Self-similar clustering of beads in a bacterial suspension

Passive particles suspended in a bath of active swimmers are known to produce cooperative phenomena such as dynamic clustering or phase separation. However, such clusters have never been reported experimentally for particles in a bacterial suspension. Here we report experimental observations of two-dimensional clustering of micron-sized beads in a bath of Burkholderia contaminans bacteria. This clustering is induced by the short-range hydrodynamic attraction produced by the swimming of bacteria between neighboring beads. The characteristic cluster size, computed from the correlation length of the fluorescence intensity pattern, shows a self-similar growth in time. We explore this coarsening dynamics by varying the radius and surface fraction of the beads and the bacterial concentration.