Dynamic clustering of passive colloids in an active bacterial bath

Active particles are autonomous entities that consume ambient free energy and convert it to some form of systematic motion. Examples of active particles range from motile bacteria and birds in the natural world to colloidal and granular systems engineered in the laboratory. Owing to their intrinsically nonequilibrium nature, collections of active particles exhibit a plethora of self-organized patterns such as swarms, flocks and turbulent flows that are not observed in thermal equilibrium. While the phenomenology and dynamics of active systems have been vibrant areas of research for the last two decades, how active particles shape the self-organization of passive, or non-self-propelled objects remains unexplored, despite the ubiquity of active-passive interactions in nature. In this talk, I will present results from recent video microscopy experiments and simulations on the self-organization of passive silica colloids immersed in dense suspensions of motile Pseudomonas aurantiaca bacteria. We observe that silica colloids that interact via purely repulsive short-ranged interactions in equilibrium, spontaneously self-assemble into dynamic clusters that form and fragment frequently in the presence of motile bacteria. Using computer simulations of mixtures of active and passive Brownian particles, we show that exclusion of bacteria from the space between two colloids creates an effective attraction that subsequently leads to clustering. Despite the short-ranged nature of the effective attraction, experimentally observed spatial correlations in particle velocities are long-ranged. Analysis of the pair diffusivity tensor reveals that bacterial modification of hydrodynamic interactions between colloids reconciles short-ranged effective attractions with long-ranged velocity correlations. Given that the nonequilibrium drive imposed by bacteria generates new inter-particle interactions, our system provides an ideal opportunity to combine the tools of colloid chemistry and genetic engineering for controlling self-assembly at the micrometer scale.