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Colloidal shear thickening presents a significant challenge because themacroscopic rheology becomes increasingly controlled by the microscopic detailsof short ranged particle interactions in the shear thickening regime. Ourmeasurements here of the first normal stress difference over a wide range ofparticle volume fraction elucidate the relative contributions from hydrodynamiclubrication and frictional contact forces, which have been debated. At moderatevolume fractions we find $N_1<0$, consistent with hydrodynamic models, howeverat higher volume fractions and shear stresses these models break down and weinstead observe dilation ($N_1>0$), indicating frictional contact networks.Remarkably, there is no signature of this transition in the viscosity, insteadthis change in the sign of $N_1$ occurs while the shear thickening remainscontinuous. These results suggest a scenario where shear thickening is drivenprimarily by the formation of frictional contacts, with hydrodynamic forcesplaying a supporting role at lower concentrations. Motivated by this picture,we introduce a simple model which combines these frictional and hydrodynamiccontributions and accurately fits the measured viscosity over a wide range ofparticle volume fraction and shear stress.

A rheological signature of frictional interactions in shear thickening suspensions