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Cooling Mechanism Controls Motility-Induced Phase Separation in Inertial Active Liquids

Manuel Mayo1, Lorenzo Caprini2, María Isabel García de Soria1, Umberto Marini Bettolo Marconi3, Pablo Maynar1, Luca Pizzoli2, and Andrea Puglisi4,2,5,*

  • *Contact author: andrea.puglisi@cnr.it
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Phys. Rev. Lett. 136, 188301 – Published 8 May, 2026

DOI: https://doi.org/10.1103/mbrl-v75b

Abstract

Motility-induced phase separation (MIPS) is a central collective phenomenon in active matter, theoretically established in the overdamped regime. We discover that the dynamical origin of MIPS is fundamentally altered by inertia, which induces a cooling mechanism absent in overdamped active matter. This conclusion is supported by an active variant of the direct simulation Monte Carlo method and by a kinetic theory for inertial self-propelled hard spheres derived from the microscopic dynamics. In contrast to the overdamped case, both analyses demonstrate that inertial MIPS can occur even without impenetrability, as it originates from a density-dependent cooling mechanism due to the coupling of density, orientation, and temperature. This mechanism emerges from the competition between activity and a density-dependent collision rate arising from spatial correlations between colliding particles. These findings open a pathway to fundamentally connect inertial active matter with granular physics.

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Phys. Rev. Lett. 136, 188301– Published 8 May, 2026
  • Received 24 November 2025
  • Accepted 8 April 2026
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