The viscoelastic relaxation time {\tau} of a polymer solution is often
measured using Capillary Breakup Extensional Rheometry (CaBER) where a droplet
is placed between two plates which are pulled apart to form a thinning
filament. For a slow plate retraction protocol, required to avoid
inertio-capillary oscillations for low-viscosity liquids, we show
experimentally that the CaBER relaxation time inferred from the exponential
thinning regime is in fact an apparent relaxation time that increases
significantly when increasing the plate diameter and the droplet volume.
Similar results are obtained with a Dripping-onto-Substrate (DoS) method. This
dependence on the flow history before the formation of the viscoelastic
filament is in contradiction with polymer models such as Oldroyd-B that predict
a filament thinning rate 1/3{\tau} which is a material property independent of
geometrical factors. We show that this is not due to artefacts such as solvent
evaporation or polymer degradation and that it cannot be universally explained
by the finite extensibility of polymer chains.Comment: 18 pages, 6 figures, presented at the ICR 2023 in Athen