Cassie-Baxter to Wenzel state wetting transition: Scaling of the front velocity

Abstract

We experimentally study the dynamics of water in the Cassie-Baxter state to Wenzel state transition on surfaces decorated with assemblies of micrometer-size square pillars arranged on a square lattice. The transition on the micro-patterned superhydrophobic polymer surfaces is followed with a high-speed camera. Detailed analysis of the movement of the liquid during this transition reveals the wetting front velocity dependence on the geometry and material properties. We show that a decrease in gap size as well as an increase in pillar height and intrinsic material hydrophobicity result in a lower front velocity. Scaling arguments based on balancing surface forces and viscous dissipation allow us to derive a relation with which we can rescale all experimentally measured front velocities, obtained for various pattern geometries and materials, on one single curve.

Influence of Heat Treatment and UV Irradiation on the Wettability of Ti35Nb10Ta Nanotubes

“…The wettability of a solid surface by a liquid is determined by the liquid's surface tension, the solid surface's roughness and the solid's surface energy. The higher contact angle values found for the dark-stored nanotubes can be explained mainly by the hydrophobicity due to the Cassie-Bexter state where the liquid does not penetrate into the hollows of the nanotube surface and, consequently, faces a composite interface consisting of both solid and vapor; and also to the non-photoinduced hydrophilicity of TiO 2 [32][33][34][35][36][37]. The nanotube morphology allowed the air still trapped inside the nanotubes and the behavior to become substantially hydrophobic [32,33].…”

“…The nanotube that formed after running the anodic oxidation process for 45 min and without heat treatment exhibited the biggest contact angle in water (140°) compared to the 47° value obtained for the polished surface or the one of 109° for the nanotubes obtained at 15 V and with heat treatment (Figure 3). Nanotubular morphology and the fluorine content are considered two factors that increase the surface´s hydrophobic nature [29][30][31][32][33]. Contact angles substantially increase for the nanotube surface; the layers with large contact angles display hydrophobic properties.…”

Controlled liquid entrapment over patterned sidewalls in confined geometries

“…The challenge in extending our approach to such systems arises from the effect of wetting dynamics since the interface can now progress in two directions and interface speed is required to predict relative movement between the two directions. We refer the readers to reports that discuss these effects [33,34]. We also note that our analysis is currently restricted to a case where liquid flows parallel to the corrugated surface, and the results would be modified if the flow direction changes.…”

Response to “Comment on ‘Experimental study of skin friction drag reduction on superhydrophobic flat plates in high Reynolds number boundary layer flow’” [Phys. Fluid 25, 079101 (2013)]

“…The time scale of the air depletion in a static (still) condition is known to be hours. [3][4][5][6] Thus, the air depletion observed in the experiment was mainly due to the flow effect. The optical method suggested by Gad-el-Hak 2 for monitoring the longevity of air is interesting.…”