Leidenfrost wheels (vol 14, pg 1188, 2018)

As reported in 1756 by Johann Gottlob Leidenfrost, volatile liquids on hot solids form "gleaming drops resembling quicksilver", a consequence of their levitation on a vapour cushion(1,2). This makes the drops spectacularly mobile, moving away as soon as they are deposited-an observation commonly attributed to gravity or surrounding airflows. This mobility has been exploited to manipulate drops, because tiny forces such as those generated on asymmetric substrates can move them in well-defined directions(3-5), a situation that also provides heat evacuation(6). Here we report that Leidenfrost droplets initially at rest on horizontal substrates self-rotate and self-propel in the direction they are rolling, in the absence of any source of asymmetry or external force. Their rapid internal flow is found to be accompanied by a tilting of their base, which creates a permanent ratchet-like mechanism, entraining the rolling liquid despite the fact that it is not in contact with its substrate.