Motility-Induced Phase Separation Is Maxwell-like Fluid with an Extended and Nonmonotonic Crossover

Understanding the mechanical properties of active suspensions is crucial for their potential applications in materials engineering. Among active phenomena with no analog in equilibrium systems, motility-induced phase separation (MIPS) in active colloidal suspensions is one of the most extensively studied. However, the mechanical properties of MIPS remain poorly understood. This Letter investigates the rheology of a suspension of active colloidal particles under constant and oscillatory shear. Systems consisting of pseudohard active Brownian particles exhibiting coexistence of dense and dilute phases behave as a viscoelastic Maxwell-like fluid at low and high frequencies, displaying exclusively shear thinning across a wide range of densities and activities. Remarkably, the crossover frequency between the storage and loss moduli is nonmonotonic, increasing with activity before the MIPS transition but decreasing with activity after the transition, in contrast to a passive analog system of attractive particles in liquid-gas phase coexistence, revealing the subtleties of how active forces and intrinsically out-of-equilibrium phases affect the mechanical properties.