What is it about?
When a polymer flow it is typically non-Newtonian. Several effects are known to most like viscoelasticity and shear thinning. However the polymer can also demonstrate normal stress differences. This lead to important effects like extrudate swelling in extrusion, rod climbing if you rotate a cylinder in a liquid, and stresses pushing rotating plates apart. Measuring normal stress differences is difficult and especially at shear rates relevant to industrial applications. Here a conventional rotational rheometer fall short due to low shear rates. This paper show a method using an injection moulding machine to provide the high flow rates.
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Why is it important?
It is very important as the key for simulating a non-Newtoniann flow is both a model and data. And reliable data at high shear rates call for measurements. The experimental results show that one can not just extrapolate data from a low shear rate region measured in a plate rheometer.
Perspectives
Normal stresses is probably the most overlooked phenomena in flow of non-Newtonian liquids. Even volcanic melts show Normal stresses and this can lead to extra pressure on the side of the magma pipe. To predict extrudate swell or die swell normal stresses are needed.
Jesper de Claville Christiansen
Aalborg University
Read the Original
This page is a summary of: Measurements of first and second normal stress differences in a polymer melt, Journal of Non-Newtonian Fluid Mechanics, January 2008, Elsevier,
DOI: 10.1016/j.jnnfm.2007.04.011.
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