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Joakim Stenhammar, Edinburgh

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Location: PS1.28

Phase behaviour 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 present in passive attractive 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 and three 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.

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