Passive colloids reveal a wet to dry crossover in active bacterial suspensions

Disentangling the role of long-ranged hydrodynamics and short-ranged steric interactions in suspensions of self-propelled particles poses a significant challenge in active matter physics. Here, by analyzing the structure and dynamics of passive colloids immersed in active suspensions of motile bacteria, we reveal the existence of two distinct regimes dominated by hydrodynamic and steric interactions respectively. In dilute bacterial suspensions, dynamic correlations between colloids evolve continuously with increasing density whereas in concentrated suspensions, they are density independent. Furthermore, in concentrated bacterial suspensions, nonequilibrium depletion interactions mediated by collisions with bacteria give rise to strong effective attraction between colloids. Simulations of active and passive Brownian particles without hydrodynamics show excellent agreement with experimental data in dense suspensions but fail to capture the observed phenomenology in dilute ones. Collectively, our findings uncover a crossover from fluid-mediated to collision-mediated interactions that not only influences the dynamics of the active fluid itself, but also reveals novel nonequilibrium phenomena in passive systems that are in contact with it.