The rheological characteristics were measured by using (24R)-MC 976 stress-controlled rheometer (MCR301: Anton Paar GmbH). A cone-plate device with the diameter of 50 mm and the cone-angle of 0.04 rad was used for the measurements for the apparent viscosity under the shear rate range from 1.00 to 1000 s−1. The relaxation behavior of each surfactant solution was measured by using a shear-rate controlled rheometer (ARES; Rheometrics) with a cone-device of the same geometry. At first, a certain initial shear rate of 70.0 s−1 was added to the fluids. After the sudden release of the shear rate, the relaxation behavior was measured. As reported in the previous study , sternum surfactant system shows multi-relaxation behaviors. Thus, the time-variation of the relaxing stress, τ (Pa) after the sudden release was fitted by a triple exponential Maxwell model as follows:equation(1)τ=τ1e-t/tr1+τ2e-t/tr2+τ3e-t/tr3τ=τ1e-t/tr1+τ2e-t/tr2+τ3e-t/tr3Here, t (s) is time and tr1 (s), tr2 (s) and tr3 (s) are relaxation times (tr1 < tr2 < tr3). τ1 (Pa), τ2 (Pa) and τ3 (Pa) are the contributions to the stress from the respective relaxations. The temperature was set at 25 °C for both experiments.