Local determination of the constitutive law of a dense suspension of non-colloidal particles through MRI

TL;DR

This study combines MRI and rheometry to analyze dense suspension flow, revealing local behaviors, concentration-dependent viscosity, and proposing a new constitutive law that explains both local and macroscopic observations.

Contribution

It introduces a local measurement approach using MRI to characterize dense suspension rheology and proposes a new purely viscous constitutive law based on these findings.

Findings

Flow localization at low velocities

Concentration-dependent viscosity follows Krieger-Dougherty law

Shear-induced migration occurs almost instantaneously

Abstract

We investigate the flowing behavior of dense suspensions of non-colloidal particles, by coupling macroscopic rheometric experiments and local velocity and concentration measurements through MRI techniques. We find that the flow is localized at low velocities, and that the material is inhomogeneous; the local laws inferred from macroscopic rheometric observations must then be reinterpreted in the light of these local observations. We show that the short time response to a velocity step allows to characterize dense suspensions locally: they have a purely viscous behavior, without any observable influence of friction. In the jammed zone, there may be a contact network, whereas in the sheared zone there are only hydrodynamic interactions: localization consists in a change in configuration at the grain scale. From the concentration and velocity profiles, we have provided for the first time local measurements of the concentration dependence of viscosity; we find a Krieger-Dougherty law $\eta(\phi)=\eta_0(1-\phi/0.605)^{-2}$. Shear induced migration is almost instantaneous and seems inconsistent with most observations: it would imply that the diffusion coefficients strongly depend on the concentration. We finally propose a simple constitutive law for dense suspensions, based on a purely viscous behavior, that accounts for all the macroscopic and local observations.