title: Scalar φ4 field theory for active-particle phase separation.
abstract: Recent theories predict phase separation among orientationally disordered active particles whose propulsion speed decreases rapidly enough with density. Coarse-grained models of this process show time-reversal symmetry (detailed balance) to be restored for uniform states, but broken by gradient terms; hence, detailed-balance violation is strongly coupled to interfacial phenomena. To explore the subtle generic physics resulting from such coupling, we here introduce 'Active Model B'. This is a scalar φ(4) field theory (or phase-field model) that minimally violates detailed balance via a leading-order square-gradient term. We find that this additional term has modest effects on coarsening dynamics, but alters the static phase diagram by creating a jump in (thermodynamic) pressure across flat interfaces. Both results are surprising, since interfacial phenomena are always strongly implicated in coarsening dynamics but are, in detailed-balance systems, irrelevant for phase equilibria.
authors.affiliation: SUPA, School of Physics and Astronomy, University of Edinburgh, Edinburgh EH9 3JZ, UK.
authors.forename: Raphael
authors.initials: R
authors.lastname: Wittkowski
authors.affiliation: SUPA, School of Physics and Astronomy, University of Edinburgh, Edinburgh EH9 3JZ, UK.
authors.forename: Adriano
authors.initials: A
authors.lastname: Tiribocchi
authors.affiliation: SUPA, School of Physics and Astronomy, University of Edinburgh, Edinburgh EH9 3JZ, UK.
authors.forename: Joakim
authors.initials: J
authors.lastname: Stenhammar
authors.affiliation: SUPA, School of Physics and Astronomy, University of Edinburgh, Edinburgh EH9 3JZ, UK.
authors.forename: Rosalind J
authors.initials: RJ
authors.lastname: Allen
authors.affiliation: SUPA, School of Physics and Astronomy, University of Edinburgh, Edinburgh EH9 3JZ, UK.
authors.forename: Davide
authors.initials: D
authors.lastname: Marenduzzo
authors.affiliation: SUPA, School of Physics and Astronomy, University of Edinburgh, Edinburgh EH9 3JZ, UK.
authors.forename: Michael E
authors.initials: ME
authors.lastname: Cates