Beware of CaBER: Filament thinning rheometry doesn't give `the' relaxation time of polymer solutions
arxiv(2023)
摘要
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 τ_e 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. Similarly, we observe that τ_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 1/3τ (τ 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.
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