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Purely-elastic instabilities in serpentine microchannels
Purely elastic instabilities are known to occur in flows with curved streamlines in viscoelastic
fluids at low Reynolds numbers. They have recently attracted renewed interest as they have
been shown to increase mixing in wavy microchannels. The onset of instability has been
proposed to be a function of the balance between curvature and normal stress effects, but
the exact form of this relation is scarcely studied, in particularly for channel flow. Here we
report the results of a combined experimental and numerical investigation of variation of the
instability threshold with the channel curvature.
The experimental study is performed for a dilute polymer solution in a wavy microchannel.
We have analyzed the critical Weissenberg number (Wi) at which the flow becomes unstable
as a function of the geometry of the channel and the properties of the polymer solution. For
this purpose we utilize a microfluidic channel of width (side-length) 25 ?m to 100 ?m and
aspect ratios (width over height) between 0.5 and 2. The channel comprises a series of half
loops of radius R which is systematically varied in the range 25 ?m < R < 2000 ?m. We
use solutions of a high molecular weight polyethylene oxide (Mw=106 and 4x106) in various
glycerine/water solvents. The instability onset is defined via visualizations of fluctuations
within the flow. We systematically investigate the effects of channel curvature and aspect
ratio and the effects of solvent viscosity, molecular weight and polymer concentration.
The numerical simulations study the creeping flow (Re = 0) for a viscoelastic fluid obeying the
upper-convected Maxwell model. Two-dimensional simulations matching the experimental
conditions show that above a critical Weissenberg number the flow becomes unsteady. The
scaling for this critical Wi is in good qualitative agreement with the experiments.
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Contact : mathilde.reyssat@espci.fr