Stress Fluctuations in a 2D Granular Couette Experiment: A Continuous Transition

Abstract

Experiments on a slowly sheared 2D granular material show a continuous transition as the packing fraction $\ensuremath{\gamma}$ passes through ${\ensuremath{\gamma}}_{c}\ensuremath{\simeq}0.776$. The mean stress, $\overline{\ensuremath{\sigma}}$, plays the role of an order parameter. As $\ensuremath{\gamma}\ensuremath{\rightarrow}{\ensuremath{\gamma}}_{c}$ from above, (1) the compressibility becomes large, (2) a slowing down of the mean velocity occurs, (3) the force distributions change, and (4) the network of stress chains changes from intermittent long radial chains near ${\ensuremath{\gamma}}_{c}$ to a tangled dense network for larger $\ensuremath{\gamma}$.