The “isothermal” compressibility of active matter

We demonstrate that the mechanically defined “isothermal” compressibility behaves as a thermodynamic-like response function for suspensions of active Brownian particles. The computed from pressure—a combination collision and unique swim pressures—is capable predicting critical point motility induced phase separation, expected mechanical stability criterion. relate this definition to static structure factor via an form thermodynamic equation find two be equivalent, would case equilibrium systems. This equivalence indicates like function, even when activity is large. Finally, we discuss importance interface defining chemical potential. Previous definitions potential are shown accurate above but breakdown in coexistence region. Inclusion pressure suggests essential determining behavior.