Large and small bubble interaction patterns in a bubble column

A visual analysis is made on the fate of a large (or ''cap'') bubble injected into a swarm of otherwise uniformly dispersed small bubbles; experiments, covering gas holdups for the swarm bubbles as high as 6%, are conducted in a two-dimensional column to ensure the distinct appearance of a single cap throughout its rise in the swarm. Specific focus is on the stability or breakup of the large bubble induced by the overtaken small bubbles. The diversity of cap-bubble interactions is observed and classified based on the observed shape distortion and/or breakup patterns into seven modes. The probability of occurrence of each mode is estimated, the results of which indicate that there exists a critical gas holdup (e.g. similar to 0.01 for tap water) beyond which the degree of cap-bubble interactions sharply increases. This general trend is found for the other liquids tested (i.e. sodium sulfite and glycerin solutions) with lower degrees of interactions and the transition being less distinct. The effects of the swarm gas holdup on the specific interaction properties are also studied: with increasing holdup, the axial location of the breakup event shows a noticeable tendency of shifting from lower to higher positions; the size ratio of two daughters after the split tends to approach unity; the dominating breakup mechanism shifts from the inherent instability of the cap itself to the bubble attachment at the cap base, which triggers the ultimate necking and ''pinching-off'', to the acceleration of the cap toward (usually two) preceding bubbles under the influence of their wakes.