On the interplay of short-range repulsive forces and long-range motility regulation

Long-ranged interactions are ubiquitous in nature, where they can be 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 self-organization of biological systems induced by mediated interactions. However, when systems undergo phase-separation, excluded-volume interactions may play an important—yet understudied—role in the dense phase. In this work, we consider assemblies of self-propelled bacteria and investigate the non-trivial interplay between phase separation induced by mediated interactions and short-range repulsive forces. When motility regulation triggers an absorbing phase transition, as is the case for chemotactic collapse, repulsive forces arrest the transition and stabilize the coexistence between finite-density phases. On the contrary, the addition of repulsive forces to a standard liquid gas phase coexistence can lead to a significant increase of the liquid density or even to the formation of a close-packed solid phase. Our results have implications both for biological systems and for the design and control of synthetic active materials.