Emergence of collective states in suspensions of swimming bacteria in confined geometries*

Geometric confinement is known to alter the swimming behavior of individual microswimmers as well as the interactions between them. This is relevant in biological systems such as swimming of bacteria in pores of soil and of spermatozoa in the confines of the cervical canal. To understand how confinement affects the collective behavior of microswimmers, we conduct experiments with suspensions of genetically modified Escherichia coli with tunable swimming velocity, in Hele-Shaw cells with a gap thickness of 7 microns. Imaging these suspensions using bright-field microscopy, we observe the emergence of three distinct collective states, dependent on the density of bacteria and their swimming velocity: a disordered state, a state characterized by lanes with long-ranged orientational nematic order, and a state of swarming clusters with short-ranged polar order. Using kinetic theory, we investigate the role of binary collisions in bringing about a transition from the disordered state to the orientationally ordered state. Thus, our experiments demonstrate how geometric confinement can give rise to novel collective states of microswimmers and help us understand the nature of interactions that are responsible for the emergence of these states.