“Asymmetric Wettability of Nanostructures Directs Leidenfrost Droplets”

A water droplet deposited onto a surface heated above a certain temperature can levitate on a thin cushion of its own vapor. This phenomenon, known as the Leidenfrost effect, is associated with dramatically reduced friction and heat exchange between the solid and liquid phases and can also lead to directionally self-propelled droplets on surfaces with asymmetric micro and macro scale features resembling surface profiles in ratchet gears. Here, we demonstrate for the first time that directionality of self-propelled Leidenfrost droplets can be achieved on surfaces with nanoscale features, namely, on tilted nanopillar arrays. High speed video analysis of bouncing Leidenfrost droplets on such asymmetrically nanostructured surfaces reveals an unexpected role of the asymmetric wettability of tilted nanopillar arrays during their intermittent contact with the Leidenfrost droplets as a key mechanism responsible for the directional droplet rebound and subsequent movement