Birefringence of Aqueous Surfactant Solution in the Vicinity of a Slit Exit

Abstract

Two dimensional jets of a visocoelastic fluid have been studied using aqueous surfactant solution of cetyltrimethylammonium bromide (CTAB) and sodium salicylate (NaSal). The solution shows strong viscoelasticity even at low concentration condition 0.01[mol/l] and 0.07[mol/l] for CTAB and NaSal, respectively. Shapes of jets issuing from slits are observed, and the birefringence of sample solution is measured along the flow direction from the inside to the outside of the slit. Elastic stresses of jets are evaluated from their shapes, and the degree of orientation of thread-like micelles is calculated from the value of birefringence. The elastic stress of the slit jets agrees with that of capillary jets, and the value of birefringence increases and is followed by decreasing inside of the slit near the exit. The increase and decrease in the value of birefringence is related to a change in the orientation of micelles caused by the contraction and expansion of the flow near the exit. The increase in the value of birefringence is also observed with die swelling.