Beware of CaBER: Filament thinning rheometry does not always give 'the' relaxation time of polymer solutions

The viscoelastic relaxation time 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 ${\tau }_e$ inferred from the exponential thinning regime is in fact an apparent relaxation time that may increase significantly when increasing the plate diameter and the droplet volume. Similarly, we observe that ${\tau }_e$ increases with the plate diameter for the classical step-strain plate separation protocol of a commercial (Haake) CaBER device and increases with the nozzle diameter for a dripping-onto-substrate (DoS) method. This dependence on the flow history before the formation of the viscoelastic filament contradicts polymer models such as Oldroyd-B that predict a filament thinning rate $1/3\tau$ ($\tau$ being the model's relaxation time), which is a material property independent of geometrical factors. We show that this is not due to artifacts such as solvent evaporation or polymer degradation and that it can be rationalized by finite extensibility effects (FENE-P model) only for a dilute polymer solution in a viscous solvent, but not for semidilute solutions in a low-viscosity solvent.