Motility-induced phase separation of self-propelled soft inertial disks.

The phase diagram of the phenomenon of motility-induced phase separation (MIPS) for a collection of self-propelled interacting disks over a large inertial range is explored using active Langevin dynamics simulation with particular emphasis on disk softness and effective size. It is shown that the parabola-like phase boundary between the homogeneous and MIPS states in the semi-log space of disk softness and effective size moves towards the hard disk limit with increase in inertia, before complete disappearance in the limit of large inertia. With increase in effective size of the disks, re-entrant phase separation, that is the system phase-separating from a homogeneous phase and eventually re-entering the homogeneous phase, is reported. The structural and the dynamical properties of the different phases are investigated in the considered inertial range. The particular shape of the phase boundary and the re-entrant behavior is explained based on several qualitative and quantitative results. Unlike most of the earlier studies on MIPS, which consider hard particle limits, our findings may be directly applicable to soft active matter for a range of physical and biological systems.