Generalized Thermodynamics of Phase Equilibria in Scalar Active Matter

Alexandre P. Solon, Joakim Stenhammar, Michael E. Cates, Yariv Kafri, Julien Tailleur

Published 2016-09-12Version 1

Motility-induced phase separation (MIPS) arises generically in fluids of self-propelled particles when interactions lead to a kinetic slowdown at high density. Starting from a continuum description of diffusive scalar active matter, we give a general prescription for phase equilibria that amounts, at a hydrodynamics scale, to extremalizing a generalized free energy. We illustrate our approach on two well known models: self-propelled particles interacting either through a density-dependent propulsion speed or via direct pairwise forces. Our theory accounts quantitatively for their phase diagrams, providing a unified description of MIPS, and explains their qualitatively different behavior when changing ensembles from isochoric to isobaric.