Chapter One - Superhydrophobicity – An Introductory Review

Abstract

Superhydrophobicity as a phenomenon has become an increasing focus of research and technological activity, where its fundamental aspects span surface chemistry, chemical physics, and cellular biology. Additionally, its significance to the behavior of natural systems, interfacial fluid dynamics, and biotechnology represents an area rapidly gaining potential importance. Detailed reviews have progressively explored superhydrophobicity from a number of viewpoints (e.g., Ma and Hill, 2006; Quéré, 2002; Shirtcliffe et al., 2010). Here, aspects underlying this wetting behavior are illustrated. It has long been recognized that surface roughness has a profound effect on wetting behavior, in particular through apparent contact angles and subsequent contact angle hysteresis (Bico et al., 2001; Quéré, 2008). Quéré (2008) points out that both chemical and structural surface heterogeneity can cause pinning of the three-phase contact line (TPL) of an advancing wetting front, whereby the difference in the advancing and receding contact angles produces a Laplace pressure and hence a force resisting further liquid advancement. Movement of the wetting front (advancing and receding) can be viewed as a kinetic process in response to changing forces at the TPL that characteristically produce jumps in the movement of this line. Rough and microstructured surfaces inherently increase hydrophobicity of hydrophobic surfaces through two very different mechanisms: a purely geometrical increase in the actual surface area with respect to its projected area generally termed the Wenzel state (Wenzel, 1936) and a composite interfacial effect arising from an air–water interface when air is trapped between microstructural features of the surface ahead of the advancing wetting front forming a Cassie–Baxter state (Cassie and Baxter, 1944), as illustrated in Figure 1. As such, these conditions represent homogeneous and heterogeneous surface wetting systems, respectively, and in both cases are derived from the result of variations in the interfacial energy of the substrate phase(s) solid or solid-vapor.