fuckyeahfluiddynamics:

When liquids hit a surface much hotter than their boiling point, a thin layer of gas can form between the drop and surface, allowing the drop to glide along. This Leidenfrost effect is what makes drops of water skitter across a hot pan. But what happens when the pan isn’t flat? The video above shows a Leidenfrost drop on a ratchet-like surface. Instead of gliding or skittering randomly, the drop self-propels toward the steepest section of the ratchet  This behavior allows researchers to design surfaces that guide the drops on an intended path. (Video credit: G. Lagubeau and D. Quéré)

(Source: annualreviews.org)