Leidenfrost drops hover and move above hot surfaces on a thin layer of their own vapor. Over a flat surface, this vapor flows radially out from under the droplet, but creating rachets in the surface forces the vapor to flow in a single direction. The vapor then acts like exhaust, generating propulsion in the droplet and making it roll. How quickly the drop moves depends both on the droplet’s size and the rachets’ aspect ratio. For a given length, deeper rachets propel a drop faster than their shallower counterparts. The droplet’s size also affects the thrust with different scalings depending on the drop’s initial size. Like all of this week’s videos, this video is an entry in the 2013 Gallery of Fluid Motion. (Video credit: A. G. Marin et al.)
Self-Propelled Droplets
