Figure 7.

(a) Measured birefringence ($\Delta n$) as a function of the strain rate ($\dot{\epsilon }$) for HA1.6 solutions in the OSCER. In the 0.1 wt. % solution the experiment is curtailed at lower $\dot{\epsilon }$ due to the onset of an inertio-elastic instability that distorts the birefringent strand (as illustrated by the inserted image for U = 0.36 m ${\mathrm{s}}^{-1}$, Re = 29, Wi = 2.4). (b) Derivatives of the curves shown in part (a) (i.e., $d\Delta n/d\dot{\epsilon }$, normalized by their maximum values) plotted against $1/\dot{\epsilon }$. The data are fitted with log-normal distributions, the peak of which can be used as a measure of the fluid relaxation time (λ). (c) Extensional viscosity (${\eta }_E$) as a function of $\dot{\epsilon }$, determined from birefringence measurements using the stress-optical rule. (d) Trouton ratio ($\mathrm{Tr}={\eta }_E/\eta$) as a function of the Weissenberg number ($\mathrm{Wi}=\dot{\epsilon }\lambda$).