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

TL;DR

This paper demonstrates that CaBER measurements of polymer relaxation times are influenced by experimental conditions and flow history, challenging the assumption that they directly reflect the true material relaxation time.

Contribution

The study reveals that CaBER relaxation times are apparent and depend on geometrical and flow parameters, highlighting the importance of finite extensibility effects in interpreting rheometry data.

Findings

CaBER relaxation time increases with plate and droplet size.

Flow history affects measured relaxation times, contradicting model predictions.

Finite extensibility explains the observed dependence in dilute solutions.

Abstract

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 is in contradiction with 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 artefacts such as solvent evaporation or polymer degradation and that it can only be rationalised by finite extensibility effects (FENE-P model) for a dilute polymer solution in a viscous solvent, but not for semi-dilute solutions in a low-viscosity solvent.