Rheology of a granular column

ABSTRACTWe study the rheology of a granular material in a confined geometry. The grains are stacked in a vertical cylinder and pushed at different driving velocities. The resistance force encountered by the bottom piston is monitored while the piston is pushing the granular column upwards. Above a critical velocity, the motion is characterized by a steady sliding and by a force level increasing rather slowly with the pushing velocity. For driving velocities under this threshold, the system undergoes a dynamic instability and then, a stick-slip motion occurs. The amplitude of the slipping events, and thus, the elastic energy release, increase strongly when the velocity decreases. The critical velocity depends on the stiffness of the driving system and on the height of the granular column. This transition can be shifted towards higher velocity values by increasing the friction at the walls of the cylinder. It is also very sensitive to the state of compaction of the grains. Moreover, the mean energy release during a stick-slip motion seems to increase as a power-law when the pushing velocity is decreased. We also show that the distribution of energy release is strongly dependent on the level of disorder in the grains (polydispersity, friction, etc.). We argue that this complex phenomenology characterizes confined granular packing in connection with arching and aging phenomena.