Effect of surfactant transport on the mobility of bubbles in liquids: An experimental and computational study

When a bubble rises in an infinite liquid containing trace amount of surfactant, the surfactant adsorbs on to the front end of the bubble and is swept by surface convection to the trailing end. As the surface concentration of surfactant at the trailing pole is greater than the concentration at the leading pole, the leading pole is of higher tension, and tugs the trailing pole, creating a Marangoni force opposing the driving force moving the bubble. Hence the retardation in the bubble terminal velocity.; We consider both theoretically and experimentally the effect of the transport rates of an added surfactant from (to) the bulk phase to (from) the bubble surface on the steady state drag of a spherical bubble rising in an infinite liquid containing the surfactant. Theoretically we calculate the drag on the bubble by solving the complete flow equations coupled with the equations of surfactant transport. Experimentally we study the effect of adding a poly-ethoxy surfactant (C_iE_j: CH₃ (CH₂) _i−1(-OCH₂CH₂)_j-OH)), C₁₂E₆ or C₁₀E₈ on the drag of an air bubble rising in a 70:30 Glycerol-Water mixture.; In the limit of the surfactant transport rates being very low compared to the hydrodynamic rates, we develop an exact theory and conduct systematic experimental studies that lets us implement a new technique to determine the kinetic exchange rate constants of the surfactant at the air-liquid interface. In the limit when the surfactant transport rates are very high compared to the hydrodynamic rates, previous research in our group hinted at the possibility of the surfactant adsorbed on the bubble surface being able to desorb and making the interface stress free again like in a surfactant free system. We verify this idea of remobilizing the surfactant laden bubble surface from our numerical simulations and in the process deduce the criteria for its possibility under realistic conditions. We corroborate these conclusions from bubble rise experiments as before where we see the drag on a bubble of any given size showing a maximum with the addition of surfactant to the bulk phase. For the first time this provides a concrete proof of this phenomenon of remobilization.