MRI Visualization of Non-uniformities in a Flowing Slurry

The trajectories of particles inside fluids during flow are difficult to predict for dense suspensions, and very hard to measure. Particle migration related to shear rate gradients and density mismatch can nonetheless have a tremendous impact on the behavior of the slurry and our understanding of this behavior is very limited. MRI method used here enables us to measure local solid volume fractions (SVF) and velocity profiles. This technique is much more versatile than the conventional optics methods where transparency is required.

In this work [Figure below], we imaged the flow through a pipe of a Newtonian fluid and density matched particles [1] [1] . We observe that particles migrate to the center of the pipe where SVF reaches a maximum constant value of 0.64 in a finite subregion of the cross-section, well within the jammed material domain. This value of 0.64 is the SVF of a random-close packing of spherical particles: essentially the particles are packed as dense as it is possible. These images clearly demonstrate a progressive compaction within the jammed region. Granular rheology models are able to predict the flow velocity profile. However, they do not successfully capture the compaction at the center of the flow [2] [2] . These experimental results, obtained for the first time, challenge our current understanding of dense suspension flow.