Coalescence of air bubbles at air-water interface

The rest time of air bubbles at flat air water and water-organic interfaces is studied in the present work. Effects of cationic and anionic surfactants, alcohol, salts and bubble-size on rest time are investigated. Wide distributions in rest times are observed in all the systems, which establishes the stochastic nature of the process. The stochastic model of Ghosh and Juvekar (2002; Chem Eng Res Des 80: 715-728) is used to fit the bubble rest time distributions. The results show that the magnitude of the rest time is determined by the strength of the interfacial repulsive force and the magnitude of surface diffusivity of the surfactant molecules. Entanglement of surfactants by hydrophobic interaction is believed to be a major factor behind the high rest time in many of the systems studied, apart from the repulsive electrostatic double layer, hydration and steric forces. The nature of the repulsion differs from system to system depending on the type of the adsorbed species. The work provides further support to the viewpoint (Ghosh and Juvekar, 2002) that the hydrodynamic drainage of the thin liquid film trapped between the bubble and the flat interface is complete once the bubble comes to a rest on the interface and the lubrication force plays a negligible role in supporting the weight of the bubble.