Conclusions
In summary, the bubble formation process and bubbling regimes for a submerged capillary nozzle was investigated. When the capillary nozzle is slightly inclined downward-pointing, continuous microbubbles can be generated via the continuous partial coalescence of larger bubbles. This process not only provides a new microbubble generation method but also has influence on the bubble size distributions in the bubble column. High-speed visualization shows that the generation of microbubbles is controlled by two processes: (I) The mother bubble is formed in a very short time and coalesces with the departed father bubble near the nozzle orifice. (II) The capillary waves form and converge at the apex of the mother bubble, leading to the pinch-off of a satellite microbubble. In previous studies with an upward-pointing capillary nozzle, the continuous coalescence of bubbles was available, but generating microbubbles was still very difficult even under very wide experimental conditions. Through comparing the formation and departure process of bubbles from the nozzle with different incline angle, we find that the angle between the directions of gas injection and drainage flow inside the coalescing bubbles can be changed from around 0o to 45-135o to realize continuous microbubble formation when the capillary nozzle is changed from upward-pointing direction to the slight inclined downward-pointing direction. A further analysis indicates that the generation of microbubbles mainly depends on the rising velocity of father bubble, growth rate of mother bubble and sizes of the parent bubbles. These parameters are affected by the gas flow rate and nozzle diameter directly in present air-water experiments. Therefore, the Bond and Weber numbers are selected to show their influences, and a scaling law for the microbubble size and a phase diagram identifying bubble coalescence with the formation of microbubbles are given accordingly. Our experiments also suggest that lower Weber and Bond numbers help microbubble generation.