Motility-induced phase separation in suspensions of active Brownian particles

Condensed Matter lunchtime seminar

Motility-induced phase separation in suspensions of active Brownian particles

  • Event time: 1:00pm
  • Event date: 3rd June 2013
  • Speaker: Joakim Stenhammar (Formerly School of Physics & Astronomy, University of Edinburgh)
  • Location: Room 2511,

Event details

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.

This is a weekly series of informal talks given primarily by members of the soft condensed matter and statistical mechanics groups, but is also open to members of other groups and external visitors. The aim of the series is to promote discussion and learning of various topics at a level suitable to the broad background of the group. Everyone is welcome to attend..

Find out more about Condensed Matter lunchtime seminars.