Time-resolved rheology on complex fluids

The aim of this work was to explore the potential of a combined rheology and X-ray scattering approach on complex fluids. Shear rates between 0.9 $\cdot$ 10${}^5$ s${}^{-1}$ and 5.6 $\cdot$ 10${}^5$ s${}^{-1}$, which are magnitudes higher than found in classical rheometry studies, were applied to a suspension of colloidal silica nanoparticles by a microfluidic jet device. Characteristic structure formation was studied along and across the flow direction with small angle X-ray scattering. The anisotropy of the diffraction patterns was evaluated by X-ray cross-correlation analysis. Furthermore, the decay of the shear-induced ordering after the cessation of the shear was quantified. With particle sizes of r = 15 nm-76.5 nm Péclet numbers of 1 to 1162 were investigated, a dynamic regime where diffusive motion is dominated by shear-dominated dynamics. For different Rayleigh nozzle sizes and geometries characteristic decay times between 25 ms and 495 ms were measured and correlated with the Péclet number of the system. The influence of electro-static forces was investigated by adding salt to the colloidal suspension, which reduced the overall ordering. The impact of the particle charge on the effective screening of the particles and the ionic strength of the suspension were explored. By modeling string-like particle distributions and comparison with the corresponding diffraction patterns and the measured shape asymmetry, it was possible to determine a variation of the volume fraction over the azimuthal angle of $\approx$$\pm$5% for the maximum ordered state in the jet. This interpretation was in good agreement with rescaled mean spherical approximation modeling.