Microscopic ergodicity breaking governs the emergence and evolution of elasticity in glass-forming nanoclay suspensions

Colloidal suspensions can undergo ergodic to nonergodic transitions that depend on the colloidal concentration, temperature, or other conditions. The transitions are typically characterized by localization of the colloids and the emergence of macroscopic elasticity. However, the relationship between these phenomena at very different length scales is not fully clear. We report a study employing x-ray photon correlation spectroscopy (XPCS) and in situ rheometry to investigate the microscopic dynamics and rheology of Laponite suspensions, composed of nanoscale discoidal colloids with concentrations from 3.25 to 3.75 wt%, which evolve over time from a fluid to a soft glass that displays aging behavior. The XPCS characterizes the particle localization during formation and aging of the soft-glass state. The fraction of localized particles f₀ increases rapidly during the early formation stage and more slowly during subsequent aging, while the localization length r_loc steadily decreases. Despite the strongly varying rates of aging at different concentrations, both f₀ and r_loc scale with the elastic shear modulus G’ in a manner independent of concentration. In the later aging stage, the scaling between r_loc and G’ agrees quantitatively with a prediction of naive mode coupling theory.