Rheometric and rheo-optical investigation on the effect of the aliphatic chain length of the surfactant on the shear thickening of dilute worm-like micellar solutions

In this study, a new way of understanding the shear-thickening phenomenon in self-assembled solutions is introduced. The near- and out-of-equilibrium behavior is investigated in four aqueous micellar solutions containing surfactants of the same family: the alkyltrimethylammonium bromide mixed with an ionic salt at equimolar concentration. Thus, the four molecules of surfactants have the same polar head but different aliphatic chain length containing 12, 14, 16, and 18 carbon atoms. In aqueous solutions, the attractive forces between the surfactant molecules depend on the length of the aliphatic chain, thus, varying this parameter will have a definite influence on the aggregation number, the shape and dimension of the micelles. According to our results, the evolution of the low shear viscosity is affected by the chain length. The rheometric measurements performed in steady and time-dependent flows show that the emergence, the range, and the amplitude of the shear thickening also depend on this parameter. However, in the domain of high shear rates, after reaching the maximum of viscosity, all flow curves superimpose irrespective of the chain length. The rheo-optic measurements confirm the apparition of the shear-induced structure (SIS); this new phase is locally oriented and the chain length affects strongly the micelles orientation and the birefringence intensity. These results undoubtedly demonstrate that the chain length plays an important role in the behavior near equilibrium and under shear flow of the micellar systems.