Hydrodynamic interactions between passive colloids in an active bacterial bath*

The dynamics of active particles has been a subject of intense scientific investigation owing to the complex nonequilibrium patterns observed in these systems. Nonequilibrium effects also have tangible consequences for interactions between active and passive objects, as evidenced by the spontaneous rotation of microscopic gears in suspensions of motile bacteria. Here, using video microscopy experiments and simulations, we show that motile bacteria have a profound impact on dynamical interactions between passive colloidal particles. By quantifying spatial velocity correlations and the short time pair diffusivity tensor for colloids, we show that bacterial motion leads to partial screening of long-ranged hydrodynamic interactions. Notably, for sufficiently dense bacterial suspensions, transverse velocity correlations develop a pronounced minimum at a characteristic length scale that is independent of the bacterial density, but depends on the bacterial species. We hypothesize that the observed length scale demarcates short-ranged interactions dominated by direct collisions between colloids and bacteria from long-ranged ones associated with hydrodynamics.