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.</p

Emergent probability fluxes in confined microbial navigation

When the motion of a motile cell is observed closely, it appears erratic, and yet the combination of nonequilibrium forces and surfaces can produce striking examples of organization in microbial systems. While most of our current understanding is based on bulk systems or idealized geometries, it remains elusive how and at which length scale self-organization emerges in complex geometries. Here, using experiments and analytical and numerical calculations, we study the motion of motile cells under controlled microfluidic conditions and demonstrate that probability flux loops organize active motion, even at the level of a single cell exploring an isolated compartment of nontrivial geometry. By accounting for the interplay of activity and interfacial forces, we find that the boundary’s curvature determines the nonequilibrium probability fluxes of the motion. We theoretically predict a universal relation between fluxes and global geometric properties that is directly confirmed by experiments. Our findings open the possibility to decipher the most probable trajectories of motile cells and may enable the design of geometries guiding their time-averaged motion