Motility-induced phase separation in suspensions of active Brownian particles

So-called active Brownian particles (ABPs) - i.e., self-propelled, non-aligning colloids whose swimming direction relaxes through thermal diffusion - constitutes a paradigmatic example of active matter, and can be seen as a minimal model of synthetic swimmers as well as motile bacteria. Recently, simulations of ABPs have demonstrated the existence of a phase transition which strongly resembles that of a gas-liquid coexistence in a system of passive particles with attractive interactions. Since the direct interaction potential between ABPs is purely repulsive, this phase transition is exclusively driven by the far-from-equilibrium microscopic dynamics of ABPs, and one would therefore not expect any generic similarities between this type of phase coexistence and those in passive systems.

In this seminar, I will discuss how a semi-thermodynamic mapping, in the form of a dynamic continuum equation for the time-evolution of the density field, can be derived directly from the microscopic ABP dynamics. A numerical solution of the equations yields quantitative agreement with domain topologies and phase-separation dynamics (growth exponents) obtained from explicit, large-scale Brownian dynamics simulations of ABPs in two dimensions. While the model weakly violates detailed balance through a non-standard interfacial energy, the effects of this violation are found to be surprisingly small. This result thus suggests unexpected analogies between phase transitions in active and passive systems, in spite of the far-from-equilibrium microscopic dynamics of the former. Finally, I will briefly discuss the qualitative differences between ABP phase separations in two and three dimensions.