Solid-liquid transition and hydrodynamic surface waves in vibrated granular layers

Experiments on thin layers of granular material subjected to vertical vibration have revealed a series of subharmonic bifurcations from flat surface to standing wave patterns. From pressure and surface dilation measurements we show that a solid-liquid-type transition precedes surface wave bifurcation, indicating that these waves are associated with the fluidlike behavior of the layer. Also, we show that depending on the excitation frequency two kinds of waves can be distinguished: hydrodynamic waves which involve lateral transfer of grains within the layer, and dilation waves for which the layer slightly dilates and compacts alternatively in time and space.