Tiny bubbles in water have large specific area and long residence time in a tank. Because of these characteristics, they are used to enhance the mass transfer between gas and liquid phase in chemical reactors, aeration tanks for water purification systems etc. Also, there are a lot of expectations in the medical filed. Microbubbles, whose diameter is less than 5 have been already used as contrast agents of ultrasound diagnosis through the injection into veins. These microbubbles are expected to be further utilized as drug delivery agents through the development of special device to control the microbubble functions. In this paper, the recent advances on these microbubble studies are explained and the microbubble generator using microchannel, which has good controllability on bubble size and numbers, are briefly introduced.

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Microbubbles have several useful functions. By utilizing the characteristics of microbubbles, the novel technologies are developed for various applications. There are some principles to produce microbubbles and several kinds of microbubble generators have been proposed. Therefore, the performance of each generator is different each other. By using the microbubble generator instead of standard bubble distributors set a gas absorber, the gas absorption rate was improved well although it depended somewhat on the microbubble generator. To recover the fine iron oxide particles from waste water, the microbubble flotation was developed. By utilizing the rapid shrinkage and dissolution of a single microbubble, the single fine particle was crystallized even in dilute solution. On the other hand, the microbubbles were agglomerated to be swarms by irradiation of ultrasound, and then those escaped rapidly from the liquid phase.
The several new microbubble technologies above were introduced in the report.

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