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Viscoelastic flow through an abrupt planar contraction geometry above acertain Weissenberg number (Wi) is well known to become unstable upstream ofthe contraction plane via a central jet separating from the walls and formingvortices in the salient corners. Here, for the first time we considerthree-dimensional (3D) viscoelastic contraction flows in a microfabricatedglass square-square contraction geometry. We employ state-of-the-artmicrotomographic particle image velocimetry to produce time-resolved andvolumetric quantification of the 3D viscoelastic instabilities arising in adilute polymer solution driven through the geometry over a wide range of Wi butat negligible Reynolds number. Based on our observations, we describe newinsights into the growth, propagation, and transient dynamics of an elasticvortex formed upstream of the 3D micro-contraction due to flow jetting towardsthe contraction. At low Wi we observe vortex growth for increasing Wi, followedby a previously unreported vortex growth plateau region. In the plateau region,the vortex circulates around the jet with a period that decreases with Wi butan amplitude that is independent of Wi. In addition, we report new out-of-planeasymmetric jetting behaviour with a phase-wise dependence on Wi. Finally, weresolve the rate-of-strain tensor D and ascribe local gradients in D as theunderlying driver of circulation via strain-hardening of the fluid in the wakeof the jet.

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