The Dynamics of Suspensions of Anisotropic and Deformable Particles in Sedimentation
(Collaboration with Eric Darve and Juan Santiago, Mechanical Engineering)

For more than a decade, our group has examined the dynamics of nonBrownian fiber suspensions noting that the flow dynamics under all types of situations is qualitatively different than that found in suspensions of spheres. The simplest difference comes in the effect of hydrodynamic interactions where a given high aspect ratio fiber can interact with many of its neighbors before it ever interacts with its opposite end (!). Such semi-dilute fiber suspensions have properties which are dramatically different than their Newtonian suspending fluid even at remarkably small volume fraction. A second profound effect in fiber suspensions is associated with the mobility or drag coffiecient depending on orientation for fibers, and thus in sedimentation, fibers move rapidly in directions perpendicular as well as parallel to gravity. The consequence of this for a sedimenting suspension of fibers is that the suspension does not remain homogeneous but spontaneously forms "clumps" or "packets" which settle more quickly than an isolated fiber. We have discovered by simulation and theory, as well as by detailed experiment, that there is a region of particle concentration where the average sediment velocity is actually larger than the isolated particle rate. This finding brings new meaning to the phrase "hindered settling function"! This sedimentation velocity in suspension is therefore critically dependent on the "clumps" which form in the suspension. In new work, we have shown that this instability is generic to all separation processes involving deformable and orientable particles. In microfluidic applications, the induced electro-osmotic flows around the orientable particles creates new effects on this instability that we are examining in the context of simulating the operation of micro-barcode readers in collaboration with Eric Darve and Juan Santiago.