Surfactant effect on the multiscale structure of bubbly flows

Small amounts of surfactant can drastically change bubble behavior. For example, a bubble in aqueous surfactant solution rises much slower than one in purified water. This phenomenon is explained by the so-called Marangoni effect caused by a nonuniform concentration distribution of surfactant along the bubble surface. In other words, a tangential shear stress appears on the bubble surface due to the surface tension variation caused by the surface concentration distribution, which results in the reduction of the rising velocity of the bubble. More interestingly, this Marangoni effect influences not only the rising velocity, but also the lateral migration in the presence of mean shear. Furthermore, these phenomena influence the multiscale nature of bubbly flows and cause a drastic change in the bubbly flow structure. In this article, we review the recent studies related to these interesting behaviors of bubbles caused by the surfactant adsorption/desorption on the bubble surface. Introduction Bubbly flows are encountered in many industrial and environmental processes, such as chemical reactors, heat exchangers and aeration systems for water purification, They are also utilized to reduce the frictional drag of a moving body in water by injecting bubbles in the turbulent boundary layer, which has recently attracted much attention from many researchers (see Sugiyama et al. 2008 and Ceccio 2010 for a comprehensive review). In these bubbly flows, small amounts of surfactant can cause dramatic changes in the flow structures through the multiscale effects of the flows. Figure 1 represents the idea of the multiscale structure of bubbly flows in aqueous surfactant solutions. Small amounts of surfactant dissolved in the water adsorb onto the bubble surface, greatly affecting the bubble rising motion. Likewise affected are bubble-bubble interactions; the coalescence of small bubbles hardly occurs in the presence of surfactant. These factors are coupled and can dramatically change the whole bubbly flow structure. For example, because of this structural change of bubbly flow and also the influence of the adsorbed surfactant on mass transfer, the total mass transfer through a bubbly plume in an aeration tank can be quite different with and without surfactant. Fig. 1 Multiscale structure of bubbly flows in aqueous surfactant solution Surfactant