Granular wave-solid state: an accident of density inversion?

Discrete element simulations are carried out to investigate the dissipation behavior of granular balls in a quasi-2D closed granular container subjected to vertical vibration. In the Density Inversion (DI) region of granular phase diagram, an unknown granular motion pattern that has never been reported before occurs scatteredly, where a dense hexagonal close-packed granular cluster is supported by arch-like undulating granular layers underneath. We call this phenomenon the granular Wave-solid State (WS). The occasionality of WS occurring in the DI region is further revealed by re-simulating 100 times on the original seven groups of excitation parameters. Similarities and differences in the dissipation behavior between WS and DI obtained by the same excitation parameters are analyzed by comparing the motion pattern of granular bed, the packing structure of dense granular cluster and the density profile, velocity profile and loss profile of vibrated granular balls. Moreover, the initial packing structures of WS and DI obtained by the same excitation parameters are compared, which indicates that the dissipation behavior of vibrated granular balls is related to their initial packing structure.