Activity-induced phase transitions in confined bacterial suspensions*

Active matter exhibits fascinating emergent ordered structures absent in passive equilibrium systems and, therefore, has been extensively studied as a model to explore nonequilibrium phase transitions. Here, we study the emergence of collective order in confined suspensions of genetically engineered Escherichia coli, whose propulsion speed can be controlled via intensity of light. The suspensions, when placed in a Hele-Shaw cell having a gap thickness of 10 microns, show various ordered phases unseen in their three dimensional counterparts. Using a bright-field microscope, we image the positions and orientations of individual bacterium through different phase transitions. We identify a transition from a disordered phase to a nematic phase characterized by a long-range orientational order and the formation of lanes. The dependence of transition dynamics on the concentration and swimming speed of bacteria is explored. Moreover, we examine the rise of bacterial polar ordering within the nematic phase. Our study provides an experimental benchmark for understanding the role of complex interplays between hydrodynamic and steric interactions responsible for the emergence of ordered phases in confined active systems.