Active Compressibility

Active systems possess the unique ability to separate into dense and dilute phases without attractive interactions. Determining aspects of their phase behavior is an on-going challenge as they are far from equilibrium, barring the use of existing thermodynamic frameworks. However, mechanical and structural order parameters remain viable options for probing these systems. Using a combination of large-scale simulation and analytic theory, we demonstrate that the mechanically defined compressibility acts as a thermodynamic-like response function in suspensions of active disks. As such, the compressibility captures aspects of active phase separation and provides a means through which mechanical, structural, and "thermodynamic" quantities can be related in these systems. This connection provides an alternate perspective regarding the existence of an active analogue to chemical potential.