Investigating the Stability of Viscoelastic Stagnation Flows in T-shaped Microchannels 
by Soulages, J., Oliveira, M.S.N., Sousa, P.C., Alves, M.A. and McKinley, G.H.

We investigate the stability of steady planar stagnation flows of a dilute polyethylene
oxide (PEO) solution using T-shaped microchannels. The precise flow rate control and
well-defined geometries achievable with microfluidic fabrication technologies enable us to
make detailed observations of the onset of elastically-driven flow asymmetries in steady
flows with strong planar elongational characteristics. We consider two different stagnation
flow geometries; corresponding to T-shaped microchannels with, and without, a recirculating
cavity region. In the former case, the stagnation point is located on a free streamline,
whereas in the absence of a recirculating cavity the stagnation point at the separating
streamline is pinned at the confining wall of the microchannel. The kinematic differences
in these two configurations affect the resulting polymeric stress fields and control the critical
conditions and spatiotemporal dynamics of the resulting viscoelastic flow instability. In
the free stagnation point flow, a strand of highly-oriented polymeric material is formed
in the region of strong planar extensional flow. This leads to a symmetry-breaking bifurcation
at moderate Weissenberg numbers followed by the onset of three-dimensional
flow at high Weissenberg numbers, which can be visualized using streak-imaging and
microparticle image velocimetry. When the stagnation point is pinned at the wall this
symmetry-breaking transition is suppressed and the flow transitions directly to a threedimensional
time-dependent flow at an intermediate flow rate. The spatial characteristics
of these purely elastic flow transitions are compared quantitatively to the predictions of
two-dimensional viscoelastic numerical simulations using a single-mode simplified Phan-
Thien-Tanner (SPTT) model.