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Molecular orientation in non-Newtonian flow of dilute polymer solutions around spheres

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Abstract

A novel approach is presented to study the benchmark problem of flow around spheres in model dilute solutions of monodisperse samples of atactic polystyrene in di-octyl phthalate. Spheres are held “stationary” on flexible cantilevers of known spring-constant, k, while the polymer solutions are pumped past at controlled flow rates, allowing access to a wide range of Deborah number. In this way the non-Newtonian forces experienced by the spheres can be measured as a function of Deborah number, while detailed observations and measurements of birefringence are made, enabling assessment of macromolecular strain and orientation. In addition the flow field around a sphere has been measured in an a-PS solution. Experiments have been performed on a single sphere and on two spheres axially aligned in the direction of flow. The extensional flow around the downstream stagnation point of the single sphere is found to play a pivotal role in the development of molecular strain and stress, resulting in flow modification and subsequent non-Newtonian behaviour. The flow birefringence in the wake is found to modify severely the flow around a second, downstream, sphere, affecting the non-Newtonian forces encountered by the second sphere. This provides an explanation for the time interval dependent terminal velocity often observed when two spheres follow the same path through viscoelastic liquids.

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Acknowledgements

We gratefully acknowledge the support of EPSRC and the EU Alfa programme. We are indebted to Dr S.P. Carrington for helpful discussions.

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  1. H.H. Wills Physics Dept., University of Bristol, Tyndall Avenue, Bristol, BS8 1TL, UK

    Simon J. Haward & Jeffrey A. Odell

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  1. Simon J. Haward
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  2. Jeffrey A. Odell
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Correspondence to Jeffrey A. Odell.

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Haward, S.J., Odell, J.A. Molecular orientation in non-Newtonian flow of dilute polymer solutions around spheres. Rheol Acta 43, 350–363 (2004). https://doi.org/10.1007/s00397-003-0350-7

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