  The objective of this work is to characterise the onset of laterally
asymmetric flow of viscoelastic solutions around a confined microfluidic
cylinder, which was encountered in a recent study [Rodrigues et al.,
$\textit{J. Non-Newton. Fluid Mech.}$ $\textbf{289}$, 104406 (2020)]. To this
end, two non-Newtonian fluids were employed in the same micro-geometry. Two
microchannels were studied, both with a cylinder of diameter 75
$\mathrm{\mu}$m, aspect ratio (channel height over width) of 0.37 and blockage
ratio (cylinder diameter over channel width) of 0.28, differing only on the
width of the pressure taps, located 500 $\mathrm{\mu}$m up- and downstream from
the respective cylinder face, on opposing walls. The working fluids consist of
two poly(ethylene oxide) (PEO) solutions: a weakly shear-thinning elastic fluid
and an elastic shear-thinning fluid. Micro-Particle Image Velocimetry
($\mathrm{\mu}$PIV) and streak imaging techniques were used to evaluate the
flow over a Weissenberg number range: $100\leq Wi\leq500$, while maintaining a
low Reynolds number, $Re<1$. The elastic shear-thinning solution showed
laterally asymmetric flow past the cylinder with both pressure tap designs,
while with the weakly shear-thinning solution asymmetric flow was only observed
with the wider pressure tap intake. In both cases, the fluids preferentially
chose the cylinder/wall gap opposing the upstream pressure tap, which was found
to influence the flow greatly, seemingly associated with time-dependent flow
and possibly the lateral flow asymmetry itself. This work brings to light the
necessary compromise between optimal pressure tap design for quality pressure
measurements and minimal flow interference, due to the increased susceptibility
of elastic microfluidic flows to flow perturbations.
