Recovery dynamics and polymer scission in capillary breakup extensional rheometry

Abstract

Capillary breakup extensional rheometry (CaBER) is a technique widely used to quantitatively measure the transient extensional rheology of a visco-elastic fluid. In this paper, we investigate some of the shortcomings of measuring the transient relaxation time through CaBER and Dripping onto Substrate (DoS)-CaBER experimentation and describe problematic conditions for which consistency of results is not achieved. Using a high molecular weight polyacrylamide polymer $(M_W=18x{10}^{6}g/\text{mol})$ in a viscous water and glycerol solution, we investigated the effect that the choice of syringe size, tubing size, tubing length and flow rate used to generate the liquid bridge in DoS-CaBER can have on the decay evolution of the fluid filament. The resulting measurements showed a sharp decrease in extensional viscosity and relaxation time with increasing strength of the shear and extensional flows within the syringe and tubing used to generate the pendant drop. These measurements highlighted the importance of considering the flow and deformation history of the polymer prior to the DoS-CaBER and CaBER stretches. In order to understand whether these observed effects were due to recoverable pre-deformation of the polymer or permanent scission of the polymer, the DoS-CaBER syringe setup was used to deposit the polymer solution into a CaBER under different loading conditions. CaBER tests were then performed with various delay times to erase the deformation history of loading. For these samples, rest times of more than 100 extensional relaxation times were required to erase the deformation history caused by the loading of the sample. Even with the pre-conditioning erased, however, unrecoverable losses in relaxation time and extensional viscosity remained. These observations indicate that polymer scission occurred in all samples where loading resulted in an extensional Weissenberg number greater than $Wi>8s^{-1}$. Next, the effect of successive CaBER stretches on a single sample and the time delay imposed between successive stretches on the fluid rheology was studied. Stretches performed immediately one after the other with no recovery time built in showed a steep decline in measured relaxation time and breakup time. However, even with post stretch delays of twenty minutes, full recovery of the initial fluid properties was not achieved suggesting that extensional flow induced scission of the polymer had occurred even in CaBER. Thus, it is clear that the effect of preconditioning a viscoelastic fluid is strong, and these factors need to be considered prior to conducting CaBER and DoS-CaBER experiments in the future.