Levich Institute Seminar Announcement, 10/17/2006
Steinman Hall, Room #312
(Chemical Engineering Conference Room)
Dr. Guillaume Ovarlez
Laboratoire des Matériaux et Structures du Génie Civil
"Local Measurements of the Constitutive Law of a Concentrated Noncolloidal Suspension Through MRI"
[This is a CCNY/Columbia NSF-IGERT Soft Materials seminar]
We study the behavior of dense suspensions of noncolloidal particles through Magnetic Resonance Imaging techniques. We measure the local velocity and concentration profiles in a Couette geometry, and also perform macroscopic rheometric experiments. We observe that the flow is localized at low velocities. We study the short time response to a velocity step, and deduce from the measurements that dense suspensions have nevertheless a purely viscous behavior, without any observable influence of a yield stress or granular friction. We also find that the material is inhomogeneous and that the shear-induced migration process at the origin of this inhomogeneity is almost instantaneous in contrast to most other observations. Our results imply that the diffusion coefficients depend strongly on the concentration. The local laws inferred from macroscopic rheometric observations must be reinterpreted in the light of these local observations. From the concentration and velocity profiles, we provide for the first time local measurements of the concentration dependence of viscosity, based on the true local shear rate and the true local concentration. We find a Krieger-Dougherty law to apply with a maximum packing fraction 60.5% and an exponent 2. We are also able to measure directly and locally the maximum packing fraction. 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.
Reference : Ovarlez, G., F. Bertrand, S. Rodts. Local determination of the constitutive law of a dense suspension of noncolloidal particles through magnetic
resonance imaging. J. Rheol. 50, 256-292 (2006).