A Study on the Condition of No Shear-Induced Structure Generation in Wormlike Micelle Solutions

We hypothesized that the shear-induced structure, SIS, of wormlike micelles solutions could be suppressed by increasing the volume concentration of micelles, because emergence of spatial heterogeneity in the micellar concentration is not induced. In order to prove this hypothesis, we prepared samples with high concentrations of surfactants and counterions, keeping their molar concentration ratios constant, and observed the occurrence of SIS. The occurrence of extremely large stress overshoot in the step shear test was used as a criterion for the occurrence of SIS. We found disappearance of the SIS by increasing the micellar concentration in all the molar ratio. Although it is known that the relaxation time of the micellar solution decreases with the increase in the surfactant molar concentration, the absence of the SIS at high surfactant molar concentration is not attributed to the reduction of the relaxation time because the SIS can be found even when the temperature is increased. The relaxation time was not the dominant factor in SIS, but the spatial movement of micelles was a necessary condition for the generation of SIS. In addition, the effective body integrability of wormlike micelles was calculated and compared with the experimental results to determine the critical effective body integrability for SIS generation.