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

Viscoelastic flow through an abrupt planar contraction geometry above a certain Weissenberg number 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 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 we observe vortex growth for increasing, 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 but an amplitude that is independent of. In addition, we report new out-of-plane asymmetric jetting behaviour with a phase-wise dependence on. Finally, we resolve the rate-of-strain tensor and ascribe local gradients in as the underlying driver of circulation via strain hardening of the fluid in the wake of the jet.