Leidenfrost wheels

The high mobility of Leidenfrost drops is commonly attributed to the lack of contact with the hot substrate, and it makes them sensitive to tiny forces, such as gravity or airflows. However, we observed that drops deposited without initial velocity on strictly horizontal substrates self-propel in random directions. This phenomenon~is triggered by confinement. PIV measurements reveal that, as the drop aspect ratio of the drop becomes of order unity, internal motions of liquid systematically switch from toroidal axisymmetric to~asymmetric~rolling. Such a flow reshapes the vapor thickness and tilts it, as evidenced by interferometric measurement, hence leading to propulsion. Droplets are thus found to generate their own dynamics and self-propel despite the absence of external field, which contributes to their legendary mobility