Heap corrugation and hexagon formation of powder under vertical vibrations.

We report free-surface instabilities in a deep bed of fine granular material of irregular shape under vertical vibrations. At low frequency of vibration, the conical heap due to convective flow becomes unstable above a critical amplitude of vibration and acquires an azimuthal dependence which makes the heap surface corrugated. At even higher amplitude, the heap is no longer stable and splits into small heaps on a hexagonal lattice. At high frequency, we observe standing waves (stripes) at the same frequency as the driving one. The main mechanism of these instabilities can be traced back to the presence of the surrounding gas, since they vanish under vacuum conditions.