Propulsion on a superhydrophobic ratchet

As shown by Linke in 2006, an evaporating Leidenfrost drop self-propels on a hot ratchet. Indeed, the vapour flow below the drop can be rectified by the asymmetric teeth of the ratchet and, therefore, entrain the levitating drop by viscosity. This motion is usually observed above the Leidenfrost temperature. We show how the use of a super-hydrophobic ratchet allows us to extend self-propulsion down to the boiling point of water, and even below. We discuss a possible explanation for this ``cold regime'' of propulsion.