Active percolation in pusher-type microswimmers

The aggregation of microorganisms in colonies and biofilms underpins a myriad of biological processes, and has crucial implications in ecology and biomedical sciences. While much of our knowledge of microbial motion is based on single-cell mechanisms or cell-cell interactions, the origin of cooperativity in microbial communities is not yet fully understood. Here, we reveal the existence of a continuum percolation transition in two model suspensions of pusher-type microswimmers: an asymmetric dumbbell and a squirmer model. Clusters of swimmers held together by hydrodynamic forces dynamically aggregate and separate. Using simulations with explicit hydrodynamics and theory, we find that as the microswimmers' filling fraction increases, the cluster size distribution approaches a scale-free form and system-spanning clusters emerge.