The images above show the development of a non-homogeneous microstructure during sedimentation in a viscoelastic solution of polyacrylamide and carboxymethyl cellulose in water. Because sedimentation velocity is a function of the suspension microstructure, these changes result in a sedimentation velocity that varies in time. The particles are 1/8" Nylon spheres, and the particle volume fraction is 30%. In a Newtonian suspending fluid, the suspension microstructure remains homogeneous, and the sedimentation velocity is constant in time through most of the settling process. Sedimentation of the same Nylon particles in a Newtonian fluid is shown below. For more information, see the paper by Bobroff and Phillips (J. Rheol. 42:1419, 1998).