Microtomographic PIV measurements of viscoelastic instabilities in a 3D micro-contraction

TL;DR

This study uses advanced 3D microtomographic PIV to investigate viscoelastic flow instabilities in a micro-contraction, revealing new vortex behaviors and flow dynamics at different Weissenberg numbers.

Contribution

First 3D volumetric measurements of viscoelastic instabilities in micro-contraction flows, uncovering novel vortex growth, plateau behavior, and asymmetric jetting phenomena.

Findings

Vortex growth increases with Wi up to a plateau.

Vortex circulation period decreases with Wi.

Out-of-plane asymmetric jetting depends on Wi.

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 (3D) 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 3D 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 3D micro-contraction 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 D and ascribe local gradients in D as the underlying driver of circulation via strain-hardening of the fluid in the wake of the jet.