In the previous chapters it was assumed, explicitly or implicitly, that the suspending medium was Newtonian, which is typical for small molecule solvents. For a suspending medium containing a polymer, it was assumed that the only effects were on the interparticle forces. However, the consequences of having a viscoelastic medium were not considered. In many technological suspensions the suspending medium is viscoelastic. Examples can be found in coatings, inks, food products, detergents, cosmetics, pharmaceuticals, filled polymers, and composites, including nanocomposites. The source of viscoelasticity is most often the presence of polymers, either in solution or as a melt, which serve as binder or thickener. Detergents containing worm-like micelles are also viscoelastic, and suspensions in such fluids will display behavior similar to those for polymers. Some products contain vesicles, liquid crystals, or other mesophases that impart viscoelasticity.
The non-Newtonian nature of the suspending medium will affect the hydrodynamics. As has been shown (Chapter 2), for a shearing suspension in a Newtonian fluid the local flow around and between particles is much more complicated than the bulk, laminar shear flow. The constant viscosity of the suspending medium, however, ensures that there is universality in the flow behavior. This does not hold for suspensions in shear thinning fluids, making their analysis more involved. Nevertheless, this problem was tackled early on, as discussed in . With viscoelastic media the situation becomes even more complex. Even during globally steady shearing flow the fluid elements near the particles are subjected to a transient motion (i.e., their motion is unsteady in a Lagrangian sense). Therefore, the time dependence of viscoelastic fluids will affect the local flow, destroying, for instance, the fore-aft symmetry of purely laminar flow around a sphere. In addition, the normal force differences in the fluid phase will affect the stress distribution on the particles, and hence their motion. Finally, seemingly anomalous behaviors occur for suspensions in viscoelastic media because the flow between particles is not simple shear flow, but includes extensional components.