(423i) Time Resolved Study of Shear-Induced Microstructures of Concentrated Silica Dispersions By SAXS

Concentrated colloidal dispersions sometimes show non-equilibrium structure formation during shear. These microstructural behaviors affect the macroscopic rheological response. However, there is still a lack of detailed understanding of the link between rheology and microstructure formation, especially regarding the onset of shear thinning and shear thickened/jammed structure. Here, we made simultaneous measurements of rheology and shear-induced structure by small angle x-ray scattering (SAXS) at Sector 8-ID-I of Advanced Photon Source. Concentrated silica dispersions were loaded in Couette cell. The x-ray scattering was measured on velocity-vorticity plane. Under moderate oscillatory shear, particles are rearranged into hexagonal packed layered structures stacking along gradient direction. This rearrangement resulted in reducing the resistance against flow, leading to shear thinning. Upon faster shear, the ordered structure breaks down. But we found that this order-to-disorder transition occurs in a different shear stress regime than the typical shear thickening state, and its occurrence is highly dependent upon oscillatory frequency, particle size and its distribution.