Microtomographic particle image velocimetry measurements of viscoelastic instabilities in a three-dimensional microcontraction

Carlson, Daniel W., Shen, Amy Q. and Haward, Simon J. (2021) Microtomographic particle image velocimetry measurements of viscoelastic instabilities in a three-dimensional microcontraction. Journal of Fluid Mechanics. (doi:10.1017/jfm.2021.620).

Abstract

Viscoelastic flow through an abrupt planar contraction geometry above a certain Weissenberg number ($Wi$) is well known to become unstable upstream of the contraction plane via a central jet separating from the walls and forming vortices in the salient corners. Here, for the first time, we consider three-dimensional (3-D) viscoelastic contraction flows in a microfabricated glass square–square contraction geometry. We employ state-of-the-art microtomographic particle image velocimetry to produce time-resolved and volumetric quantification of the 3-D viscoelastic instabilities arising in a dilute polymer solution driven through the geometry over a wide range of $Wi$ but at negligible Reynolds number. Based on our observations, we describe new insights into the growth, propagation and transient dynamics of an elastic vortex formed upstream of the 3-D microcontraction due to flow jetting towards the contraction. At low $Wi$ we observe vortex growth for increasing $Wi$, followed by a previously unreported vortex growth plateau region. In the plateau region, the vortex circulates around the jet with a period that decreases with $Wi$ but an amplitude that is independent of $Wi$. In addition, we report new out-of-plane asymmetric jetting behaviour with a phase-wise dependence on $Wi$. Finally, we resolve the rate-of-strain tensor $\boldsymbol{\mathsf{D}}$ and ascribe local gradients in $\boldsymbol{\mathsf{D}}$ as the underlying driver of circulation via strain hardening of the fluid in the wake of the jet.

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Contributors

Author: Daniel W. Carlson

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