(10o) The Transition Kinetics of Bacterial Collective Motions

Teaching Interests:  Thermodynamics and statistical mechanics, fluid mechanics.

Few things seem to capture our imagination more reliably than the collective motions of flocking birds, schooling fish or swarming insects. Such collective motions represent a universal feature of active fluidsâ??a novel class of non-equilibrium complex fluids. While the collective motions of active fluids in steady states have been extensively studied in recent years, the kinetic route to these striking motions is still largely unknown. Here, we study the transition kinetics of the collective motions of active fluids by using a new light-controlled bacterial strain, whose swimming behaviors can be reversibly switched between swimming and tumbling modes by blue light. We investigate the initiation of collective motions, particularly the formation of coherent vortex structures, in bacterial suspensions when the bacteria are switched from tumbling to swimming modes. Velocity correlations, energy spectrum and vortex statistics are measured for quantifying the kinetics of this non-equilibrium â??phase transitionâ?. We map the phase diagram of active fluids by systematically varying bacterial concentrations and the ratio of tumblers and swimmers. The transition kinetics at different points of the phase diagram are explored.