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 coherent scattering data via X-ray Photon Correlation Spectroscopy (XPCS). This combination of rheology and scattering allows for time-resolved observations of the microstructural dynamics as yielding occurs, which can be linked back to the applied rheological deformation to form structure-property relations. Under sufficiently small applied creep stresses, examination of the correlation in the flow direction reveals that the scattering response recorrelates with its predeformed state, indicating nearly complete microstructural recovery, and the dynamics of the system under these conditions slows considerably. Conversely, larger creep stresses increase the speed of the dynamics under both applied creep and recovery. The data show a strong connection between the microstructural dynamics and the acquisition of unrecoverable strain. By comparing this relationship to that predicted from homogeneous, affine shearing, we find that the yielding transition in concentrated colloidal systems is highly heterogeneous on the microstructural level.SignificanceThe flow and deformation behavior of colloidal glasses are important to a wide range of potential applications, but direct connections between the macroscopic flow/deformation and microscopic structure or dynamics have been difficult to come by. In this work, we utilize simultaneous stress-controlled rheology and x-ray scattering to bridge this gap. By probing the onset of yielding in a colloidal glass, we determine that the transition from recoverable to unrecoverable deformation is strongly linked to the loss of structural memory and the acceleration of the nanoscale fluctuations of the glass.