Active Crystallization

Motility-induced phase separation (MIPS), the phenomena in which purely repulsive active particles undergo a liquid-gas phase separation, is among the simplest and widely studied examples of a nonequilibrium phase transition. In this talk, we reveal that nearly the entirety of the MIPS phase boundary of three dimensional active Brownian particles is in fact metastable with respect order-disorder coexistence -- the active crystallization transition. At an activity just above the MIPS critical point, the solid-fluid coexistence curve intersects the liquid-gas binodal, forming a triple point where solid, liquid and gas may coexist. The nearly close-packed density of the active solid phase results in a remarkably narrow transition pathway for nucleating an active crystal from a disordered fluid. However, this structural barrier is surmountable at high activity and fluid densities near maximal packing, where the liquid remains decidedly mobile (non-glassy) and local density fluctuations resembling active crystals become increasing probable. This nucleation landscape results in a diminishing lifetime of liquid-gas coexistence with increasing activity as nucleation of the active within the liquid phase becomes remarkably facile.