  We report quantitative evidence of mixing-layer elastic instability in a
viscoelastic fluid flow between two widely spaced obstacles hindering a channel
flow at $Re\ll1$ and $Wi\gg1$. Two mixing layers with nonuniform shear velocity
profiles are formed in the region between the obstacles. The mixing-layer
instability arises in the vicinity of an inflection point on the shear velocity
profile with a steep variation in the elastic stress. The instability results
in an intermittent appearance of small vortices in the mixing layers and an
amplification of spatio-temporal averaged vorticity in the elastic turbulence
regime. The latter is characterized through scaling of friction factor with
$Wi$, and both pressure and velocity spectra. Furthermore, the observations
reported provide improved understanding of the stability of the mixing layer in
a viscoelastic fluid at large elasticity, i.e. $Wi\gg1$ and $Re\ll1$, and
oppose the current view of suppression of vorticity solely by polymer
additives.
