Drop Impact on Two-Tier Monostable Superrepellent Surfaces.

Superrepellency is a favorable nonwetting scenario featuring a dramatic reduction of the solid/liquid contact area. The robustness of superhydrophobicity plays a central role in self-cleaning and anti-icing. Drop impacts happen ubiquitously in natural environments and often cause a notable extension of the solid/liquid contact area. This is associated with an enhanced affinity between water and the microtextures and therefore leads to irreversible breakdowns in the superhydrophobicity. This problem remains a major challenge and limits the practical applications of superrepellent materials. In order to find a solution, in this paper, a repeated Cassie-Wenzel-Cassie wetting state transition is studied at the microscale when a drop impacts a two-tier superhydrophobic surface. In this case, the surface is completely dry without any liquid residue after the drop rebounds. The present results exhibit a striking contrast to the conventional perspective. The influence of geometrical parameters of the textured surface on the spreading and retracting behaviors of the impact drops is quantified, as well as the time-dependence scaling laws. From a practical point of view, it is demonstrated that the self-cleaning and dropwise condensation may significantly benefit from this repeated wetting transition. Dirt particles or small droplets in deep textures are able to be taken away so that the functionality and the robustness of the superhydrophobicity may be significantly strengthened. The results reported in this study facilitate the design of functional superrepellent materials.