Exploring the onset of the nonlinear start-up response of wormlike micelles through rheology and rheo-microscopy: banding & scission

This study aims to offer new insights and perspective into the linear and nonlinear rheology of a wormlike micellar solution containing a well known surfactant, cetylpyridinium chloride, and a binding salt, diclofenac sodium. The wormlike micelles (WLMs) mesoscopic lengths were evaluated through rheology. The micrometer long micelles, disclosed also with cryogenic -Electron Microscopy (cryo-EM), were subjected to start-up flow experiments, both with a rotational rheometer and with a linear strain controlled shear cell coupled with a microscope. In the nonlinear regime, two Weissenberg numbers can be defined, one related to the terminal relaxation time, Wid, and one connected to the breaking and reforming time, Wib. When Wid < 1, the measured stress response is monotonically increasing up to a steady state value, and the resulting velocity profile is stable. When Wid > 1, the WLMs behave as polymer chains in fast flows, aligning and stretching in the flow direction. When a characteristic shear rate is reached, WLMs show a very pronounced strain hardening behaviour, with a stress peak typical of an elastic chain response. When Wib >> 1, the stress peak appears at the same strain units above a characteristic shear rate value. A sudden stress decrease manifests after the peak, suggesting a breakage phenomenon. The strain at which the stress peak appears permits the evaluation of the WLMs scission energy. The reconstruction of the velocity profiles along the gap of the sample, thanks to a home made rheo-optical device, and the complete flow curve of the system, built with the help of a bio-printer used as a capillary rheometer, suggest that a shear banding phenomenon appears when Wid is roughly equal to one. No direct connection between banding and scission has been experimentally found.