A rheological signature of frictional interactions in shear thickening suspensions

TL;DR

This study reveals that shear thickening in suspensions involves a transition from hydrodynamic to frictional interactions, evidenced by changes in the normal stress difference, and introduces a model combining these effects for better understanding.

Contribution

The paper provides the first measurements of the normal stress difference across various volume fractions, clarifies the role of frictional contacts in shear thickening, and proposes a combined model for viscosity.

Findings

Normal stress difference transitions from negative to positive with increasing volume fraction.

Frictional contacts dominate shear thickening at high volume fractions.

A combined hydrodynamic and frictional model accurately fits experimental viscosity data.

Abstract

Colloidal shear thickening presents a significant challenge because the macroscopic rheology becomes increasingly controlled by the microscopic details of short ranged particle interactions in the shear thickening regime. Our measurements here of the first normal stress difference over a wide range of particle volume fraction elucidate the relative contributions from hydrodynamic lubrication and frictional contact forces, which have been debated. At moderate volume fractions we find $N_1<0$, consistent with hydrodynamic models, however at higher volume fractions and shear stresses these models break down and we instead observe dilation ($N_1>0$), indicating frictional contact networks. Remarkably, there is no signature of this transition in the viscosity, instead this change in the sign of $N_1$ 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.