Demonstration of upper and lower Newtonian fluid behaviour in a pseudoplastic fluid

Abstract

THE most common type of non-Newtonian fluid is the pseudoplastic fluid. The pseudoplastic fluid is characterised by a constant viscosity at very low shear rates, a viscosity which decreases with shear rate at intermediate shear rates, and an apparently constant viscosity at very high shear rates. That is The apparent viscosity defined by then decreases with shear rate from η₀ to η_∞ and the pseudoplastic fluid exhibits a lower and upper region of Newtonian behaviour. Examples which illustrate the zero shear viscosity and the decreasing viscosity region for pseudoplastic fluids are very common and include almost all polymer melts and solutions. η_∞ is generally not well defined. Often the data available only approach the high shear rate limit and η_∞ is either determined by extrapolation or is set equal to zero which is a suggested theoretical value. Data which illustrate the three regions of pseudoplastic behaviour are rare and indeed are not quoted in the standard reference texts on rheology. The purpose of this communication is to present such data for a polymer solution and to compare the results to the models which are available for pseudoplastic fluids.