Rheo-XPCS study of microstructural training and memory of nanocolloidal soft glasses through oscillatory shear

We report a study combining x-ray photon correlation spectroscopy with in situ rheology to investigate training and memory effects in soft glasses composed of bi-disperse nanocolloidal silica particles subject to periodic shear. During a training process consisting of a rectangular wave of strain, the x-ray speckle patterns undergo an evolution in which the degree of correlation in the microstructure between adjacent strain cycles steadily increases to a plateau. Simultaneously, the stress shows a steadily evolving hysteresis. The number of cycles required to reach the plateau depends approximately linearly on the scattering wave vector, indicating a process in which microscopic reversibility is reached at progressively shorter length scales during the training. Memory of the training is illustrated by applying a set of reading strain oscillations with different amplitudes to a well-trained glass. Both the degree of correlation in the microstructures and the change in stress between adjacent reading cycles vary non-monotonically with the reading amplitude, with the correlation showing a peak at the training amplitude.