Kinetics of phase transitions in passive and active matters
University of Minnesota
A phase transition is an abrupt change between different structures of passive or active constitutes, with snow formation, graphite-diamond transition and emergence of bird flocking as common examples. The kinetics of a phase transition is essential to understand its microscopic mechanism and to control the transition rate, intermediate state and the final structure morphology. However, it is challenging to predict the kinetics pathway duo to complex interaction and the many-body nature of these processes. Here I study the kinetics of phase transitions in a passive system, crystal-crystal transitions in colloids, and in an active system, swarming transition in E. coli suspensions. In the colloidal crystal-crystal transitions, we employed diameter-tunable microgel spheres to trigger a transition from square lattice to triangular lattice. Under static condition, video microscopy directly demonstrated a two-step nucleation process with an intermediate liquid state. This kinetic pathway was altered to a one-step nucleation with an external flow applied. The interfacial energy and the crystalline strain were shown to be responsible for the initial kinetics of the nucleation. For the active phase transition, we used gene-mutated light-control E. coli, whose locomotion can be controlled by blue light, to switch on/off their collective motions. The emergence of bacterial swarming was showed as a barrier-crossing behavior, in analogue to nucleation. The behavior of incubation time deviated from that in equilibrium, indicating a different physical origin of the transition barrier in this system. The results shed new light on microscopic mechanisms of phase transitions in active systems.