Anisotropic flocculation in shear thickening colloid-polymer suspension via simultaneous observation of rheology and X-ray scattering

Abstract

A shear thickening fluid, showing a significant increase in viscosity under shear stress, is a promising material for shock absorption applications. Colloidal dispersions of silica nanoparticles and polyethylene oxide polymers with large molecular weights also exhibit a shear thickening phenomenon, which is manifested by a transformation from liquid to gel, called shake-gel. To understand the gelation mechanism, small-angle X-ray scattering (SAXS) and ultra-small-angle X-ray scattering (USAXS) were performed with an X-ray transmission cell, which was used as a Couette cell in a rheometer. This setup enables simultaneous rheological measurements with SAXS and USAXS, providing real-time observation of the change from liquid to gel. An anisotropic increase of the scattering spectra indicated the formation of aligned floc structures. This result implies that a polymer chain recombined the network and transformed into the large flocs aligned in the flow direction. Additionally, the shifted peaks from the silica cluster indicated that the aligned flocs were compressed by the shear stress. The present work paves the way for the advanced design of shake-gel materials.