Dynamical Facilitation in Active Glass Formers: Role of Morphology and Persistence

TL;DR

This study uses large-scale simulations to explore how persistent active forces influence dynamical facilitation and cooperative relaxation in active glass-forming systems, revealing non-monotonic behavior and morphological changes.

Contribution

It demonstrates that activity modifies facilitation pathways in active glasses, showing a non-monotonic dependence on persistence time and a scaling collapse of facilitation length.

Findings

Facilitation length scales with persistence length, indicating diffusive-like coupling.

Core morphology changes with activity but retains internal plasticity.

Shell acts as a scaffold supporting axial deformation and transport.

Abstract

Understanding dynamical facilitation in nonequilibrium glass-forming systems driven by active forces remains an open challenge. In particular, it is unclear whether facilitation survives in active glasses, where persistent self-propulsion breaks detailed balance and introduces directional memory. Here, we use large-scale simulations of a two-dimensional athermal Ornstein-Uhlenbeck particle model to investigate how persistent active forcing modifies cooperative relaxation. We analyze the morphology of cooperatively rearranging regions (CRRs) and the spatial transport of mobility excitations. A spatially resolved core-shell decomposition reveals distinct responses of the core and shell to activity: the core undergoes global morphological changes while retaining internal plasticity, whereas the shell acts as a rigid scaffold that supports primarily axial deformation and facilitates transport. Dynamical observables, including modal displacement, shell occupation probability, and facilitation length, exhibit a pronounced non-monotonic dependence on persistence time. This behavior reflects the competition between persistence and effective noise, leading to either coherent or trapping-dominated dynamics at large persistence, depending on temperature. Despite significant morphological changes, the facilitation length shows an approximate scaling collapse when rescaled by the persistence length, $l_p=\sqrt{T_{\mathrm{eff}}\tau_p}$. This is consistent with a diffusive-like time-length coupling, $\xi_{\mathrm{fac}} \sim \tau_{\alpha}^{1/2}$, indicating that activity reshapes facilitation pathways without altering their large-scale transport character. Our results support a generalized facilitation framework for active glass formers.