Effect of non-ideality of wave plate in polarization imaging method for rheo-optical studies on CTAB/NaSal aqueous solution

Abstract

Polarization imaging method, PIM, can provide us with two-dimensionally spatial distributions of birefringence and extinction angle. In a previous study, we reported a new rheo-optical apparatus based on conbination of PIM and a conventional rheometer. The appratus was very effective to visualize the stress field of shear induced sturucture. However, there was a problem in the quantitative evaluation of birefringence, which can be attributed to non-ideality of the wave plate for the circuler polarizer. In this paper, we improved the wave plate and reports a new data for a wormlike micelle solution which is composed of 30 mol/m3 cetyltrimethylammonium bromide and 60 mol/m3 sodium salicylate. While the previous optical system provided us with 2-fold symmetry images, the new one provided us with almost uniform birefringence distributions. The spatially mean value of the birefringence distribution, [Δn], in low strain range in the new optical system meets the stress-optical rule, and the stress-optical coefficient is consistent to literature value in linear regime. In non-linear regime where the finite extensibility effect significantly contribute to the stress, the stress-optical coefficient can be described as a unique function of orientation factor of micelles.