Rheological aging of viscoelastic glasses

Glassy materials age; i.e., their rheology and dynamical properties depend on the time spent waiting before a measurement is taken. Furthermore, aging occurs through two relaxation processes: i.) primary, which involves activated events involving compact regions, and ii.) secondary, which is a faster relaxation governed by string-like clusters. There is a fundamental interest in understanding how both primary and secondary relaxations affect aging of glassy materials, which also applies to biological matter such as biocondensates and jammed actin networks. In this talk, we present a theory of aging for viscoelastic glasses that incorporates both relaxation processes. Using random first-order transition theory, we consider the local fluctuations in the driving force to account for the glassy material's heterogeneity and non-exponential relaxation rates. We also examine the nonequilibrium, protocol-dependent properties of the system. Our model demonstrates how the stress response and other mechanical properties vary with age. We also discuss general insights into the physics of aging motivated by biological materials.