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.

Keywords

Biotechnology; Natural systems; Superhydrophobicity; Three-phase contact line; Wettability Glossary TPL    three-phase contact line S-L-V    solid-liquid-vapour phases CMC    critical micelle concentration 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.
Figure 1 Contact angles on structured surfaces in the classic Wenzel (W) and Cassie-Baxter (C-B) states, respectively, where r is the physical amplification of surface area due to roughness and fS is the fractional area in contact with air. Adapted from Shirtcliffe et al. (2010). Marmur characterized superhydrophocity according to two criteria (Marmur, 2004), a very high contact angle and very low drop roll-off angle, as well as addressing the transition between these states in terms of their metastability and its impact on superhydrophobicity (Marmur, 2003). Since the free energy increases with increasing contact angle, the state that is most stable is represented by that with lowest contact angle. Gao and Yan (2009) and Quéré (2008) point out that due to local energy minima, a water drop cannot only assume multiple energy states but also coexist on a particular surface, and from the energy barrier for the transition Cassie-Baxter to the Wenzel state, surface geometry influences the interfacial energy requirement to reach equilibrium whether homogeneous or heterogeneous wetting is involved. Pinning of the advancing TPL is also dependent on the topology (feature size, spacing, and shape). Shirtcliffe et al. (2010) and McHale (2007) note that this also determines the observed contact angle and consistency with the Wenzel and Cassie-Baxter models since the incremental advancing area is assumed to characterize the surface overall. And, as a result, features on either side of the TPL do not influence the contact angle over the increment but must be of sufficiently small size to ensure that an average of the overall topology is sampled by the TPL at any one time, as shown by McHale (2007) and Shirtcliffe et al. (2010) for a random surface, illustrated in Figure 2.
Figure 2 Advancing wetting front across homogeneous zones 1 and 2 with respective contact angles ?1,2 and across a microscopically random surface (1,2) with contact angle ?c-B. Interestingly, these studies also illustrate that a systematic variation in topological pattern within the bounds of a TPL yields a driving force for liquid transport not reliant on capillary suction (Figure 3), which may have implications for microfluidic device design. Transition between a Cassie-Baxter suspended state and complete surface immersional Wenzel wetting readily occurs through the imposition of external forces whether mechanical, hydrostatic, or compositional or through the size of the drop and its internal Laplace pressure (Ishino et al., 2004; Lafuma and Quéré, 2003). The enhancement of superhydrophocity, through a hierarchical surface roughness where a nanoscale topology is superimposed on surface microscale roughness, has now been widely studied. In many cases, this has been related to fractal behavior (Bottiglione and Carbone, 2013). Nosonovsky (2007), in investigating the instability of the Cassie-Baxter wetting state and its transformation to Wenzel wetting, established hierarchical roughness as a criterion enhancing the composite wetting state, which was then shown to apply to both natural and synthetic surfaces and suggested as a general requirement when other destabilizing scale-dependent effects are present such as in microprocesses. Noting the transition to a stable Cassie state under imposed vibration on the Lotus leaf surface (Boreyko and Chen, 2009), Boreyko and Collier (2013) then examined the obverse situation where drops in the Wenzel condition can assume the Cassie-Baxter state through a dewetting transition. Here, they show that pinned Wenzel drops cannot be totally stretched during a transition to reduce their contact angle below a critical curvature since pinch-off of the drop surface occurs, resulting in a partial liquid Wenzel droplet remaining on the surface. They thereby provide a condition for a Wenzel dewetting where the receding contact angle needs to be moderately large (~90°).
Figure 3 Influence of a variation in surface topology producing an additional wetting force and fluid flow. Adapted from Shirtcliffe et al. (2010). Contact angle and movement at the TPL (wetting or dewetting) have been noted as a response to forces at the intersection of interfaces between the three principal phases (S-L-V). Within this line of contact, where the feature size of surface topology exerts influence, an additional line tension (Rusanov et al., 2004) exists akin to a "one dimensional surface tension" that may influence the contact angle and hence the Wenzel and Cassie-Baxter models and their respective transitions, if the droplet and feature size are small. Amirfazli and Neumann (2004) point out, in a detailed analysis of the status of line tension, that line tension influences contact...