  In this experimental and numerical study, we revisit the question of the
onset of the elastic regime in viscoelastic pinch-off. This is relevant for all
modern filament thinning techniques which aim at measuring the extensional
properties of low-viscosity polymer solutions such as the Slow Retraction
Method (SRM) in Capillary Breakup Extensional Rheometry (CaBER) as well as the
dripping method where a drop detaches from a nozzle. In these techniques, a
stable liquid bridge is slowly brought to its stability threshold where
capillary-driven thinning starts, slowing down dramatically at a critical
radius $h_1$ marking the onset of the elastic regime where the bridge becomes a
filament with elasto-capillary thinning dynamics. While a theoretical scaling
for this transition radius exists for the classical step-strain CaBER protocol,
where polymer chains stretch without relaxing during the fast plate separation,
we show that it is not necessarily valid for a slow protocol such as in SRM
since polymer chains only start stretching (beyond their equilibrium coiled
configuration) when the bridge thinning rate becomes comparable to the inverse
of their relaxation time. We derive a universal scaling for $h_1$ valid for
both low and high-viscosity polymer solution which is validated by both CaBER
(SRM) experiments with different polymer solutions, plate diameters and sample
volumes and by numerical simulations using the FENE-P model.
