From motility-induced phase-separation to glassiness in dense active matter

Emergent behavior of dense living materials shows coexistence of both equilibrium and non-equilibrium features, such as glassy states and motility-induced phase separation. Here, the authors study transition between these two phases in a model of dense, disordered, soft active materials and find that the mechanism leading to fluidization from the glassy phase do not have an equilibrium counterpart. Dense active systems are widespread in nature, examples range from bacterial colonies to biological tissues. Dense clusters of active particles can be obtained by increasing the packing fraction of the system or taking advantage of a peculiar phenomenon named motility-induced phase separation (MIPS). In this work, we explore the phase diagram of a two-dimensional model of active glass and show that disordered active materials develop a rich collective behaviour encompassing both MIPS and glassiness. We find that, although the glassy state is almost indistinguishable from that of equilibrium glasses, the mechanisms leading to its fluidization do not have any equilibrium counterpart. Our results can be rationalized in terms of a crossover between a low-activity regime, where glassy dynamics is controlled by an effective temperature, and a high-activity regime, which drives the system towards MIPS.

Automated summary

This study investigates the transition between glassy and fluid states in dense active materials, revealing that the mechanisms leading to fluidization in active systems do not have an equilibrium counterpart.