  Spontaneous phase separation, or demixing, is important in biological
phenomena such as cell sorting. In particle-based models, an open question is
whether differences in diffusivity can drive such demixing. While
differential-diffusivity-induced phase separation occurs in mixtures with a
packing fraction up to $0.7$ [Weber et al. Phys Rev Lett 2016], here we
investigate whether demixing persists at even higher densities relevant for
cells. For particle packing fractions between $0.7$ and $1.0$ the system
demixes, but at packing fractions above unity the system remains mixed,
exposing re-entrant behavior in the phase diagram. We also find that a
confluent Voronoi model for tissues does not phase separate, consistent with
the highest-density particle-based simulations.
