Effect of Cohesion on the Time between Avalanches On A Slowly-Driven Conical Bead Pile

A conical bead pile subject to slow driving is used as a model critical system to experimentally investigate variations in avalanche size and time between events. The pile is composed of roughly 20 000 steel spheres, 3 mm in diameter, atop a circular base. We add one bead at a time to the apex of the pile; avalanches are measured through changes in pile mass. We investigate the dynamic response of the pile by recording avalanches off the pile over the course of tens of thousands of bead drops. The avalanching behavior is studied at different drop heights and different amounts of cohesion between the beads. At low cohesion, the statistical properties of the avalanches, including probability of particular avalanche sizes, are well-characterized by universal power laws and scaling functions. As cohesion increases, we observe a deviation from the power law behavior. Here we focus on the time between events, both in terms of the time interval between two events both above a threshold size, and also the waiting time after a given avalanche until the next event of any size. At both high and low cohesion, the experimental results match well with a mean-field model of slip avalanches [Dahmen, Nat Phys 7, 554 (2011)].