S-Shaped Flow Curves of Shear Thickening Suspensions: Direct Observation of Frictional Rheology

TL;DR

This study reveals that shear thickening in granular suspensions results from the formation of a frictional force network, leading to an S-shaped flow curve and a discontinuous transition between flow states.

Contribution

It provides direct experimental evidence linking frictional interactions to shear thickening and characterizes the S-shaped flow curve in granular suspensions.

Findings

Shear thickening is caused by frictional force network formation.

The flow curve exhibits an S-shape with a negative slope region.

No shear banding occurs at intermediate stresses.

Abstract

We study the rheological behavior of concentrated granular suspensions of simple spherical particles. Under controlled stress, the system exhibits an S-shaped flow curve (stress vs. shear rate) with a negative slope in between the low-viscosity Newtonian regime and the shear thickened regime. Under controlled shear rate, a discontinuous transition between the two states is observed. Stress visualization experiments with a novel fluorescent probe suggest that friction is at the origin of shear thickening. Stress visualization shows that the stress in the system remains homogeneous (no shear banding) if a stress is imposed that is intermediate between the high and low-stress branches. The S-shaped shear thickening is then due to the discontinuous formation of a frictional force network between particles upon increasing the stress.