04. Boiling and Multi-Phase Flow

Experiments on contaminated Taylor flows in a square microchannel were conducted to investigate the effects of the concentration and the type of surfactant on the shape of bubbles. Nitrogen and water were used for the gas and liquid phases, respectively. Triton X-100 and sodium dodecyl sulfate (SDS) of 99% purity and 90% purity were used as surfactant. The cross section of the microchannel was square and its width was 200 μm. The volume fluxes of the gas and liquid phases were set at 0.54 and 0.44 m/s, respectively. Taylor flows were formed by mixing the two phases at a T-junction. Flow images were obtained by a microscope and a high-speed video camera. An image processing method was applied to the bubble images to obtain the bubble velocity, the bubble length, the curvatures of bubble nose and tail. The curvature of the bubble nose region increased with increasing the surfactant concentration and was found to be well correlated with the surface coverage ratio. It was also proportional to the bubble length at low surfactant concentrations, while at high surfactant concentrations it was independent of the concentration. The bubble velocity decreased with increasing the nose curvature. Its trend depended on the type of surfactant. The surface tension for the adsorption-desorption equilibrium plays a key role to correlate the bubble velocity under the surfactant effect. The tail curvature steeply increased at low concentrations, but tended to be constant at high concentrations. This trend is due to that the bubble tail was completely covered with surfactant transferred toward the tail by the interfacial advection. The trail curvature showed a strong correlation with the surface coverage ratio as well as the nose curvature.