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The importance of initial extension rate on elasto-capillary thinning of dilute polymer solutions
Journal article   Open access   Peer reviewed

The importance of initial extension rate on elasto-capillary thinning of dilute polymer solutions

Ann Aisling, Renee Saraka and Nicolas J. Alvarez
Journal of non-Newtonian fluid mechanics, v 333, 105321
24 Sep 2024
DOI: https://doi.org/10.1016/j.jnnfm.2024.105321
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https://doi.org/10.1016/j.jnnfm.2024.105321View
Published, Version of Record (VoR)Open Access via Drexel Libraries Read and Publish Program 2024CC BY-NC-ND V4.0,  Open

Abstract

CaBER Capillary breakup rheometry Chain stretch Elasto-capillary thinning FENE Polymer physics Relaxation time Stress relaxation
This work focuses on inferring the molecular state of the polymer chain required to induce stress relaxation and the accurate measure of the polymer’s longest relaxation time in uniaxial stretching of dilute polymer solutions. This work is facilitated by the discovery that constant velocity applied at early times leads to initial constant extension rate before reaching the Rayleigh–Plateau instability. Such constant rate experiments are used to correlate initial stretching kinematics with the thinning dynamics in the final thinning regime. We show that there is a minimum initial strain-rate required to induce rate independent elastic effects, and measure the longest relaxation time of the material. Below the minimum extension rate, insufficient stretching of the chain is observed before capillary instability, such that the polymer stress is comparable to the capillary stress at long times and stress relaxation is not achieved. Above the minimum strain-rate, the chain reaches a critical stretch before instability, such that during the unstable filament thinning the polymer stress is significantly larger than the capillary stress and rate-independent stress relaxation is observed. Using a single relaxation mode FENE model, we show that the minimum strain rate leads to a required initial stretch of the chain before reaching the Rayleigh Plateau limit. These results indicate that the chain conformation before entering the Rayleigh Instability Regime, and the stretching induced during the instability, determines the elastic behavior of the filament. Lastly, this work introduces a characteristic dimensionless group, called the stretchability factor, that can be used to quantitatively compare different materials based on the overall material deformation/kinematic behavior, not just the relaxation time. Overall, these results demonstrate a useful methodology to study the stretching of dilute solutions using a constant velocity stretching scheme. •This work highlights the importance of chain stretch in the visco-capillary stretching regime to achieve rate-independent elasto-capillary thinning.•The stretching rate in the visco-capillary regime must be larger than 2/3 the inverse relaxation time to accurately measure the longest relaxation time of the material in the elasto-capillary thinning regime.•The strain required to achieve elasto-capillary thinning is a strong function of the initial stretching rate and concentration, and was shown to correlate to the material relaxation time.•In this work, we show that a variable stretching rate, coupled with a direct measure of axial and radial filament strain allows for a kinematic description of the material, which is denoted as the stretchability factor.

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