4.7 Article

Coalescence of surface bubbles: The crucial role of motion-induced dynamic adsorption layer

Journal

ADVANCES IN COLLOID AND INTERFACE SCIENCE
Volume 317, Issue -, Pages -

Publisher

ELSEVIER
DOI: 10.1016/j.cis.2023.102916

Keywords

Bubble; Coalescence; Dynamic adsorption layer; Foam film; Interferometry; Drainage; Direct numerical simulations

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This study provides the first quantitative proof of the influence of dynamic adsorption layers on the drainage dynamics of single foam films formed under dynamic conditions. By measuring the drainage dynamics of single foam films formed by air bubbles colliding against the interface between n-octanol solutions and air, it was found that the morphology of the single film during drainage is closely related to the state of the dynamic adsorption layer during the rising and bouncing steps of the bubble. It is demonstrated that the lifetime of surface bubbles is intimately related to the history of their formation.
The formation of motion-induced dynamic adsorption layers of surfactants at the surface of rising bubbles is a widely accepted phenomenon. Although their existence and formation kinetics have been theoretically postu-lated and confirmed in many experimental reports, the investigations primarily remain qualitative in nature. In this paper we present results that, to the best of our knowledge, provide a first quantitative proof of the influence of the dynamic adsorption layer on drainage dynamics of a single foam film formed under dynamic conditions. This is achieved by measuring the drainage dynamics of single foam films, formed by air bubbles of millimetric size colliding against the interface between n-octanol solutions and air. This was repeated for a total of five different surfactant concentrations and two different liquid column heights. All three steps preceding foam film rupture, namely the rising, bouncing and drainage steps, were sequentially examined. In particular, the morphology of the single film formed during the drainage step was analyzed considering the rising and bouncing history of the bubble. It was found that, depending on the motion-induced state of adsorption layer at the bubble surface during the rising and the bouncing steps, single foam film drainage dynamics can be spectacularly different. Using Direct Numerical Simulations (DNS), it was revealed that surfactant redistribution can occur at the bubble surface as a result of the bouncing dynamics (approach-bounce cycles), strongly affecting the interfacial mobility, and leading to slower rates of foam film drainage. Since the bouncing amplitude directly depends on the rising velocity, which correlates in turn with the adsorption layer of surfactants at the bubble surface during the rising step, it is demonstrated that the lifetime of surface bubbles should intimately be related to the history of their formation.

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