Investigation of the yielding transition in concentrated colloidal systems via rheo-XPCS

We probe the microstructural yielding dynamics of a concentrated colloidal system by performing creep/recovery tests with simultaneous collection of scattering data via X-ray Photon Correlation Spectroscopy (XPCS). This combination allows for time-resolved observations of the microstructural dynamics as yielding occurs, allowing for the formation of time-resolved structure-property relations. To more accurately track the non-equilibrium processes which occur under yielding, we utilize two-time correlation functions, which provide additional time-resolved information that is inaccessible via more typical one-time correlations. Under sufficiently small applied creep stresses, the correlation in the flow direction reveals that the scattering response recorrelates with its pre-deformed state, indicating nearly-complete microstructural recovery, and the dynamics of the system slow considerably. Conversely, larger creep stresses increase the speed of the dynamics under both applied creep and recovery. The correlation data show a strong connection between the microstructural dynamics and the acquisition of unrecoverable strain, and suggest that the yielding transition in concentrated colloidal systems is highly heterogeneous on the microstructural level.