Microbubble smallness limited by conjunctions

•Reason discovered why bubbles are not as small as expected.•They grow by conjunction at aerator exit.•Revealed by very high-speed camera and long-distance macro-lens.•Different from larger bubbles: high surface energy and slow ascent.

Microbubbles, small gas bubbles of diameter less than 1 mm, are desirable in many important chemical and process engineering applications. Their behaviour differs from that of large bubbles due to very high surface energy and slow motion velocity. Unfortunately, producing them efficiently and economically has been until recently a problem. Paper discusses one of the reasons, apparently not yet recognised in the available literature. Because of the slow ascent motion associated with the small size, the microbubble, after leaving the aerator exit, inevitably gets into close proximity with the microbubble that was formed at the same exit earlier. Quite often, they merge to form a single substantially larger bubble. The conjunctions are so fast and take place at a scale so small that they have apparently escaped attention. Author discovered this limiting effect to bubble smallness by using a long-distance macro-lens and a high-speed camera.