Viscoelastic secondary flows in curved microchannels*

The flow of viscoelastic fluids is well-known to develop purely elastic instabilities in curved geometries in the absence of inertia. Below the critical shear rate at which the instability is triggered, a steady, secondary flow driven by the first normal stress difference and the curvature of the streamlines develops in the cross-section of the channel. For channels of constant curvature and square cross-section, numerical calculations have shown that this flow takes the shape of two counter-rotating vortices. We present the first experimental visualization evidence and characterization of this steady secondary flow. Using a dilute solution of polymer, we capture the nature of the flow by performing confocal imaging of the stream-dyed fluid in the channel cross-section. We show that the observed dye transport is in good qualitative agreement with the flow lines computed numerically. We then use micro-PIV techniques to measure the components of the flow velocity in the plane of the microchannel, half-way between the top and bottom walls. We show that the measured streamlines and the relative velocity magnitude of the secondary flow are in quantitative agreement with the numerical results.