Rheo-SAXS Observation of Structural Change in Shear Thickening Polymer Gel

Background:<br/>Shear thickening fluid (STF), often termed as a dilatant fluid, is a non-Newtonian fluid characterized by significant increase in viscosity when the applied shear rate above a critical value. It shows wide application prospects in shock absorption, body protection, and damper systems. However, these viscosity changes occur in a dynamic process, in which shear stress is applied, and the viscosity returns to its original state when the shear stress is removed. This reversible phase change makes it difficult to measure the dynamic microstructural changes that lead to the shear thickening phenomena. In this study, we performed a rheo-SAXS, which is simultaneous measurements of viscosity with small-angle X-ray scattering (SAXS) or ultra-small-angle X-ray scattering (USAXS) using an X-ray transmission solution cell. This real-time observation of the structural changes in colloidal silica solution showed the aggregation process of polymers and compressing colloidal particles responsible for the ST.<br/><br/>Experiment:<br/>The silica colloid sample was prepared by dispersing a silica sol (Snowtex ST-S, Nissan Chemical) with a particle size of 9 nm and PEG solution [1]. The viscosity and SAXS/USAXS were measured simultaneously using a rheometer (ONRH-1, Ohna Tech Inc.) in combination with a polycarbonate X-ray transmission Couette cell. The obtained spectra were resolved in the XY direction to elucidate anisotropic changes with respect to shear flow, and were plotted against the scattering vector Q. The experiments were performed at the Sumitomo Electric Beamline BL16 at the Kyushu Synchrotron Light Source Center and BL40XU, BL19B2 at SPring-8.<br/><br/>Results:<br/>A viscosity measurement of the silica colloidal sample showed sharp increase of viscosity at a critical shear rate of 10 [1/s], indicating phase change of ST. SAXS spectra were acquired while rotating at several constant shear rate before and after ST transition, compared obtained spectral intensity. At shear above the critical shear rate, the spectral intensity centered at 0.01 1/nm increases in USAXS, indicating the formation of a floc of about 600 nm. SAXS spectra also showed similar anisotropy, with the 50 nm spacing intensity decreasing only in the velocity direction. The results suggest that the polymer network was restructured, forming larger flocs, and the silica clusters were compressed in the flow.<br/><br/>Acknowledgments:<br/>This work was supported by Innovative Science and Technology Initiative for Security Grant Number JPJ004596, ATLA, and AKENHI Grant Number JP25107002, JSPS, Japan.<br/><br/>[1] Kamibayashi, M., Ogura, H. & Otsubo, Y. <i>J. Colloid Interface Sci.</i> <b>321</b>, 294–301 (2008).