Continuous formation of microbubbles during partial coalescence of bubbles from a submerged capillary nozzle

Shape Oscillation-Induced Early Detachment of Bubble from a Submerged Microcapillary Nozzle

Bubble growth and detachment are basic phenomena in boiling and water splitting processes and are essential for their heat transfer performance and hydrogen production efficiency. In this study, we investigate the formation and detachment of bubbles from a submerged capillary nozzle. Here, we report a new bubbling regime and term it unstable bubbling, where early and normal bubble detachments coexist. The early detachment leads to the generation of many finer bubbles, ranging from one-fifth to one-half of the volume of the permanently detached bubbles driven by buoyancy. The occurrence conditions for unstable bubbling are identified by developing a regime map. The visualization of bubble-induced liquid flow and energy analysis suggest that the oscillating detached bubble provides excess surface and kinetic energies to the growing bubble, supporting early bubble detachment. Two dimensionless numbers are derived considering the effect of these energies, which can be applied to successfully predict the occurrence of early detachment. The present results not only provide physical insight into bubble interactions but also offer potential techniques for generating fine bubbles.