Activity controls fragility: A Random First Order Transition Theory for an active glass

TL;DR

This paper extends the RFOT theory to active glasses, explaining how activity influences glass fragility and reconciling contradictory simulation results through a unified analytical framework.

Contribution

It introduces an active RFOT framework that quantitatively predicts how activity parameters affect glassy behavior, resolving previous contradictions.

Findings

Self-propulsion force inhibits glassiness consistently.

Persistence time's effect on glassiness depends on microscopic details.

The theory accurately fits existing simulation data and makes testable predictions.

Abstract

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, remodelled by activity. However, different simulations of dense assemblies of active particles, parametrised by a self-propulsion force, $f_0$, and persistence time, $\tau_p$, 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 using an effective medium approach - that 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 $f_0$ always inhibits glassiness, the effect of $\tau_p$ 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 towards a more rigorous analytical treatment of active glass.