Microscopic signatures of yielding in concentrated nanoemulsions under large-amplitude oscillatory shear

Yielding occurs when a solid is stressed beyond its elastic limit. Signatures of yielding at the nano-to-microscale are irreversible changes to the material's structure. In amorphous solids, the intrinsic disorder makes identifying these microstructural changes difficult. We describe x-ray photon correlation spectroscopy experiments on a series of concentrated oil-in-water nanoemulsions subjected to in situ large-amplitude oscillatory shear in which we characterize the shear-induced droplet dynamics associated with yielding [1]. The dynamics include irreversible rearrangements 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. We observe a power-law distribution in the size of regions undergoing rearrangement. The values of the onset strains for re-arrangement correlate with the concentration-dependent macroscopic yielding behavior. Specifically, they occur below the strains at which the nanoemulsions become effectively fluidized and, except for the lowest concentration sample in the study, significantly above the threshold strain for nonlinear rheological response.

[1] M. C. Rogers et al., Phys. Rev. Mater. 2, 096601 (2018).