Surface coverage ratio of contaminated Taylor bubbles in a square microchannel

Abstract

Experiments on contaminated Taylor flows in a microchannel were carried out to investigate the surface coverage ratio for different surfactant properties. Nitrogen and water were used for the gas and liquid phases, respectively. Triton X-100, sodium dodecyl sulfate (SDS) of 90% purity and SDS of 99% purity were used as surfactant. The cross section of the microchannel was square and its width was 200μm. The gas and liquid volumetric fluxes were 0.54 and 0.50 m/s, respectively. Taylor flows were formed by mixing the two phases at the T-junction. Shapes of Taylor bubbles were observed using a microscope and a high-speed video camera. The liquid film thickness changed in the film region, demonstrating that a steep gradient of the interfacial surfactant concentration was formed there. The interface was therefore separated into the clean interface region and the immobilized region by detecting the change in the liquid film thickness. The surface coverage ratio estimated from bubble shapes agreed with that measured from the reduction in the bulk concentration due to surfactant separation by bubbles. The surface coverage ratio increased with increasing the bulk concentration and the increasing rate was smaller than that for the adsorption–desorption equilibrium because of the presence of interfacial advection. The data, however, could be fitted by modifying the functional form for the ad-desorption equilibrium. The bulk concentration normalized by the concentration for the breakdown of the logarithmic relation between the surface tension and the concentration given by the isotherm worked for correlating the surface coverage ratio for the three different surfactants, which can be understood from the order of magnitude of the Marangoni stress.