Aging in thermal active glasses

Abstract

It is well established that glassy materials can undergo out-of-equilibrium aging, i.e., their properties gradually change over time. There is rapidly growing evidence that dense active and living systems also exhibit many features of glassy behavior, but it is still largely unknown if and how physical aging is manifested in such non-Hamiltonian glassy materials. Here we show, by means of computer simulations, that the aging dynamics of active thermal glasses is governed by a complex interplay of different relaxation mechanisms. Notably, we identify a time-dependent competition between thermal and active effects, which gives rise to an explicitly age-dependent effective temperature. As a consequence, the often-invoked mapping between an active system and a passive one with a unique, higher effective temperature rigorously breaks down upon aging. Moreover, unlike passive aging phenomenology, we find that the degree of dynamic heterogeneity in active aging systems is relatively small and remarkably constant with age. We attribute these differences to activity-enhanced cage breaking, which modifies both the quantitative and qualitative nature of the aging process in active glassy matter.