Presheared Extensional Flow of Dilute Polymer Solutions

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Abstract
Dilute solutions of polyethylene oxide (PEO) and polyacrylamide (PAM) were subjected to shearing and then extensional motion in order to determine the influence of prior shear on the extensional resistance of these fluids. The work was originally motivated by the explanation of drag reduction by polymer additives and so the tests were conducted in a flow cell designed to simulate the ejection event (burst) in a turbulent boundary layerflow. The results show that prior shear had a considerable effect on the PEO solutions, causing significant extensional resistance at strain rates of order 10 2 s −1 , while rates one order higher were necessary to generate an extensional effect when there was no prior shear. For the PAM solutions, on the other hand, extensional resistance depended only weakly on prior shear, and onset strain rates and resistance magnitudes depended more markedly on the ionic strength of the solvent. Tests with NaCl and CaCl 2 in the solvent identicated that Na and Ca ions affect the extension of PAM molecules in different ways. A flow visualization study revealed that the flow patterns for water and PEO solutions were the same but those for PAM solutions were always unsteady.

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