Volume 142, Number 5, June 2023
|Number of page(s)||7|
|Section||Biological and soft matter physics|
|Published online||05 June 2023|
Towards a liquid-state theory for active matter(a)
1 DAMTP, Centre for Mathematical Sciences, University of Cambridge - Wilberforce Road, Cambridge CB3 0WA, UK
2 Department of Physics and Materials Science, University of Luxembourg - L-1511 Luxembourg, Luxembourg
3 St John's College, University of Cambridge - Cambridge CB2 1TP, UK
(b) E-mail: email@example.com (corresponding author)
Received: 25 January 2023
Accepted: 22 May 2023
In equilibrium, the collective behaviour of particles interacting via steep, short-ranged potentials is well captured by the virial expansion of the free energy at low density. Here, we extend this approach beyond equilibrium to the case of active matter with self-propelled particles. Given that active systems do not admit any free-energy description in general, our aim is to build the dynamics of the coarse-grained density from first principles without any equilibrium assumption. Starting from microscopic equations of motion, we obtain the hierarchy of density correlations, which we close with an ansatz for the two-point density valid in the dilute regime at small activity. This closure yields the nonlinear dynamics of the one-point density, with hydrodynamic coefficients depending explicitly on microscopic interactions, by analogy with the equilibrium virial expansion. This dynamics admits a spinodal instability for purely repulsive interactions, a signature of motility-induced phase separation. Therefore, although our approach should be restricted to dilute, weakly active systems a priori, it actually captures the features of a broader class of active matter.
© 2023 The author(s)
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