  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.
