Experiments on a slowly sheared 2D granular material show a continuous transition as the packing fraction $\gamma$ passes through ${\gamma }_c\simeq 0.776$. The mean stress, $\overline{\sigma }$, plays the role of an order parameter. As $\gamma \to {\gamma }_c$ 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 ${\gamma }_c$ to a tangled dense network for larger $\gamma$.

• Received 6 October 1998

DOI:https://doi.org/10.1103/PhysRevLett.82.5241