(168x) Buoyancy-Driven Motion of Air Bubbles in the Presence of Soluble Surfactants

We present our experimental results for the effect of bulk-soluble surfactants on the buoyancy-driven motion of an air bubble rising in square and rectangular channels filled with an immiscible liquid. Bulk fluids such as glycerol-water solutions, carboxymethyl cellulose solutions, and silicone oils and surfactants such as Triton X-100, sodium dodecyl sulfate (SDS), Tween-80 at various concentrations are used. When the bulk fluid is Newtonian, small bubbles are nearly spherical. As the bubble volume increases, the bubbles become prolate with fore and aft symmetry. The terminal velocity also increases as the bubble volume increases. As the bubble size becomes comparable to the capillary size, the terminal velocity reduces and for very large bubble volumes, the bubble velocity is independent of the bubble size. As the Weber number for the flow is increased, the bubble loses its fore and aft symmetry and develops a negative curvature at the rear of the bubble. At very low bulk surfactant concentrations, the equilibrium surface tension is the same as the clean surface tension. However, the bubbles rising through such a surfactant solution show reduced mobility due to increased Marangoni stresses. As the bulk surfactant concentration is increased, the mobility of the bubbles is increased due to surfactant remobilization effect. The effect of the channel geometry on these dynamics are explored and presented.