Contact Forces in Motility-Regulated Active Matter.

Long-range interactions are ubiquitous in nature, where they are mediated by diffusive fields at the cellular scale or by visual cues for groups of animals. Short-range forces, which are paradigmatic in physics, can thus often be neglected when modeling the collective behaviors of biological systems induced by mediated interactions. However, when self-organization leads to the emergence of dense phases, we show that excluded-volume interactions play an important and versatile role. We consider assemblies of active particles that undergo either condensation or phase separation due to motility regulation and show that short-range repulsive forces can induce opposite effects. When motility regulation triggers an absorbing phase transition, such as a chemotactic collapse, repulsive forces oppose the formation of condensates and stabilize instead the coexistence between finite-density phases. In contrast, when motility regulation induces liquid-gas coexistence, repulsive forces can, counterintuitively, lead to a significant increase in the liquid density.