Motion of Particles Settling in a Viscoelastic Fluid

In this paper, we will attempt to extract some principles concerning the flow-induced anisotropy which develops in the settling of particles in Newtonian and viscoelastic fluids. Our point-of-view is that the local microstructure, which is determined by the hydrodynamic interactions between neighboring particles, determines the global anisotropic structure of a fluidized suspension. The particle pair interactions which develop are due to inertia and normal stresses and they are maximally different in Newtonian and viscoelastic liquids. This difference is caused by a reversal in the sign of the pressure at low speeds. At much larger supercritical speeds, inertia can again cause the particles in a viscoelastic fluid to line up across the stream as in a Newtonian fluid at low speeds.