Re-entrant demixing behavior due to differences in diffusivity in active matter at intermediate and high densities

Spontaneous phase separation, or demixing, is an emergent behavior important in biological phenomena such as cell sorting/patterning, and the formation of non-membrane bound organelles. In particulate matter, differences in size, shape and persistent motion have all been shown to cause large-scale demixing. An open question is whether differences in diffusivity, i.e. the magnitude of translational noise, between particle types – which is possible in active matter out of equilibrium – can drive demixing. Recently, researchers found that in a two-species particle-based packing, differential diffusivity drives complete phase separation up to a packing fraction of 0.7. We use an improved particle-based simulation with a faster run time to investigate whether this demixing persists at higher densities, approaching the limit of confluency. We find that for particle packing fractions between 0.7 and 1.0, the system demixes for certain diffusivity ratios. However, we observe that the system remains mixed at higher packing fractions, exposing reentrant behavior in the phase diagram. This result suggests the importance of free-space between particles to this mechanism of activity-based demixing. Using a Voronoi model, we examine a confluent system with differential diffusivity and find no evidence of phase-separation, consistent with the highest-density particle-based simulations.