  We experimentally investigate the self-sustained, star-shaped oscillations of
Leidenfrost drops. The drops levitate on a cushion of evaporated vapor over a
heated, curved surface. We observe modes with $n = 2-13$ lobes around the drop
periphery. We find that the wavelength of the oscillations depends only on the
capillary length of the liquid, and is independent of the drop radius and
substrate temperature. However, the number of observed modes depends
sensitively on the liquid viscosity. The dominant frequency of pressure
variations in the vapor layer is approximately twice the drop oscillation
frequency, consistent with a parametric forcing mechanism. Our results show
that the star-shaped oscillations are driven by capillary waves of a
characteristic wavelength beneath the drop, and that the waves are generated by
a large shear stress at the liquid-vapor interface.
