(289e) Imaging the Onset Kinetics of the Swarming Transition Using Light-Controlled Bacteria

Active fluids are a novel class of nonequilibrium soft materials, which are composed of a large number of self-propelled particles. These particles collectively form coherent clusters at high densities and low noise levels, as illustrated vividly by the striking patterns of flocking birds, schooling fishes and swarming bacteria. Although the disorder-coherent transition of active fluids has been extensively studied, its very nature is still on heated debate and the transition kinetics has not been explored. Here, using a mutated E. coli strain, whose locomotion can be reversibly controlled by light, we experimentally study the swarming transition in active fluids and explore its kinetic pathway. Particularly, we trigger bacterial swarming by tuning the light intensity and image the dynamics of coherent clusters in concentrated bacterial suspensions. We systematically map the phase diagram of bacterial swarming with respect to the concentration n and the faction of tumblers f. We find that the transition points follow n*f=12. The continuous nature of the phase transition is revealed by the absence of hysteresis loop and zero incubation times in the swarming transition. We further investigate the formation of swarming cluster, dynamics of correlation length and energy spectrum. Our study sheds light on the phase transition of active matter and the emergent properties of many-body nonequilibrium systems.