Heterogeneity of dilute polymer solutions

Abstract

Dilute macromolecular solution often exhibit flow characteristics which are remarkably different from those of the Newtonian solvent. Drag may be decreased in turbulent shear flow and increased in flow around cylinders and through porous media. In dilute polymer solutions turbulent clouds will disperse more slowly, jets become more stable, and Taylor and bath-tub vortices may be supressed. Some of these effects may be explained by assuming the macromolecules stretch in strong flows. But often the effects occur in weak flows or flows in which substantial stretching is incompatible with the kinematics, and further the onset of the non-Newtonian behaviour often depends on polymer concentration. It is proposed here that “fresh” solutions of macromolecules are heterogeneous. When dilute solutions are prepared from concentrated master solutions, strings are created by the mixing process and these strings from a network in the fluid. The difficulty in obtaining repeatable experimental results, as well as the disappearance of many non-Newtonian effects in aged solutions which retain drag reducing ability in turbulent shear flow, may be explained by this model. If the strings of polymer solutions are sufficiently long compared with their thickness, the network will move with the surrounding fluid. At low rates of strain a dilute network will hardly affect the solvent properties. Under high rates of strain, of sufficient duration, strain hardening will cause the elasticity of the strings to predominate; the fluid will behave like a concentrated polymer solution with “diluted” elastic properties. The similarity of the kinematics of concentrated and dilute solutions could be explained by such a liquid network model. Rheological equations may be easily contructed for such polymer solution networks if those of the concentrated “master” solutions are known. Many rheological equations for concentrated polymer solutions are, however, found to have some deficiencies when the liquid network model is applied to the sink flow through an orifice.