Imaging the emergence of collective swarming in light-controlled bacteria

Collective motions of active matter as illustrated by bird flocks, fish schools and bacterial swarms demonstrate the intriguing emergent behaviors of living systems. While moving independently at low density, active entities move collectively with its neighbors at high density, exhibiting orientational order at a scale larger than their individual sizes. Although such a disorder-order nonequilibrium phase transition has been previously studied, detailed kinetics of this transition has not been explored in experiments. Here, using engineered E. coli, whose locomotion can be reversibly controlled by light, we experimentally study the kinetic pathway of the swarming transition in 3D bacterial suspensions. We trigger bacterial swarming by tuning light intensity and image the emergence of collective motion. We map the phase diagram of bacterial swarming as functions of bacterial concentration, swimmer velocity and the number fraction of active swimmers. Moreover, we find that the swarming transition occurs in a nucleation manner and characterize the incubation time of the transition. Our results reveal the microscopic dynamics of the emergence of bacterial swarming and provide insights into the nonequilibrium phase transition in active matter.