Imaging the emergence of collective swarming using light-controlled bacteria*

Collective motions of active fluids emerge at high densities and low noises, as illustrated vividly by bird flocks, fish schools and bacterial swarming. Although the disorder-order transition of active fluids has been extensively studied, the detailed kinetics of this transition have not been systematically explored in experiments. Here, using an engineered E. coli strain, whose locomotion can be reversibly controlled by light, we experimentally study the swarming transition in bacterial suspensions and explore its kinetic pathway. Particularly, we trigger bacterial swarming by tuning light intensity and image the dynamics of the emergence of collective motions in concentrated bacterial suspensions. We map the phase diagram of bacterial swarming as a function of total bacterial concentrations and the fractions of active swimmers. We find that, when the fraction of active swimmers is low, the transition shows an incubation period, indicating a finite energy barrier for the formation of swarming clusters. As the fraction of active swimmers increases, the incubation disappears. The kinetics of the transition are similar to that of spinodal decomposition. Our study provides new insights on non-equilibrium phase transition in active matter.