Dispersion of passive colloids suspended in an active suspension

We study the transport of passive colloids immersed in a bath of non-interacting run-and-tumble microswimmers using a minimal model for colloid-swimmer interactions. Stochastic simulations are employed to estimate the effective diffusivity of a single suspended colloid, which we estimate in terms of the relevant non-dimensional groups, namely the ratio of the swimmer persistence length to the colloid radius, the colloid-swimmer mobility ratio, and the mean number density of swimmers. We also propose a semi-analytical model for the colloid diffusivity in terms of the variance and correlation time of the fluctuating active force resulting from swimmer collisions. In addition, we also analyze interactions between pairs of colloids suspended in an active bath, where our simulations confirm the existence of an effective attractive force, which is the active analog of the depletion force well-known in passive systems. We study the dependence of this active depletion force in terms of the system parameters and explore its role in driving emergent collective dynamics in systems of multiple colloids as recently reported in various experiments.