Viscoelastic properties of polymer solutions

Abstract

In order to measure the wall normal stresses of viscoelastic solutions, slit dies have been designed. An advantage of using the slit dies, instead of circular tubes, is that pressure transducers can be mounted flush with solid wall, so that “pressure-hole” errors, if any, can be eliminated completely in the measurements of wall normal stresses. Although the geometry of the slit die is different from that of circular tubes, the flow through a thin slit die can, to a good approximation, be considered as one-dimensional by making the aspect ratio of a slot large enough. The slit dies designed for the present study have aspect ratios of 100, 50, 30, and 20. Three pressure transducers were flush-mounted on the long side of the rectangular slot along the longitudinal center line of the die, and measurements of wall normal stresses were made with aqueous solutions of polyacrylamide (ET 597) of various concentrations. Then, exit pressures were determined by extrapolating the straight line portion of the axial pressure distributions to the duct exit. It has been found that the exit pressure increases with solution concentration at a fixed shear rate, and it also increases with shear rate of a given concentration of solution. The exit pressure measurements were then used to completely determine the primary normal stress difference, by using the measurements of axial normal stresses reported in part I of this series.