The boundary condition at the air–liquid interface and its effect on film drainage between colliding bubbles

• Published in: Current opinion in colloid & interface science Vol. 50; no. C; p. 101374
• Main Authors: Liu, Bo, Manica, Rogerio, Xu, Zhenghe, Liu, Qingxia
• Format: Journal Article
• Language: English
• Published: United Kingdom Elsevier Ltd 01.12.2020; Elsevier
• Subjects: Air–water interface mobility; Bubble coalescence; Mobile and immobile boundary conditions; Salt; Surfactant; Thin film drainage; Salt; Bubble coalescence; Surfactant; Air–water interface mobility; Thin film drainage; Mobile and immobile boundary conditions
• ISSN: 1359-0294; 1879-0399
• DOI: 10.1016/j.cocis.2020.07.003

The drainage of thin liquid films between colliding bubbles is strongly influenced by the boundary conditions at the air–liquid interface. Theoretically, the interface should not resist any tangential stress (fully mobile) in a clean water system, resulting in very fast film drainage and coalescence between bubbles within milliseconds. In reality, under most experimental and industrial conditions, the presence of impurities or surfactants can immobilize the interface and significantly hinder bubble coalescence by several orders of magnitude. In this opinion, we introduce the recent progress on understanding the boundary conditions at the air–water interface, and how they may affect the outcome of bubble collisions. The transition from mobile to immobile boundary conditions in the presence of contaminations is discussed. Despite the considerable recent progress, there are still experimental and theoretical challenges remaining on this topic, for example, finding the mechanism for hindered bubble coalescence by high salt concentrations.