Viscosity of a dense suspension in Couette flow

We study the rheology of a granular paste, i.e. a dense suspension of non-Brownian particles, quantitatively at steady state, in a cylindrical Couette cell. Previous studies have shown a discrepancy between local and global measurements of the viscosity for these materials, making it impossible to predict their resistance to flow. Using both MRI investigation techniques and classical rheology studies, we show that agreement between local and global measurements can be obtained, provided the migration of particles inside the gap is taken into account. As found by Leighton & Acrivos (J. Fluid Mech. vol. 181, 1987, p. 415), the migration leads to a particle density gradient in the flow, the highly sheared regions being less dense in particles. Here, by comparing the local viscosity and particle density measurements from MRI with the macroscopic relation between viscosity and the volume fraction, it is shown that global and local measurements agree with each other. This consequently allows us to define a viscosity for dense suspensions.