Shear and extensional rheology of aqueous solutions of cocamidopropyl betaine and sodium dodecyl sulfate mixture

Abstract

The paper presents the results obtained for aqueous solutions of a mixture of cocamidopropyl betaine (CAPB) and sodium dodecyl sulfate (SDS) in rheological tests in the shear and extensional flow. The rheological measurements in the extensional flow were conducted using a capillary breakup extensional rheometer. The description of the mechanical spectra required the application of the multi-mode Maxwell model, which demonstrates that stress relaxation occurs via reptation – as in entangled polymer solutions. The rheological properties of aqueous CAPB/SDS solutions in the extensional flow were also found to be qualitatively consistent with the properties of entangled polymer solutions. The ratio of the extensional relaxation time to the transient shear relaxation time was considerably lower than one, and the range of elasto-capillary thinning appeared at a Weissenberg number of 2/3. The difference between polymer solutions and CAPB/SDS solutions is noted at late times, when the transient extensional viscosity of the solutions of polymers and wormilke cationic surfactants reaches a constant value, whereas in CAPB/SDS solutions it drops rapidly. The decrease in transient extensional viscosity in this range may be due to the break-up of wormlike micelles under the influence of high transient extensional stresses.