Rod-assisted heterogeneous nucleation in active suspensions

Motility induced phase separation as well as the nucleation process in active particle systems has gained extensive research attention very recently. Most studies so far have considered homogeneous cases without the influence of foreign seeds or impurities; however, the heterogeneous nucleation process, widely studied in passive systems, has not been systematically investigated yet. Here we study the heterogeneous nucleation process and phase behaviors of a suspension of active Brownian particles by introducing a rod-like passive seed. We found that such a seed can exponentially accelerate the nucleation rate and thus readily induce phase separation of a dilute active system, while a homogeneous one with the same volume fraction still maintains a single phase. It is observed that the seed would automatically detach from the dense phase after the completion of phase separation instead of staying inside as an impurity. Interestingly, we found that the phase behavior is re-entrant with the activity: single-phase states exist at both high and low activities, with phase separated states in between. Our results demonstrate that heterogeneous nucleation in an active system can show novel behaviors with respect to its passive counterpart, and pave the way for more future studies in relevant fields.