  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.
