Flow Property and Micellar Structures in Capillary Flows of Surfactant Solutions

Flow property, which is expressed by the relation between wall shear stress and apparent shear rate, is examined in the flows of surfactant solutions through various capillaries. Furthermore, small angle light scattering (SALS) experiments are carried out in the flow through a slit cell. In the present experiments, aqueous solutions of Cetyltrimethylammonium bromide (CTAB) and Sodium Salicylate (NaSal) are used. The flow curve has a break point and its slope below the bending point significantly depends on the ratio of salt concentration CS to surfactant concentration C D. From the experiments on the effect of capillary diameter, it is found that eqimolar solution for CD= 0.03M exhibits a remarkable diameter dependence below the break point while the solution for CD= 0.03M and CS /CD = 7.7 exhibits a diameter dependence only in a narrow region at intermediate shear rates. In the SALS experiments the butterfly pattern is observed for CS / CD=1 and the four-fold symmetry pattern for CS /CD = 7.7. The characteristic scattering pattern is destroyed just after the startup of the flow at high shear rates where the flow curves overlaps regardless of salt concentration and length and diameter of capillary. This fact suggests that micellar network structures are broken at the high shear rates.