Stress fluctuations and shear thickening in dense granular suspensions

TL;DR

This study experimentally explores how oil viscosity influences stress fluctuations and shear thickening in dense granular suspensions, revealing a transition from frictional to lubrication-dominated interactions and changes in force correlation structures.

Contribution

It provides new experimental evidence linking oil viscosity to rheological behavior and stress fluctuations, and compares results to the Wyart-Cates model to elucidate dynamic jamming states.

Findings

Thickening strength and stress fluctuations weaken with higher oil viscosity.

A transition from frictional to lubrication interactions occurs as viscosity increases.

Stress fluctuation distributions change from asymmetric Gamma to symmetric Gaussian with shear rate.

Abstract

We experimentally investigate the rheology and stress fluctuations of granules densely suspended in silicone oil. We find that both thickening strength and stress fluctuations significantly weaken with oil viscosity $\eta_0$. Comparison of our rheological results to the Wyart-Cates model for describing different dynamic jamming states suggests a transition from frictional contacts to lubrication interactions as $\eta_0$ increases. To clarify the contribution from viscous interactions to the rheology, we systematically measure stress fluctuations in various flow states. Reduction of stress fluctuations with $\eta_0$ indicates that a strong lubrication layer greatly inhibits force correlations among particles. Measuring stress fluctuations in the strong shear thickening regime, we observe a crossover from asymmetric Gamma to symmetric Gaussian distributions and associated with it a decrease of lateral (radial) correlation length $\xi$ with increasing shear rate.