Complex Fluid Dynamics
Butler Research Group
Rods sedimenting in a viscous suspension exhibit fascinating dynamics
that are represented in the video shown on the left (click the video
to play). At relatively short times after starting from a random
configuration (shown during the first rotation of the box in the
video), the rods tend to align in the direction of gravity. Since the
sedimentation rate of a rod aligned with gravity exceeds that of a rod
in any other orientation, the overall bulk sedimentation rate
increases. The rod alignment attains a steady distribution at the time
of the second rotation of the box, yet the sedimentation rate
continues to increase in time due to the demixing, or formation of
clusters, of the particles.
An instability that couples spatial inhomogenieties and rotational dynamics causes the demixing and the associated enhancement of the sedimentation rate. Depending upon the concentration, the rate of sedimentation of the collection of rods can exceed the maximum sedimentation rate of a single particle; in contrast, the sedimentation of a suspension of spheres is always slower than that of a single sphere.
Experiments verify both that the suspension of rods demixes during sedimentation and that this instability results in an enhanced rate of sedimentation. The rods used in the experiments (see the two figures above) are placed in the sedimentation vessel containing a viscous, Newtonian fluid (as shown on the far right). Due to the difference in density with the fluid, the particles sediment. Measurements are facilitated by imaging the rods, which are fluorescent and refractive-index matched with the fluid, within a laser sheet as shown. An example result, on the right, shows regions of high and low density of particles. Processing many such images provides information on the distribution of the rods, their rate of sedimentation, and more.