Self-excitation of Leidenfrost drops and consequences on their stability

Abstract

Significance A volatile liquid placed on a hot plate is known to levitate on a cushion of vapor. Such drops have unusual behaviors, among which is the ability to transform in pulsating stars that sporadically appear for a few seconds. We report that the vapor cushion of Leidenfrost drops spontaneously vibrates, which can trigger the pulsating modes via a Faraday instability. The mediating waves and subsequent peripheric undulations resonate at well-defined drop sizes, which explains their transient nature and the mode hierarchy. These effects illustrate the unique nature of a situation combining hydrodynamics, aerodynamics, and phase change. Beyond the light shed on the origin of Leidenfrost stars, levitating drops also provide toy models for the Faraday instability in open, globular, small-size systems. Volatile liquids (water, alcohol, etc.) poured on hot solids levitate above a layer of vapor. Unexpectedly, these so-called Leidenfrost drops often suddenly start to oscillate with star shapes, a phenomenon first reported about 140 y ago. Similar shapes are known to be triggered when a liquid is subjected to an external periodic forcing, but the unforced Leidenfrost case remains unsolved. We show that the levitating drops are excited by an intrinsic periodic forcing arising from a vibration of the vapor cushion. We discuss the frequency of the vibrations and how they can excite surface standing waves possibly amplified under geometric conditions of resonance—an ensemble of observations that provide a plausible scenario for the origin, mode selection, and sporadic nature of the Leidenfrost stars.