Activity-induced dynamics in dense binary suspensions of shakers

We study the effect of passive particles on the dynamics of dense active suspensions in three dimensions using Stokesian dynamics simulations. The active particles are modelled as shakers, which--though not self-propelling--move in the presence of other particles due to hydrodynamic and steric interactions. At volume fractions above 40%, both the shakers and passive particles fluctuate vigorously in space, and their mean square displacements show crossovers from short-time ballistic to long-time diffusive dynamics. Comparing to suspensions composed solely of shakers, the dynamics of active-passive mixtures can increase by an order of magnitude in the dense regime considered. We investigate the origin of the enhanced dynamics and suggest that they could stem from an activity-induced local correlation in the orientation of shakers.