(372e) A Rheological Signature of Frictional Interactions in Shear Thickening Suspensions

We explore the relative contributions from hydrodynamic lubrication and frictional contact forces to colloidal shear thickening by measuring both the viscosity and first normal stress difference in suspensions of silica spheres over a wide range of volume fractions. At moderate volume fractions we find N₁<0, consistent with hydrodynamic models, however at higher volume fractions and shear stresses these models break down and we instead observe dilation (N₁>0), indicating frictional contact networks. Remarkably, there is no signature of this transition in the viscosity, instead this change in the sign of N₁ occurs while the shear thickening remains continuous.

These results suggest a scenario where shear thickening is driven primarily by the formation of frictional contacts, with hydrodynamic forces playing a supporting role at lower concentrations. Motivated by this picture, we introduce a simple model which combines these frictional and hydrodynamic contributions and accurately fits the measured viscosity over a wide range of particle volume fraction and shear stress.