Rheo-SANS investigation of acicular-precipitated calcium carbonate colloidal suspensions through the shear thickening transition.

The rheology and particle alignment of suspensions of anisotropic-precipitated calcium carbonate particles of various aspect ratios is investigated using small-angle neutron scattering simultaneous with rheological measurement (Rheo-SANS). Rheo-SANS experiments were performed at concentrations from dilute to those exhibiting continuous and discontinuous shear thickening behavior. Long axis flow alignment is evident in all systems over the range of shear rates investigated. The flow alignment is shown to increase with particle aspect ratio, the applied shear stress (up to the point of shear thickening), and particle loading. For samples exhibiting shear thickening behavior, the highest degree of flow alignment occurs at the critical stress associated with the onset of shear thickening. Irrespective of particle aspect ratio, a gradual reduction in flow alignment with increasing applied stress is observed beyond the critical stress in continuously shear thickening samples, whereas discontinuously shear thickening samples maintain nearly the same degree of flow alignment within the shear thickening regime. The results are shown to be consistent with hydrocluster formation as the mechanism of shear thickening. The critical volume fraction for discontinuous shear thickening is shown to coincide with the equilibrium isotropic-nematic phase transition at higher aspect ratios. [ABSTRACT FROM AUTHOR]