Distinct regimes of dynamic clustering in mixtures of passive colloids and motile bacteria

In a recent study, we have shown that passive colloids exhibit steady state dynamic clustering in dense suspensions of motile bacteria (Gokhale*, Li*, et al., arXiv:2110.02294, 2021). Here, by performing experiments over a wide range of bacterial densities, we uncover the existence of two dynamical regimes associated with dynamic clustering. In dilute bacterial suspensions, the mean cluster size decreases with increasing bacterial densities, whereas for dense suspensions, it increases with bacterial density. By analyzing colloid trajectories, we show that colloid displacements at short and intermediate times are strongly non-Gaussian in dilute suspensions, and Gaussian in dense ones. Further analysis reveals that the non-Gaussian displacements result from local spatial heterogeneity in bacterial flow fields, that is averaged out at high bacterial densities. Collectively, our experiments show that clustering at low bacterial densities is dominated by heterogeneous flow fields, whereas clustering at high densities is dominated by torque-induced effective attractions between colloids.