A random first-order transition theory for an active glass

Understanding how activity affects the physics of dense glassy suspensions is of fundamental interest in a variety of cellular and tissue contexts. However, current simulations of dense active systems make qualitatively different predictions about the influence of activity on characteristic features of glass, such as fragility. Acknowledging the need for a broad theoretical framework, we extend random first-order transition (RFOT) theory to active glasses, based on an effective independent-particle treatment. We find that the analytic model predicts that the behavior of the active glass is strongly influenced by the microscopic details of activity. This not only resolves the apparent contradictions posed by previous studies but provides a number of testable predictions, some of which we verify using numerical simulations.How does nonequilibrium activity modify the approach to a glass? This is an important question, since many experiments reveal the near-glassy nature of the cell interior, remodeled by activity. However, different simulations of dense assemblies of active particles, parametrized by a self-propulsion force, f0, and persistence time, τp, appear to make contradictory predictions about the influence of activity on characteristic features of glass, such as fragility. This calls for a broad conceptual framework to understand active glasses; here, we extend the random first-order transition (RFOT) theory to a dense assembly of self-propelled particles. We compute the active contribution to the configurational entropy through an effective model of a single particle in a caging potential. This simple active extension of RFOT provides excellent quantitative fits to existing simulation results. We find that whereas f0 always inhibits glassiness, the effect of τp is more subtle and depends on the microscopic details of activity. In doing so, the theory automatically resolves the apparent contradiction between the simulation models. The theory also makes several testable predictions, which we verify by both existing and new simulation data, and should be viewed as a step toward a more rigorous analytical treatment of active glass.