Influence of Pins on the dynamics of a sheared granular system*

We use molecular dynamics simulations to study a two dimensional 50:50 binary mixture of purely repulsive harmonic disks of radii 1:1.4. Via top and bottom walls of frozen particles we shear the system at constant shear rate.  We simulate at zero temperature and apply dissipative interactions depending on relative velocities.  We investigate how the dynamics is influenced by the addition of fixed miniscule disks of radius 0.004 placed on a square lattice. We find that at the absence of pins the velocity profile shows long time fluctuations about the linear velocity field. In the presence of pins, and similarly at higher shear rate, the velocity profile  does not have the long time fluctuations and can approximated with a model of B. Tighe. We will show also the long time shear stress as functions of packing fraction, shear rate and pressure for the quasistatic case as well as for fast shear rate. Furthermore, we will show preliminary results for the case of applying constant pressure on the top wall. We find that the local pressure profile is increasingly non-uniform with increasing number of pins. To study dynamical heterogeneities we determine D2min. We find that the probability of larger D2min is larger near a pin and decreases with distance from the pin. For distances approximately smaller than 10 this decrease is independent of the number of pins.