|Title||Three-gradient regular solution model for simple liquids wetting complex surface topologies|
|Author(s)||Akerboom, Sabine; Kamperman, Marleen; Leermakers, Frans A.M.|
|Source||Beilstein Journal of Nanotechnology 7 (2016). - ISSN 2190-4286 - p. 1377 - 1396.|
Physical Chemistry and Soft Matter
|Publication type||Refereed Article in a scientific journal|
|Keyword(s)||Inverse opal - Regular solution model - Self-consistent field theory - Surface topology - Wetting|
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 θY <90°) to hydrophobic (effective advancing contact angle θ > 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 θ. Therefore, air entrapment cannot be the main reason why the contact angle θ 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 θ 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 θkink smaller than the angle θY. Even for θkink > θY, 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 θkink, determines the final observed contact angle.