Shear thickening corresponds to an increase of the viscosity as a function of the shear rate. It is observed in many concentrated suspensions in nature and industry: water or oil saturated sediments, crystal-bearing magma, fresh concrete, silica suspensions, cornstarch mixtures.
In this talk, I will explain why the viscosity of a non Brownian dispersion may increase under shear stress. By developing new experimental procedures based on quartz-tuning fork atomic force microscopy, we have measured the pairwise frictional profile between approaching pairs of polyvinyl chloride and cornstarch particles in solvent. We report a clear transition from a low-friction regime, where pairs of particles support a finite normal load, while interacting purely hydrodynamically, to a high-friction regime characterized by hard repulsive contact between the particles and sliding friction. Critically, we show that the normal stress needed to enter the frictional regime at nanoscale matches the critical stress at which shear thickening occurs for macroscopic suspensions. Our experiments bridge nano and macroscales and provide long needed demonstration of the role of frictional forces in discontinuous shear thickening.
In a second part, I will reveal how shear-thickening suspensions flow, shedding light onto yet non understood complex dynamics reported in the literature. When shear thickening is important, we show the existence of density fluctuations that appear as periodic waves moving in the direction of flow and breaking azimuthal symmetry. They come with strong normal stress fluctuations of same periodicity. The flow includes small areas of normal stresses of the order of tens of kPa as well as areas of normal stresses of the order of a hundred Pa. These stress inhomogeneities could play an important role in the damage caused by thickening fluids in the industry.