Dynamic clustering of passive colloids in an active bacterial bath

Active or self-propelled particles such as motile bacteria often exhibit exotic forms of self-organization on account of their intrinsically nonequilibrium dynamics. Further, it is known that these nonequilibrium dynamics can be harnessed to manipulate passive objects such as microscopic gears and motors. Here, using video microscopy experiments and numerical simulations, we show that the nonequilibrium fluctuations in a bath of motile Pseudomonas aurantiaca bacteria can spontaneously drive the self-assembly of suspended passive colloidal silica particles. In contrast to the phase separation between active and passive particles reported in previous computational studies, we observe a dynamic clustering phenomenon with frequent formation and fragmentation events. We demonstrate that the mean cluster size increases with increasing bacterial density. Moreover, we extract an effective attractive interaction energy scale from the distribution of bond lifetimes and show that it correlates well with the mean cluster size. We hypothesize that a local transient circulation of the bacterial velocity field around colloidal particles is responsible for the observed attractive interactions.