Ultrasonics is well known for its many functions in information extraction, such as non-destructive evaluation and medical diagnosis. Less known, but also wide spread, are its functions in material processing, such as ultrasonic cleaning, emulsification, and more recently, nano-technologies and high-intensity focused ultrasound.
Ultrasound exhibited processing applications in chemistry early back in the 1920s; now, such applications have grown to such a large extent that they have gained the official title of Sonochemistry. In chemistry, ultrasonics can help enhance yields, speed up processes, switch pathways and stimulate reactions which would not occur without the presence of the ultrasonic wave. The origin of these applications is nevertheless a phenomenon of physics, the cavitation. The abundant instances of successful applications, however, are mainly restricted to research laboratories. Sonochemistry has been handicapped since its institution by the lack of satisfactory equipment for industrial batch or flow production. Simple magnification of the laboratory equipment using ultrasonic transducers seems to be inadequate for large scale production. Recent years have seen some progress in the scale-up effort and yet optimization remains wanting.
So YING Chongfu, BAI Lixin, LI Chao and DENG Jingjun of Institute of Acoustics, Chinese Academy of Sciences carried out a series of studies and made trails to improve the situation, based on an approach proposed by Moholkar and his co-workers with some modification of their own.
They propose a model of the field which hypothesizes that most violent cavitation bubbles originate from the vibrating surface and there-from fast drift to the near liquid region. These bubbles are chiefly responsible for the practical applications of cavitation for a large power input to the transducer. During migration they become weakened. Cavitation bubbles are also produced in the liquid region by the acoustic wave directly emitted by the transducer, but these bubbles are weak due to the shielding effect of the bubbles clinging to the transducer vibrating surface. Consequently, only weak cavitation bubbles exist in the far liquid region.
This research result was published on the recently issued journal of SCIENCE CHINA-PHYSICS MECHANICS & ASTRONOMY (2011, 54 (1): 74-78).