Viscous mechanism for Leidenfrost propulsion on a ratchet

An evaporating drop placed on a ratchet shaped substrate self-propels, as discovered by Linke et al. in 2006. Sublimating platelets of dry ice do the same, and we discuss a possible viscous mechanism for these motions. We report that the flow of vapor below the levitating material is rectified by the asymmetric teeth of the ratchet, in the direction of descending slopes along each tooth. As a consequence, the resulting viscous stress can entrain the material in the same direction, and we discuss the resulting self-propelling force.