Active depletion forces and colloidal clustering in an active bath

We analyze active depletion interactions between a pair of colloids suspended in an active bath of non-interacting run-and-tumble micro-swimmers using a minimal model for colloid-swimmer interactions. Using stochastic simulations, we confirm the existence of an effective attractive force, which is the active analog of the classic passive depletion force arising in suspensions of polymer coils. We use simulations to characterize the dependence of this active depletion force on colloid separation distance, density of the active bath, and mean run length of the micro-swimmers. In suspensions of multiple colloids, these interactions tend to drive colloidal clustering and aggregation, the kinetics of which we study for colloids subject to short-ranged adhesive interactions.