Vortex interactions between the bubble pairs rising side by side in viscous liquids*

The present study focuses on the importance of the vortex structures during the interactions between the bubble pairs rising side by side. Specifically, we identify that if there are standing eddies behind the bubble rear, the bubble pairs will repel each other after being released, and whether they will show a subsequent repeated-bounce or not depends on the rising behavior of the isolated bubble and the fluid properties. Moreover, the repeated-bounce behavior is actually a manifestation of the zigzag motion of isolated bubble. By contraries, the bubbles tend to attract each other after being released if there are no standing eddies behind them.However, whether they will coalesce or bounce after "kissing" is dependent on the development of the vortex structures. A thorough investigation explains why different bubble interactions are observed in water by changing the bubble size or their separated distance in the experiments from the aspect of the wake structure. Moreover, different vertical magnetic fields are further applied onto the bubble pairs to verify that it is actually the vortex interactions rather than the collision velocity that produces different bubble interactions