Generalized thermodynamics of phase equilibria in scalar active matter

AP Solon and J Stenhammar and ME Cates and Y Kafri and J Tailleur, PHYSICAL REVIEW E, 97, 020602 (2018).

DOI: 10.1103/PhysRevE.97.020602

Motility-induced phase separation (MIPS) arises generically in fluids of self-propelled particles when interactions lead to a kinetic slowdown at high densities. Starting from a continuum description of scalar active matter akin to a generalized Cahn-Hilliard equation, we give a general prescription for the mean densities of coexisting phases in flux-free steady states that amounts, at a hydrodynamics scale, to extremizing an effective 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.

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