Surfactant effect on the buoyancy-driven motion of bubbles and drops in a tube.

The effect of surfactants on the buoyancy-driven motion of bubbles and drops in a vertical tube is experimentally examined. The terminal velocities of fluid particles are measured and their steady shapes are quantitatively characterized in systems with various bulk-phase concentrations of surfactant. In the case of air bubbles, the presence of surfactant retards the motion of small bubbles due to the development of adverse Marangoni stresses, whereas it enhances the motion of large bubbles by allowing them to deform away from the tube wall more easily. For viscous drops, the surfactant-enhanced regime of particle motion becomes more pronounced in the sense that the terminal velocity becomes more sensitive to surfactant concentration, whereas the surfactant effect in the surfactant-retarded regime becomes weaker.