Shear-induced structure change in shear-banding of a wormlike micellar solution in concentric cylinder flow

Abstract

The occurrence of shear-banding in wormlike micelle solutions has been observed by the birefringence distribution, velocity distribution, and other techniques. In the observation method of the birefringence distribution, however, the birefringence distribution is not enough to understand the banding structure and to examine the micelles' condition such as the generation of the shear-induced structure (SIS) but the orientation distribution is also required. In this study, a new technique to evaluate the distribution of the birefringence, the orientation angle, and the stress-optical coefficient along the radial direction was developed for this purpose. The stress optical coefficient is used as a key factor to recognize the micelles' structure, and the micelle condition in each band is examined. The visualized image by the crossed-Nicol method was precisely quantified by the new image analysis technique using the Hue value as color information. The radial distribution of the orientation angle was evaluated by the extinction position as a function of the distance from the inner wall that was quantified from the dark area shape in the whole circumferences observation image of the concentric cylinder flow. The distribution and the transient behavior of the stress-optical coefficient were calculated based on these results. The shape of the shear-banding visualized by the crossed-Nicol method coincides with the distribution of the stress-optical coefficient, and the SIS is generated only in the inner band. During the temporal stress oscillation phenomenon, the band thickness does not change, but the stress-optical coefficient of the inner SIS-band is synchronized with the stress oscillation in the opposite phase.