Microorganism Billiards

The presence of boundaries can have many different consequences on the locomotion of microorganisms. Recent experiments and numerical simulations have shown that certain types of microorganisms have a particular interaction with a wall: either through active (flagellar contact with the surface) or passive (hydrodynamic) interactions, the body rotates away from the surface and then departs at a critical angle. We explore the billiard-like motion of such a body as it swims in confined domains. The dynamics of swimming inside a regular polygon is characterized, where stable periodic or unstable chaotic trajectories are determined by the angle of departure. We also explore the dynamics of swimming in an array of obstacles. The results may provide insight on entrapment and sorting of microorganisms and other active particles.