Subdiffusive Behavior in a Two-Dimensional Granular Assembly under Shear

We study a compact ensemble of beads in a two-dimensional sheared configuration by means of molecular dynamics. The rheology obtained for low shear rates shows a plateau and the presence of sticking agglomerates of particles. The lifetime of these agglomerates can stand several thousands time units. Also, our analysis of the mean square displacement shows that it grows like <(Delta r(tau))(2)> similar to tau(beta) with beta < 1.0. This last corresponds to a subdiffusive flow in the quasi- static regime characterized by the presence of vortices. Here, we detail how the observed agglomerates of particles deform the flow and we also give the parameter region where vortices can appear.