Shell Crystallization in Granular Hopper Flow

An interesting phenomenon that a layer of crystallized shell formed at the container wall during hopper flow is observed experimentally and is investigated in DEM simulation. Different from shear or vibration driven granular crystallization, our simulation shows during the hopper flow the shell layer is formed spontaneously from the stagnant zone at the base and grows at a constant rate to the top with no external drive. The growth rate of the shell is found linearly proportional to the rate of the hopper flow. This shell is static and served as a new wall, which changes the flow profiles and its stress properties, and in turn guarantees a constant flow rate.