Cavitation bubble clouds can develop various structures in the ultrasound field, which has been studying for decades. Robert Mettin’s classification based on visual aspects of acoustic cavitation is very useful but not widely used. Among them, the Smoker is a special type of cavitation structure in Mettin’s classification. Many professors have researched Smoker from several aspects. However, Smoker is still not thoroughly investigated.  

So BAI Lixin, YING Chongfu, LI Chao and DENG Jingjun from the Institute of Acoustics, Chinese Academy of Sciences conducted an experimental investigation on the structures and evolution of Smoker in a 20 kHz ultrasonic field by means of high-speed photography.

The experimental design consisted of the ultrasonic cavitation devices, the imaging and illumination system, and a movable transparent reflector plate. Through this experiment, it was found that Smoker only appeared on a surface (a transducer surface or a reflector plate surface) in the liquid with strong acoustic fields. If the surface was located in a cavitation band, Smokers could be produced more easily. In the experiment, the “spine-plume” structure of Smoker on a transducer surface was discovered. The phenomenon was that some large cavitation bubbles aligned themselves with the central line and formed the spine of Smoker which was surrounded by countless small cavitation bubbles. As a result, plume structures were formed. Meanwhile, driven by the secondary Bjerknes forces, small bubbles could create a dendritic structure. The evolution of cavitation structure from Flare (a kind of cavitation structure ejected from radiating surface into the liquid) to Smoker was uncovered. When a Flare was close to a Smoker, the Flare might merge into the plume structures of the Smoker, or develop a double-tipped Smoker. A double-tipped Smoker hardly split into two Smokers, while two separate Smokers tended to merge as one. The large bubbles in middle of the two separate Smokers drew them, making Smokers merge together.

This research result will be published on the issued Ultrasonics Sonochemistry (Volume 19, Issue 4, July 2012, Pages 762–766).