Collective motion of interacting swimmers in impure flows

Micro-swimmers such as bacteria or spermatozoa navigate through viscous fluids interdispersed with much smaller fibers and other inert micro-particles. We look at the locomotion and interaction of individual micro-swimmers in such an impure viscous fluid. We utilize the Brinkman approximation to capture the effect of sparse and stationary obstacles that are represented via a single resistance parameter. We note the effect of the Brinkman resistance in the pair interaction of pusher and puller swimmers by comparing the trajectories, attraction, disturbance fluids-flows and effective speed-up or slow-down in the motion. We discuss the implications this has for collective behavior as well as motion near surfaces. Lastly, we present large-scale simulations of thousands of interacting microswimmers, pushers and pullers, in Brinkman flows and analyze the properties of the collective dynamics.