Reversible self-propelled Leidenfrost droplets on ratchet surfaces

We investigate the self-propelled motion of the Leidenfrost droplets on hot surfaces with ratchet like topology. It is found that on hot ratchet surfaces with the certain geometry parameters, the droplets move in the direction towards the steep side of the teeth; as the surface temperature rises, droplets are observed to self propel to the right-angle side direction. Furthermore, there exists a temperature threshold to trigger the motion, at which the droplets do not move in either direction but stay at the stagnation state. A physical model is proposed to analyze the observation in this paper.