Rheo-SAXS investigation of shear-thinning behaviour of very anisometric repulsive disc-likeclay suspensions

Abstract

Aqueous suspensions of swelling clay minerals exhibit a rich and complex rheologicalbehaviour. In particular, these repulsive systems display strong shear-thinning at very lowvolume fractions in both the isotropic and gel states. In this paper, we investigate theevolution with shear of the orientational distribution of aqueous clay suspensions bysynchrotron-based rheo-SAXS experiments using a Couette device. Measurements in radialand tangential configurations were carried out for two swelling clay minerals of similarmorphology and size, Wyoming montmorillonite and Idaho beidellite. The shear evolutionof the small angle x-ray scattering (SAXS) patterns displays significantly different featuresfor these two minerals. The detailed analysis of the angular dependence of the SAXSpatterns in both directions provides the average Euler angles of the statistical effectiveparticle in the shear plane. We show that for both samples, the average orientation isfully controlled by the local shear stress around the particle. We then apply aneffective approach to take into account multiple hydrodynamic interactions inthe system. Using such an approach, it is possible to calculate the evolution ofviscosity as a function of shear rate from the knowledge of the average orientationof the particles. The viscosity thus recalculated almost perfectly matches themeasured values as long as collective effects are not too important in the system.