Microscopic signatures of yielding in concentrated nanoemulsions under large-amplitude oscillatory shear
Here, we report x-ray photon correlation spectroscopy (XPCS) experiments on a series of concentrated oil-in-water nanoemulsions with varying droplet volume fraction subjected to in situ steady-state large-amplitude oscillatory shear (LAOS). The shear strain causes periodic echoes in the x-ray speckle patterns that lead to peaks in the intensity autocorrelation function. Above an onset strain amplitude that depends on nanoemulsion concentration, the peaks become attenuated, signaling spatially heterogeneous, shear-induced droplet dynamics. These dynamics include irreversible re- arrangements among the droplets that occur in some regions of the nanoemulsions during a given shear cycle and residual strain-like displacements in those regions that do not re-arrange. The wave-vector dependence of the peak attenuation indicates a power-law distribution in the size of regions undergoing shear-induced rearrangement that is similar to that observed previously in LAOS-XPCS measurements on concentrated nanocolloidal gels. The values of the onset strains for re-arrangement correlate with the concentration-dependent macroscopic yielding behavior of the nanoemulsions. Specically, they occur below the strains at which the nanoemulsions become effectively uidized and, except for the lowest concentration nanoemulsion in the study, signicantly above the threshold strain for nonlinear rheological response.