No large-scale demixing due to differences in diffusivity at high densities*

Spontaneous phase-separation, or demixing, is an emergent behavior important in biological phenomena such as cell sorting. 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 can drive demixing. Recently, researchers found that in particle-based packings, higher densities drive differential-diffusivity-induced phase separation up to a packing fraction of 0.7. We investigate whether this demixing persists at higher densities. For particle packing fractions between 0.7 and 1.0, we find the system demixes for certain diffusivity ratios. However, we observe that the system remains mixed at packing fractions above unity, exposing re-entrant behavior in the phase diagram. These changes in phase are associated with specific features in the interaction and active contributions to the total pressure. 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.