Unexpected behaviour of microbubbles immediately at the aerator exit

Abstract

Microbubbles (of diameter less than 1 mm) are desirable for many engineering applications. Until quite recently, generating them efficiently was a problem. A solution was found in excitation of gas supply by inexpensive and reliable nomoving-part fluidic oscillators. Strangely, generated microbubbles are larger than expected. The author studied their formation using a high-speed camera with a long distance macro-lens. The images revealed the cause being a conjunction of bubbles, mostly taking place within the very proximity of the aerator exit. Small bubbles move slowly and do not vacate fast enough from the position in which they were generated. They thus get into contact with the next generated bubble. Another cause of slow departure is the increased drag caused by shape oscillation driven by energy released in the conjunction. The conjunctions may also occur due to lateral bubble motion if the Reynolds number exceeds the vortex shedding threshold.