Friction properties of superhydrophobic ridges

The extreme mobility of droplets on non-wetting materials implies the necessity of controlling their motion, direction or speed. In this paper, we show how ridges allow us to tune drop friction. Depending on the liquid speed and viscosity, two regimes emerge: fast drops with low viscosity dynamically deform and undergo inertial friction, so that their velocity is eventually fixed by the deformations induced by the ridges; in contrast, viscous drops hardly interact with the texture, so that their velocity is classically limited by viscous dissipation, as on a flat substrate. The transition between these two regimes reveals spectacular morphological changes: drops with intermediate viscosity elongate and adopt worm-like shapes, which we qualitatively describe.