A reentrancy of motility-induced phase separation in overdamped active Brownian particles

Abstract: In a system of Self-Propelled Particles (SPPs), the combination of self-propulsion and excluded volume effects can result in a phase separation called Motility-Induced Phase Separation (MIPS). Previous studies reported that MIPS is one of the phenomena so-called "reentrant phase separation" ,i.e., MIPS is suppressed when the P\'eclet number $Pe$ (dimensionless self-propelled speed) is sufficiently large. We used a fundamental model of SPPs, i.e., overdamped Active Brownian Partcles (ABPs), to investigate the mechanism of the reentrancy of MIPS. We expect that elucidating the conditions under which MIPS occur is important, since MIPS is a phenomenon that can occur in a wide range of SPPs systems, and the potential applications of MIPS can also be wide range. Detailed investigation of particle motion revealed that a entire particle cluster deforms due to multiple slip deformation (known as plastic deformation in materials science). As $Pe$ increases, the frequency of occurrence of slip-lines increases, and the particle motion becomes fluid-like. Therefore, the shape of the cluster becomes unstable and the number of particles in the cluster decreases. Let $\bf{f}{LA}$ be the local spatial average of the self-propelled force generated by the particles. The observation of the inhomogeneity in the magnitude and direction of $\bf{f}{LA}$ shows that $\bf{f}{LA}$ is large on the cluster surface and generally orients toward the cluster inside. We determined that $\bf{f}{LA}$ generates stress on the cluster, and it causes the multiple slip deformation.