Why Leidenfrost droplets spontaneously oscillate into star shapes

Settled in 2017 by X. Ma, J.-J. Liétor-Santos, J. C. Burton.

Leidenfrost's basic vapor-levitation effect dates to 1756, but the striking star-shaped, multi-lobed oscillation of the levitated drop was first documented by Holter and Glasscock in 1952 and remained only qualitatively understood for decades. The puzzle was why the number of lobes and the pattern's wavelength were so regular despite depending in complicated ways on drop size, viscosity, and substrate temperature. Ma, Lietor-Santos, and Burton's 2017 study showed the oscillations are driven by capillary waves of a characteristic wavelength set by the liquid's capillary length, excited by shear stress at the liquid-vapor interface through a parametric (Faraday-wave-like) forcing mechanism at roughly twice the drop's oscillation frequency, which explained why wavelength is independent of drop radius and temperature while the number of visible lobes depends on viscosity.

A drop of water hovering on its own steam, untouched by the hot plate beneath it, spontaneously organizes itself into a spinning, many-pointed star.

Explanations that circulated

• The star shape is caused by irregularities or dirt on the hot plate surface
• The oscillation frequency depends mainly on the drop's size or the plate's temperature

References

• Star-shaped oscillations of Leidenfrost drops, Xiaolei Ma, Juan-Jose Lietor-Santos, Justin C. Burton, Physical Review Fluids, 2017