Diffusion Transients in Motility Induced Phase Separation

We propose a diffusion based approach to motility induced phase separation, by numerically investigating the normal diffusion regime of a two-dimensional athermal suspension of active micro-swimmers, modeled as Janus particles with fixed self-propulsion speed and weakly fluctuating orientation. By the term micro-swimmers we refer to either motile micro-organisms, like bacteria, or their synthetic counterparts, typically two-faced colloidal particles [for this reason called Janus particles (JP)], both capable of self-propulsion under non-equilibrium conditions. The system investigated here consisted of identical, impenetrable active particles; only after undergoing MIPS, the suspension can be regarded as dynamically heterogeneous. When the diffusion constant is plotted versus the overall suspension packing fraction, the relevant diffusion constant traces a hysteresis loop with sharp jumps in correspondence with the binodal and spinodal of the gaseous phase. Moreover, even under steady-state conditions, the particle displacement distributions exhibit non-Gaussian normal diffusion with transient fat (thin) tails in the presence (absence) of phase separation.