). In Wenzel’s case, the contact angle hysteresis cannot be accurately determined given that the water droplets cannot slide on the surface. In Cassie’s case, however, it is possible to accurately measure the contact angle hysteresis because liquids do slide off the surface.