Viscoelastic flow in an obstructed microchannel at very high Weissenberg number

Detailed measurements of shear-thinning viscoelastic aqueous solutions, with relatively low zero- shear viscosity allowing for uncharacteristically high Weissenberg number in microchannel flow are presented. Herein we examine the flow behaviour resulting from a 50um post placed in the channel centreline. Micro Particle Image Velocimetry measurements show the onset of an upstream insta- bility at a critical Elasticity number corresponding to polymer concentrations above 25ppm. The local viscosity ratio in the region upstream of the post is proposed as a driving mechanism for the instability which resembles the Kaye effect. Additionally, the classical instability owing to separa- tion and vortex formation downstream of the post in Newtonian flow is suppressed and a stable wake is observed instead at the Elasticity numbers considered.