Intruder dynamics in granular flow

The dynamics and kinematics of a single intruder particle in two dimensional, disordered packings of frictionless bidisperse discs interacting through linear and non-linear contact forces are investigated using computer simulations. To initiate motion of the intruder requires a threshold force to be exceeded which depends on the pressure of the packing. Beyond this critical force, for a given driving force, the average intruder velocity scales with the excess driving force. The velocity curves for different pressures are found to obey an universal scaling relation that depends on the initial packing pressure. We also find that the critical, threshold force is related to the shear modulus, thereby connecting the microrheological properties of the intruder at the grain scale to the bulk, macroscopic mechanical properties of the packing.