Slowly sheared dense granular flows: Crystallization and nonunique final states

Simultaneous time-resolved measurements of internal structure, granular volume, and boundary shear force are reported for dense granular packing steadily sheared under a fixed normal load. We identify features of the crystallization transition for a deep shear flow, whose height-dependent local mean velocity spans more than five orders of magnitude. This structural change is accompanied by a significant decrease of granular volume and shear force, with a more rapid falloff of particle velocity with depth than occurs in the disordered state. Boundary conditions can have a profound influence on the crystallization of the entire packing. We find that continuously sheared flow can exhibit nonunique final states even under identical boundary conditions; a few cycles of oscillatory pretreatment can initiate states that evolve into either a crystallized or a disordered final state after long-term unidirectional shearing. On the other hand, the disordered state can be stabilized after being sufficiently compacted by unidirectional shear. These experiments raise interesting questions about how prior history is recorded in the internal structure of granular packings, affecting their instantaneous rheology and long-term evolution in response to shear.