Leidenfrost on a ratchet

As discovered by Leidenfrost, liquids placed on very hot solids levitate on a cushion of their own vapour. These model hovercrafts are remarkably mobile: placed on a hot ratchet, a droplet not only levitates, but also self-propels, in a well-defined direction, at a well-defined velocity (typically, 10 cm s ^-1 ). The challenge is to understand the origin of the phenomenon, which contrasts with other situations where an asymmetry in the solid/liquid contact was used to generate liquid self-propulsion. We consider Leidenfrost solids that directly sublimate on hot substrates, and show that they also self-propel on ratchets. This leads to a scenario for the motion: the vapour flow escaping below the Leidenfrost body gets rectified by the presence of asymmetric textures, so that a directional thrust drives the levitating material. Using fishing lines to catch drops, we measure the force acting on them, and discuss both the driving force and the special friction generated by the textures.