Transition to Steady State in Sheared Dense Granular Materials

Jamming and shear-thinning are interesting, time-dependent behaviors found in granular materials; however, most prior research has focused on the steady-state behavior under different shear rates. We study the transition phenomenon from unsteady to steady state by using a Couette cell and controlling the torque and speed of the inner cylinder. When controlling the torque, the system cannot reach a steady state when it is below a critical stress. When controlling the speed of the boundary, the shear stress at the wall increases slowly over a period of time that depends on the initial state of the bed, wall friction, shear rate, and flow along the free surface. At steady state, the stress decreases at the highest rotation speeds. Simulations show a recirculation cell driven by gravity and the free surface, which results in the increasing stress observed in the measurements. The effective friction of the inner wall matters. When using the smooth cylinder, the system needs more time to reach a steady state than using the rough cylinder. At steady state, its wall stress decreases more significantly at the highest rotation speeds compared to the rough cylinder.