Effect of Molar Concentration Ratio on the Flow Properties of Rod-Like Micellar Solutions Passing through Small Orifices

The flow properties of rod-like micellar (surfactant) solutions passing through circular orifices with an inner diameter of 100 µm to 1.0 mm were investigated in this study. Rod-like micellar solutions comprising a cationic surfactant [oleyl bis(2-hydroxyethyl)methylammonium chloride; Lipothoquad O/12] and a counterion (sodium salicylate; NaSal) were used. The molar concentration ratio was varied from 0.10 to 100 to change the rheological properties of the test fluid, which were evaluated using a strain-controlled rheometer and a capillary viscometer. All the rod-like micellar solutions exhibited non-Newtonian viscosity except those with molar concentration ratios of 0.10 and 0.15, which instead exhibited Newtonian viscosity consistent with that of water. In the dynamic viscoelasticity measurements, the relaxation times of the rod-like micellar solutions with molar concentration ratios other than 0.10, 0.15, and 100 were calculated by extrapolating slopes 1 and 2. For each orifice, the experimental results with water alone agreed with theoretical predictions within the experimental errors (thereby demonstrating the validity of the experimental setup). In dimensionless graphs arranged by generalized Reynolds number, the dimensionless pressure drops for the molar concentration ratios of 0.10, 0.15, 50, and 100 agreed well with the experimental (predicted) values for water. For the other rod-like-micellar solutions, the dimensionless pressure drop was larger than that for water. In other words, the rheological and flow properties were found to change with the molar concentration ratio. To discuss the experimental results in depth, the flow resistivity was calculated and was largest for the molar concentration ratio of 1.0. The increase in pressure drop was also largest for molar concentration ratio of 1.0. The Weissenberg number was used to summarize the experimental results in terms of elastic properties, and the characteristic increase in pressure drop was found to occur at a Weissenberg number on the order of 10 0 , at which elasticity was strongly expressed.