Collective motion and solid-liquid type transitions in vibrated granular layers

Experiments on thin layers of granular material subjected to vertical vibration reveal a series of subharmonic bifurcations from flat surface to standing wave patterns. From pressure and surface layer dilation measurements we show that a solid-liquid type transition precedes surface wave bifurcation, indicating that these waves are associated with the fluid-like behavior of the layer. Other structures observed, like the flat with kinks state, are linked to an inverse liquid-solid type transition.