Oscillating and star-shaped drops levitated by an airflow

TL;DR

This study explores the hydrodynamic mechanisms behind oscillating and star-shaped drops levitated by airflow, demonstrating that thermal effects are not essential and providing experimental and numerical insights into their instability and formation.

Contribution

The paper introduces a controlled airflow method to study drop oscillations, revealing a purely hydrodynamic origin of star-shaped drops without thermal influences.

Findings

Instability occurs above a critical airflow rate, especially for small drops.

Numerical simulations qualitatively match experimental observations.

Thermal effects are not necessary for star drop formation.

Abstract

We investigate the spontaneous oscillations of drops levitated above an air cushion, eventually inducing a breaking of axisymmetry and the appearance of `star drops'. This is strongly reminiscent of the Leidenfrost stars that are observed for drops floating above a hot substrate. The key advantage of this work is that we inject the airflow at a constant rate below the drop, thus eliminating thermal effects and allowing for a better control of the flow rate. We perform experiments with drops of different viscosities and observe stable states, oscillations and chimney instabilities. We find that for a given drop size the instability appears above a critical flow rate, where the latter is largest for small drops. All these observations are reproduced by numerical simulations, where we treat the drop using potential flow and the gas as a viscous lubrication layer. Qualitatively, the onset of instability agrees with the experimental results, although the typical flow rates are too large by a factor 10. Our results demonstrate that thermal effects are not important for the formation of star drops, and strongly suggest a purely hydrodynamic mechanism for the formation of Leidenfrost stars.