SHEAR THICKENING BEHAVIOR OF A RHEOPECTIC MICELLAR SOLUTION - SALT EFFECTS

The influence of NaSal and NaBr concentration on the shear thickening behavior of aqueous micellar solutions of cetyltrimethylammonium bromide (CTAB) and sodium salicylate (NaSal) is studied experimentally. Flow birefringence decay at different stages during the stress growth period supports the idea that the shear thickening phenomenon is due to a continuous kinetic coagulation during which large micelles or micellar structures are formed. This process is independent of the flow direction as evidenced by the results of oscillating flow experiments. Initially, when the NaSal concentration increases, the critical shear rate decreases and the magnitude of the shear thickening increases. Subsequently, this trend is reversed when the NaSal to CTAB ratio exceeds unity. Moreover, consistent with these observations, the birefringence relaxation time and dissociation time of the shear thickening structures also increase at first and then decrease with the NaSal/CTAB ratio, with their maximum located at the unit NaSal/ CTAB ratio. The temperature dependence of the relaxation time obeys an Arrhenius law. In addition, substantial variations, which depend strongly on the NaSal/CTAB ratio, are observed in these characteristic features of the shear thickening phenomenon upon addition of NaBr. These results may be rationalized on the basis of the influence of counterions on micellar lifetime and micellar size, in both the equilibrium and shear thickening states.