University of California at Davis
California, U. S. A.
The behavior of particles in viscoelastic fluids differs dramatically from that in Newtonian fluids: Sedimentation in bulk suspensions is accompanied by the formation of large-scale heterogeneities in particle concentration; small clusters of sedimenting spheres form vertical chains and fall in rows; and spheres sedimenting near a wall are attracted to the wall, while spheres in pressure-driven flow through a channel move away from the wall and toward the channel center. We are performing simulations and sedimentation experiments in order to understand these phenomena. In our simulations, the ``viscoelastic fluid'' is represented as a suspension of bead-and-spring dumbbells in a Newtonian solvent. The dumbbells impart to the medium a finite memory and the ability to store and release energy. These dumbbell suspensions show negligible shear-thinning, and hence fall into the class of constant-viscosity, elastic fluids known as Boger fluids. Results will be presented for sedimentation of one and two spheres and for sedimentation of non-spherical particles in such a medium. Particle motion near walls, both in stagnant fluids and pressure-driven flows, will also be discussed. For our experiments, we have videotaped the motion of small groups of spheres sedimenting in non-Newtonian polymer solutions, and we have also used nuclear magnetic resonance imaging (NMRI) to monitor the microstructure of concentrated suspensions during sedimentation. Our simulation results show good qualitative agreement with experimental observations, and provide insight into why particles in complex fluids behave as they do.