Detecting wormlike micellar microstructure using extensional rheology.

We report experiments on two series of wormlike micellar solutions that comprise octyl trimethylammonium bromide/sodium oleate (OTAB/NaOA) and cetylpyridinium chloride/sodium salicylate (CPCl/NaSal) using a capillary breakup extensional rheometer (CaBER) and a dripping onto substrate (DoS) technique. These wormlike micellar systems show a peak in zero shear viscosity or in the shear relaxation time beyond a critical surfactant or salt concentration. Prior cryogenic transmission electron microscopy imaging has indicated that the system based on OTAB/NaOA experiences a transition from linear to shorter linear micelles (L-L) beyond the viscosity peak. However, the linear wormlike micelles based on CPCl/NaSal form branched networks beyond the viscosity peak (L-B). In this work, we investigate whether the extensional rheology is sensitive to these two different microstructural transitions. In particular, we attempt to develop a criterion based on the extensional flow parameters in order to distinguish these two systems from each other. In addition, we probe the possibility of flow-induced micellar breakage imposed by capillary breakup extensional experiments. Our results indicate that at a given zero shear viscosity, the branched wormlike micelles based on CPCl/NaSal exhibit a longer filament lifetime tfil than the linear wormlike micelles. However, beyond the critical concentration, the linear wormlike micelles based on OTAB/NaOA exhibit significantly shorter filament lifetimes than their linear counterparts at concentrations below the viscosity peak. More importantly, we show that the filament lifetime scales differently with zero shear viscosity η0 in these two systems. For L-B transition, t fil ∝ η 0 1.1 below the viscosity peak and t fil ∝ η 0 0.76 above the viscosity peak. However, for L-L transition, this scaling remains the same for concentrations below and above the viscosity peak and reads as t fil ∝ η 0 0.5 . These findings provide a novel criterion to distinguish these two types of microstructural transitions in wormlike micelles. On the other hand, we demonstrate that the Trouton ratio, filament lifetime, and the extensional relaxation time measured by CaBER technique are all lower than the ones obtained by DoS method. The latter result provides the first evidence for strong nonlinear effects (e.g., flow-induced micellar breakage) in capillary breakup extensional flows of wormlike micelles. [ABSTRACT FROM AUTHOR]