Effect of Molar Concentration on the Flow Behavior of Micellar Solutions Passing through Small Slits

Abstract

The flow behavior of three types of surfactant solution (namely, Lipoquad C-50, Lipothoquad O/12, and cetyltrimethylammonium bromide [CTAB] with an NaSal counterion) passing through a slit was investigated by means of birefringence correlated with the degree of micelle orientation. With Lipoquad C-50, a high birefringence appeared in the slit section and upstream. With Lipothoquad O/12, the birefringence was low. With CTAB, the birefringence varied considerably with elapsed time. Different experimental results were thus obtained depending on the type of surfactant solution. To discuss the experimental results in depth, concentrations of 0.1× and 10× (2×) were used. With 0.1× concentration of all surfactant solutions, no birefringence appeared. With 10× concentration of Lipoquad C-50, there was no sign of the birefringence that was present with 1× concentration. With 10× concentration of Lipothoquad O/12, a birefringence was present that was not present with 1× concentration, and the distribution of the birefringence was almost the same as that with 1× concentration of Lipoquad C-50. The case with 2× concentration of CTAB was similar to that with 1× concentration. Moreover, the aforementioned results depended on the slit width or contraction ratio. To organize the experimental results systematically,the degree of spatial constraint (DSC) was defined, corresponding to the number of micelles occupying a unit volume. The DSC estimations showed that the birefringence appeared only when the DSC was between 5.9 × 10−6 and 1.2 × 10−5 mol (Lipoquad C-50 and Lipothoquad O/12) and between 1.8 × 10−6 and 7.3 × 10−6 mol (CTAB). Thus, the flow-induced structure generated by flow through an abrupt contraction and expansion was found to depend on a certain degree of micelle constraint.