Ostwald-like ripening in the two-dimensional clustering of passive particles induced by swimming bacteria

Clustering passive particles by active agents is a promising route for fabrication of colloidal structures. Here, we report the dynamic clustering of micrometric beads in a suspension of motile bacteria. We characterize the coarsening dynamics for various bead sizes, surface fractions and bacterial concentrations. We show that the time scale $\tau$ for the onset of clustering is governed by the time of first encounter of diffusing beads. At large time ($t \gg \tau$), we observe a robust cluster growth as $t^{1/3}$, similar to the Ostwald ripening mechanism. From bead tracking measurements, we extract the short-range bacteria-induced attractive force at the origin of this clustering.