Flow behavior of viscoelastic fluids

Article By:

Baird, Donald G. Department of Chemical Engineering, Virginia Polytechnic Institute and State University, Blacksburg, Virginia.

Last reviewed:2011

DOI:https://doi.org/10.1036/1097-8542.YB110147

Viscoelastic fluids are a common form of non-newtonian fluid. They can exhibit a response that resembles that of an elastic solid under some circumstances, or the response of a viscous liquid under other circumstances. Typically, fluids that exhibit this behavior are macromolecular in nature (that is, they have high molecular weight), such as polymeric fluids (melts and solutions) used to make plastic articles, food systems such as dough used to make bread and pasta, and biological fluids such as synovial fluids found in joints. The macromolecular nature of polymeric molecules along with physical interactions called entanglements lead to the elastic behavior (the fluids resemble a mass of live worms). Deformed molecules are driven by thermal motions to return to their undeformed states, giving the bulk fluid elastic recovery. This article attempts to provide a basic introduction to flow phenomena that are associated with the viscoelastic nature of fluids, such as rod-climbing, die or extrudate swell, entrance pressure losses, melt fracture, and draw resonance. In the case of polymeric fluids, these flow phenomena have a direct influence on the processing behavior and in some cases on the performance of the polymer in a given application.

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