Stress fluctuations in continuously sheared dense granular materials.
At high solid concentrations, computer simulations of sheared granular materials comprised of randomly arranged, monodisperse, smooth, inelastic spherical particles flowing in a Couette geometry show large fluctuations in the normal stress at the walls. These fluctuations are characterized by a marked asymmetric amplitude distribution similar to those observed in recent experiments. In these systems the particles' mean square displacement in the shear direction is observed to vanish locally which indicates the formation of crystallized regions. However, there are other regions in the system with nonzero values for the mean square displacement in the shear direction. This observation indicates that a sheared monodisperse granular material initially in a disordered state could evolve to a system in which the crystal phase is formed largely with a well-defined interface between different phases. The periodic phase transition is observed between the compressed, highly ordered crystalline state and the dilated, less ordered state of the layer of particles adjacent to the wall, which may explain the stick-slip behavior which occurred in the experiments.