Three-gradient regular solution model for simple liquids wetting complex surface topologies.

ABSTRACT: We use regular solution theory and implement a three-gradient model for a liquid/vapour system in contact with a complex surface topology to study the shape of a liquid drop in advancing and receding wetting scenarios. More specifically, we study droplets on an inverse opal: spherical cavities in a hexagonal pattern. In line with experimental data, we find that the surface may switch from hydrophilic (contact angle on a smooth surface ?_Y < 90°) to hydrophobic (effective advancing contact angle ? > 90°). Both the Wenzel wetting state, that is cavities under the liquid are filled, as well as the Cassie-Baxter wetting state, that is air entrapment in the cavities under the liquid, were observed using our approach, without a discontinuity in the water front shape or in the water advancing contact angle ?. Therefore, air entrapment cannot be the main reason why the contact angle ? for an advancing water front varies. Rather, the contact line is pinned and curved due to the surface structures, inducing curvature perpendicular to the plane in which the contact angle ? is observed, and the contact line does not move in a continuous way, but via depinning transitions. The pinning is not limited to kinks in the surface with angles ?_kink smaller than the angle ?_Y. Even for ?_kink > ?_Y, contact line pinning is found. Therefore, the full 3D-structure of the inverse opal, rather than a simple parameter such as the wetting state or ?_kink, determines the final observed contact angle.

SUBMITTER: Akerboom S 

PROVIDER: S-EPMC5082679 | biostudies-literature | 2016

REPOSITORIES: biostudies-literature