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

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 ( C 8 TAB ) . The ratio of NaOA to C 8 TAB 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 4 wt % 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 4 wt % demonstrated some degree of strain hardening of the extensional viscosity, however, beyond 4 wt % little strain hardening is observed. The maximum value of the Trouton ratio is found to decay rapidly with increasing micelle concentration, starting from values of nearly Tr ≈ 1000 at a concentration of 2 wt % and approaching an asymptote close to Newtonian limit of Tr ≈ 3 for concentrations of 4 wt % and above. These results are most likely due to the additional stress relief mechanisms available to branched micelles which appear to be extremely efficient in extensional flows. These stress relief mechanisms include the fast and fluid sliding of branch points along the length of the micelle and the increased occurrence of “ghost-like” crossing of micelles at entanglement points with increasing surfactant concentration. These observations demonstrate how sensitive the extensional rheology of wormlike micelles is to branching.