Effect of Convection Rolls in Motility-Induced Phase Separation of Active Janus Particles.

We numerically study motility-induced phase separation of active particles in two-dimensional convection rolls. We analyze local packing-fraction distributions, density fluctuations, the corresponding phase diagrams, and diffusivity curves to characterize the interplay between self-propulsion, global packing fraction, and advection strength. In the weak-flow regime, the system exhibits phase separation characterized by bimodal density distributions, slowly decaying density fluctuations, and a sharp reduction in diffusivity. Increasing advection suppresses clustering by enhancing particle transport and reducing trapping, leading to a shift in the critical self-propulsion velocity for motility-induced phase separation and a shrinkage of the spinodal region. Beyond the intuitive suppression of clustering by weak-flow advection, our results reveal several nontrivial phenomena, including a reentrant phase behavior where extremely high self-propulsion hinders motility-induced phase separation by facilitating particle escape from dense regions. We also observe that the density distributions strongly depend on roll periodicity. These findings demonstrate that convection rolls provide an effective means to control nonequilibrium collective behavior in active matter.