Nanostructured superhydrophobic surfaces

Abstract

Superhydrophobic surfaces play an important role in many applications, such as contamination prevention, biocompatibility, enhanced lubricity and durability of materials [1–9]. Hydrophobicity is measured by the contact angle of pure water on a clean solid surface. When the contact angle is greater than 150◦, the surface is considered to be superhydrophobic. The water– solid contact angle varies with the surface energy and roughness of the solid surface. The surface energy of a solid is determined by the surface chemistry, which in turn depends on the chemical composition and atomic arrangements at or near the surface. Structure relaxation, surface restructuring, and composition segregation all can reduce the surface energy and, thus, result in an increase in contact angle. However, such a reduction in surface energy would result in a limited increase in contact angle, and is insufficient to make a surface superhydrophobic. Usually trifluoromethyl carbon (−CF3) containing diblock polymers, surfactants, or coupling agents are self-assembled on the solid surface to form a monolayer [10]. Surfaces terminated with trifluoromethyl carbon ligands possesses highest electron–fluorine affinity and consequently the lowest surface energy. Contact angle decreases when the −CF3 monolayer is less than close-packed, but even on the fully packed monolayers, contact angle on a smooth surface generally does not exceed 120◦. A further increase in contact angle and hydrophobicity, will require an increase in surface roughness [11]. The idea is conceptually straightforward; increased roughness will result in an increase in true surface area and, thus, lead to an increased nominal surface energy. Accordingly, the contact angle varies as dictated by Young’s equation. It has been demonstrated that the contact angle will increase with increased roughness of a hydrophobic surface, whereas the contact angle will decrease with increased roughness of a hydrophilic surface. This relationship was established by Wenzel and referred to as Wenzel’s law [12, 13]. However, Young’s equation and Wenzel’s law can only be used if the water droplet has complete contact with the rough surface. In reality, the contact between water and a hydrophobic rough surface cannot reach 100% and some air bubbles will be trapped at the interface. Consequently, the Young’s equation should be modified