Wetting on Hydrophobic Rough Surfaces:  To Be Heterogeneous or Not To Be?

Abstract

Equilibrium wetting on rough surfaces is discussed in terms of the “competition” between complete liquid penetration into the roughness grooves and entrapment of air bubbles inside the grooves underneath the liquid. The former is the homogeneous wetting regime, usually described by the Wenzel equation. The latter is the heterogeneous wetting regime that is described by the Cassie−Baxter equation. Understanding this “competition” is essential for the design of ultrahydrophobic surfaces. The present discussion puts the Wenzel and Cassie−Baxter equations into proper mathematical−thermodynamic perspective and defines the conditions for determining the transition between the homogeneous and heterogeneous wetting regimes. In particular, a new condition that is necessary for the existence of the heterogeneous wetting regime is added. It is demonstrated that when this condition is violated, the homogeneous wetting regime is in effect, even though the Cassie−Baxter equation may be satisfied.

Highly Polar Linkers (Urea, Carbamate, Thiocarbamate) for Superoleophobic/Superhydrophobic or Oleophobic/Hydrophilic Properties

“…[ 42,43 ] In the Wenzel state, a liquid droplet enters all the surface asperities. [ [43][44][45] In the Cassie-Baxter state, a liquid droplet is only in contact with the top of the surface roughness and also on air fractions trapped between the liquid droplet and the substrate. The Cassie-Baxter can predict superhydrophobic properties with low H water if the air fraction is suffi ciently important.…”

Fabrics with Tunable Oleophobicity

“…between the liquid and the textured substrate becomes equal to the equilibrium contact angle " (given by Young's relation [22] ) for the three phase contact line. [23][24][25] The existence of points on the surface that enables the condition ! = " [26] to be fulfilled is a necessary, though not sufficient condition, [19,24,25,27,28] for the formation of a composite interface, such as the one shown in Fig.…”

Design of a robust superhydrophobic surface: thermodynamic and kinetic analysis

“…Existing approaches at surface design involve b/a ratio less than the critical limit 12,17,19,21,39,40 Although the critical limit is wellknown, the domains of dependent parameters, namely c/a ratio and θY have not yet been investigated. It can be shown that the critical limit assumes a positive real value, if and only if the Young's contact angle exceeds 90° (equation 8).…”