  Secondary flows are ubiquitous in channel flows, where small velocity
components perpendicular to the main velocity appear due to the complexity of
the channel geometry and/or that of the flow itself such as from inertial or
non-Newtonian effects, etc. We investigate here the inertialess secondary flow
of viscoelastic fluids in curved microchannels of rectangular cross-section and
constant but alternating curvature: the so-called "serpentine channel"
geometry. Numerical calculations (Poole et al, 2013) have shown that in this
geometry, in the absence of elastic instabilities, a steady secondary flow
develops that takes the shape of two counter-rotating vortices in the plane of
the channel cross-section. We present the first experimental visualization
evidence and characterization of these steady secondary flows, using a
complementarity of microPIV in the plane of the channel, and confocal
visualisation of dye-stream transport in the cross-sectional plane. We show
that the measured streamlines and the relative velocity magnitude of the
secondary flows are in qualitative agreement with the numerical results. In
addition to our techniques being broadly applicable to the characterisation of
three-dimensional flow structures in microchannels, our results are important
for understanding the onset of instability in serpentine viscoelastic flows.
