Leidenfrost self-propulsion on ratchets

Settled in 2006 by Heiner Linke, Brian J. Alemán, Lindsay D. Melling, et al..

The Leidenfrost effect, where a liquid drop levitates on its own vapor cushion above a very hot surface, has been known since 1756, but for 250 years the vapor layer was seen mainly as a curiosity that slows evaporation. In 2006 Linke and coworkers asked whether the vapor flow underneath a Leidenfrost drop could be harnessed for propulsion, and showed that a sawtooth-ratchet-textured hot surface rectifies the escaping vapor flow, pushing millimeter-to-centimeter drops of various liquids (and even dry ice pucks) in a controlled direction at speeds up to several centimeters per second. The same paper and later work established the mechanism as a viscous drag from the confined, asymmetric vapor channel rather than surface tension gradients, settling the question convincingly.

A drop of water skating on its own vapor cushion can be steered like a tiny self-driving boat just by cutting saw-teeth into the hotplate beneath it.

Explanations that circulated

• Droplet motion on a ratchet is just gravity pulling it downhill along the sawtooth — the effect works and is directional even on level or slightly uphill ratchets, showing it is driven by viscous shear from vapor flowing asymmetrically beneath the drop, not simple gravity.

References

• Self-Propelled Leidenfrost Droplets, H. Linke, B. J. Alemán, L. D. Melling, M. J. Taormina, M. J. Francis, C. C. Dow-Hygelund, V. Narayanan, R. P. Taylor, A. Stout, Physical Review Letters, 2006