Bouncing or sticky droplets:: Impalement transitions on superhydrophobic micropatterned surfaces

When a liquid drop impinges a hydrophobic rough surface it can either bounce off the surface ( fakir droplets) or be impaled and strongly stuck on it ( Wenzel droplets). The analysis of drop impact and quasi-static "loading" experiments on model microfabricated surfaces allows to clearly identify the forces hindering the impalement transitions. A simple semi-quantitative model is proposed to account for the observed relation between the surface topography and the robustness of fakir non-wetting states. Motivated by potential applications in microfluidics and in the fabrication of self-cleaning surfaces, we finally propose some guidelines to design robust superhydrophobic surfaces.