STRUCTURE AND RHEOLOGY OF WORMLIKE MICELLES

In a semi-dilute aqueous solution under certain conditions, surfactant molecules will self assemble to form wormlike micelles. The micelles are dynamic in structure since they can break and reform, providing an additional mode of relaxation. The viscoelastic properties of the wor mlike micelles can be predicted using simple rheological models. For m any surfactant solutions the mechanical data can be related to the opt ical data by the stress-optical rule. From the viscoelastic data it is possible to estimate the breaking time of the micelle. The techniques of birefringence and small angle light scattering are used to study t he microstructure of a surfactant solution under simple shear and exte nsional flow The sample under investigation is a solution of cetyltrim ethylammonium bromide and sodium salicylate in water, with a salt to s urfactant ratio of 7.7. Below a critical shear rate, the birefringence increases linearly with shear rate and the stress-optical rule is val id. The SALS patterns reveal distinctive butterfly patterns indicating that scattering is a result of concentration fluctuations that modera tely couple to the flow. However, above a critical shear rate the bire fringence plateaus and the stress-optical rule is no longer valid. SAL S patterns show both a bright streak and a butterfly pattern. The brig ht streak is caused by elongated structures aligned in the direction o f the flow. The oriented structures occur when the characteristic time of flow is faster than the breaking time of the micelles.