Stress fluctuations in a 2D granular Couette experiment: A continuous transition

Experiments on a slowly sheared 2D granular material show a continuous tran sition as the packing fraction gamma passes through gamma(c) similar or equ al to 0.776. The mean stress, <(sigma)over bar>, plays the role of an order parameter. As gamma --> gamma(c) from above, (1) the compressibility becom es large, (2) a slowing down of the mean velocity occurs, (3) the force dis tributions change, and (4) the network of stress chains changes from interm ittent long radial chains near gamma(c) to a tangled dense network for larg er gamma.