Avalanches and force chains in a granular stick-slip experiment

We perform a stick-slip experiment to characterize avalanches for granular materials. In our experiment, a constant speed stage pulls a slider which rests on a vertical bed of circular photoelastic particles in a 2D system. The stage is connected to the slider by a spring. We measure the force on the spring by a force sensor attached to the spring. We study the avalanche size statistics, and other seismicity laws of slip avalanches. We analyze the effect of macroscopic system's parameters such as driving rate, the spring stiffness, pressure and system size. We identify a crackling to periodic transition as we change driving rate and system stiffness. We provide a phase diagram of different regimes and the scaling laws of the transition.
From a more local point of view and by using high speed cameras and the photoelastic properties of our particles, we characterize the local stress change and flow of particles during avalanches. By image processing we detect the avalanches, as evolving force chains, and study their statistics in time and space. The PDFs of avalanches obey power laws both at global and local scales, but with different exponents.