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.

气液界面处的边界条件强烈影响着碰撞气泡之间的液膜排水。从理论上讲，在清洁的水系统中，界面不应承受任何切向应力（完全可移动），从而导致薄膜在极短时间内迅速排出并在气泡之间聚结。实际上，在大多数实验和工业条件下，杂质或表面活性剂的存在可以使界面固定化，并显着阻碍气泡聚结几个数量级。根据这一观点，我们介绍了了解空气-水界面边界条件的最新进展，以及它们如何影响气泡碰撞的结果。讨论了在存在污染物的情况下从移动边界条件到固定边界条件的转变。