  We investigate the curvature-dependence of the visco-elastic Taylor-Couette
instability. The radius of curvature is changed over almost a decade and the
critical Weissenberg numbers of the first linear instability are determined.
Experiments are performed with a variety of polymer solutions and the scaling
of the critical Weissenberg number with the curvature against the prediction of
the Pakdel-McKinley criterion is assessed. We revisit the linear stability
analysis based on the Oldroyd-B model and find, surprisingly, that the
experimentally observed scaling is not as clearly recovered. We extend the
constitutive equation to a two-mode model by incorporating the PTT model into
our analysis to reproduce the rheological behaviour of our fluid, but still
find no agreement between the linear stability analysis and experiments. We
also demonstrate that that conclusion is not altered by the presence of inertia
or viscous heating. The Pakdel-McKinley criterion, on the other hand, shows a
very good agreement with the data.
