Distinguishing between linear and branched wormlike micelle solutions using extensional rheology measurements

Abstract

A filament stretching extensional rheometer and capillary breakup extensional rheometer is used to measure the effect of branching on the extensional rheology of a series of wormlike micelle solutions. The experiments are performed using a series of linear and branched wormlike micelle solutions consisting of sodium oleate (NaOA) and octyl trimethyl ammonium bromide (C8TAB). The ratio of NaOA to C8TAB is fixed at 70∕30 while the total surfactant concentration is varied. The shear rheology of wormlike micelle solutions is found to demonstrate a maximum in shear viscosity at 4wt% followed by a sharp decrease in viscosity with increasing surfactant concentration. It has been shown through cryo-transmission electron microscopy imaging that the maximum in the shear viscosity for these fluids corresponds to the transition from linear entangled to branched micelles. The extensional rheology measurements for all of the wormlike micelle solutions below 4wt% demonstrated some degree of strain hardening of the exten...