The effect of interface mobility on coalescence dynamics*

When a bubble rises through a liquid towards a deformable free interface, it can bounce or coalesce.   The mobility of the two interfaces has a dramatic effect on the coalescence dynamics.  By using a system with air, water and perfluorocarbon liquid (PP11) we can vary the mobility of the various interfaces from immobile to fully mobile. The particular mobility is verified by measuring the terminal velocity of small bubbles rising at low Reynolds numbers. We use high-speed video imaging to track the bubble approach and coalescence. The film drainage time is found to be two orders of magnitude shorter for fully mobile interfaces. We use a theoretical model to verify the importance of hydrodynamics and surface mobility on the measured film drainage times*.
*Vakarelski et al., Langmuir, 34, 2096-2108 (2018).