Non-exponential stress relaxations in arrested gels*

Non-exponential stress relaxation processes occur ubiquitously in arrested soft materials such as glasses, colloidal suspensions, biological networks, and highly associated polymer networks. Despite this ubiquity, the microscopic origins of such relaxation processes remain unclear. Here, we directly study this phenomenon through a combination of rheology and x-ray photon correlation spectroscopy on a model arrested associative gel. By investigating the microscopic relaxation processes in the gel in quiescent and in driven conditions, we find that these slow relaxation processes are governed by an interplay of microscopic fluctuations in the elastic stresses accrued during arrest, and of the elastic avalanches generated by small mechanical perturbations onto the system. We thus show that non-exponential stress relaxations in strongly arrested materials are a signature of non-linear relaxation processes governed by internal stress heterogeneities, and a manifestation of the fractal potential energy landscapes underlying such materials.