Forces at superhydrophobic and superamphiphobic surfaces

Forces exerted at surfaces and interphases due to formation of gaseous (air or vapor) bridges describe the extreme liquid repellence in superhydrophobicity (SH) and amphiphobicity. The neighboring research areas of liquid capillary bridges and that of interactions between hydrophobic surfaces are highly valuable reference systems. We review recent findings with particular focus on the three-phase contact line and surface forces. Although macroscopic contact angles (>150°), low contact angle hysteresis (<10°, but can be high; parahydrophobic or petal type) and low roll-off angle (≤5–10°) are adequate criteria for SH and superamphiphobicity (SA) for most studies, a detailed understanding requires a view related to mechanisms. Experimental studies of liquid drop–substrate and particle–substrate adhesion in hydrophobic, SH, and SA systems are summarized by relating measured forces to the wetting tension, γcosθ. A low wetting tension value is a necessary but not sufficient criterion for SH and SA systems. The picture emerging from detailed force distance studies is that extreme liquid repellence in SH and SA systems is a progression of liquid repellence due to hydrophobicity, in which force curves can be explained by capillary forces of constant volume of the gaseous capillary. In SH and SA, neither the capillary force equation assuming constant volume nor constant pressure of the gaseous capillary explains experimental force measurements as the capillary increases in both volume and pressure. In recent experimental studies, a transition is observed into nonconstant volume and pressure which suggests an SH and SA wetting transition from constant volume or pressure to a capillary growth as driven by the γA and the PV works but also by forces at the three-phase contact SLV (solid-liquid-vapor) line, viz. pinning forces, Fpin and Fdepin, and line energy, (τL)SLV, terms. Supported by calculations of the different contributions, we suggest this transition being an appropriate definition for the onset of (appreciable) SH and SA.