Velocity and stress fields of polymeric liquids flowing in a periodically constricted channel: Part 2. Observations of non-Newtonian behavior

Abstract

The apparatus described in the preceding Part 1 of this work has been used to investigate the flow behavior of a 20% polystyrene solution in a periodically constricted channel. Local velocity and stress measurements made over a range of flow rates, corresponding to creeping flow and shear-rate based Weissenberg numbers as large as 15, exhibited clear departures from Newtonian behavior. These departures included normal stress growth delay, local maxima and sign reversal in the shear stress near the flow cell surfaces, and significant deviation of the axial velocity profile from Newtonian predictions. Finite element simulations for creeping flow of the generalized Newtonian, upper convected Maxwell and White-Metzner fluids predict all of these features to some extent, but in general fail to describe the overall behavior of the fluid. A simplified analysis, using the Maxwell model with a shear-thinning velocity profile, indicates that the most striking non-Newtonian effect, the shear stress sign reversal, is associated with elastic recoil as fluid elements near the wall move from a region of high shear rate into a region of low shear rate. That this can occur only if the shear rates along the wall exceed the Newtonian predictions is consistent with our observations.