Local transient rheological behavior of concentrated suspensions

Abstract

This paper reports experiments on the shear transient response of\nconcentrated non-Brownian suspensions. The shear viscosity of the suspensions\nis measured using a wide-gap Couette rheometer equipped with a Particle Image\nVelocimetry (PIV) device that allows measuring the velocity field. The\nsuspensions made of PMMA particles (31$\\mu$m in diameter) suspended in a\nNewtonian index- and density-matched liquid are transparent enough to allow an\naccurate measurement of the local velocity for particle concentrations as high\nas 50%. In the wide-gap Couette cell, the shear induced particle migration is\nevidenced by the measurement of the time evolution of the flow profile. A\npeculiar radial zone in the gap is identified where the viscosity remains\nconstant. At this special location, the local particle volume fraction is taken\nto be the mean particle concentration. The local shear transient response of\nthe suspensions when the shear flow is reversed is measured at this point where\nthe particle volume fraction is well defined. The local rheological\nmeasurements presented here confirm the macroscopic measurements of\nGadala-Maria and Acrivos (1980). After shear reversal, the viscosity undergoes\na step-like reduction, decreases slower and passes through a minimum before\nincreasing again to reach a plateau. Upon varying the particle concentration,\nwe have been able to show that the minimum and the plateau viscosities do not\nobey the same scaling law with respect to the particle volume fraction. These\nexperimental results are consistent with the scaling predicted by Mills and\nSnabre (2009) and with the results of numerical simulation performed on random\nsuspensions [Sierou and Brady (2001)]. The minimum seems to be associated with\nthe viscosity of an isotropic suspension, or at least of a suspension whose\nparticles do not interact through non-hydrodynamic forces, while the plateau\nvalue would correspond to the viscosity of a suspension structured by the shear\nwhere the non-hydrodynamic forces play a crucial role.\n