3.4.1 Coalescence
The change in bubble size distribution may be related to the change of bubble coalescence and fragmentation. Therefore, the investigation of bubble coalescence is of great significance for the stable rising of bubbles. Figure 14 shows typical coalescence process of two bubbles. In that case a bubble accelerated to impact a previous bubble, two bubbles will experience (1) bubble collision ; (2) formation and drainage of liquid film between bubbles ; (3) rupture of the liquid film until the three stages of coalescence.29 Due to the fast velocity of the large bubble below, it catches up with a neighboring small bubble above at 2 ms, while the two bubbles deform (gourd shape) at 4 ms. A large bubble catches up with a neighboring small bubble above at 2 ms due to the fast velocity of large bubble below, and the two bubbles deform to a gourd shape at 4 ms. The liquid between the two bubbles is squeezed out and formed a thin liquid film. The two bubbles break the liquid film limit at 7ms to complete the coalescing. The coalesced large bubbles are continuously deformed under the action of surface tension and inertial force.30