Pressure and Phase Equilibria in Interacting Active Brownian Spheres

AP Solon and J Stenhammar and R Wittkowski and M Kardar and Y Kafri and ME Cates and J Tailleur, PHYSICAL REVIEW LETTERS, 114, 198301 (2015).

DOI: 10.1103/PhysRevLett.114.198301

We derive a microscopic expression for the mechanical pressure P in a system of spherical active Brownian particles at density rho. Our exact result relates P, defined as the force per unit area on a bounding wall, to bulk correlation functions evaluated far away from the wall. It shows that (i) P(rho) is a state function, independent of the particle-wall interaction; (ii) interactions contribute two terms to P, one encoding the slow-down that drives motility-induced phase separation, and the other a direct contribution well known for passive systems; and (iii) P is equal in coexisting phases. We discuss the consequences of these results for the motility-induced phase separation of active Brownian particles and show that the densities at coexistence do not satisfy a Maxwell construction on P.

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