ELONGATIONAL FLOW BEHAVIOR OF CETYLTRIMETHYLAMMONIUM BROMIDE SODIUM-SALICYLATE SURFACTANT SOLUTIONS

The elongational flow behavior of cetyltrimethylammonium bromide (CTAB)/sodium salicylate (NaSal) solutions is investigated using a Rheometrics RFX fluid analyzer. For dilute solutions, the elongational stress slowly increases with time upon the application of a constant strain rate and eventually arrives at a fluctuating plateau region, where the elongation viscosity is several orders of magnitude larger than the shear viscosity at a comparable shear rate. The time required for the stress growth is inversely proportional to the strain rate and decreases dramatically with the surfactant concentration. At higher concentrations, the ratio of the elongational to shear viscosities at low strain rates decreases toward the Trouton value of 3. It is suggested that elongational flow promotes side-to-side collisions between micelles which result in micelle growth. Furthermore, the salt (NaSal) concentration is found to be a key variable influencing the micellar self-assembly in both elongational and shear flow fields.