Free radicals are generated by the collapse of ultrasound-induced cavitation bubbles when they are forcefully compressed by dynamic stimuli. Radical generation occurs as a result of the extremely high temperatures induced by adiabatic compression during the violent collapse process. It is generally believed that extreme conditions are required for this type of radical generation. However, we have demonstrated free-radical generation from the collapse of microbubbles (diameter = <50 microm) in the absence of a harsh dynamic stimulus. In contrast to ultrasound-induced cavitation bubbles, which collapse violently after microseconds, the microbubbles collapsed softly under water after several minutes. Electron spin-resonance spectroscopy confirmed free-radical generation by the collapsing microbubbles. The increase of the surface charges (zeta potentials) of the microbubbles, which were measured during their collapse, supported the hypothesis that the significant increase in ion concentration around the shrinking gas-water interface provided the mechanism for radical generation. This technique of radical generation from collapsing microbubbles could be employed in numerous engineering applications, including wastewater treatment.

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Hydroxyl radicals are strong oxidants in aqueous solution, reacting rapidly with a wide range of dissolved compounds. In contrast, ozone is a highly selective oxidant. Understanding the process by which ozone is transformed into hydroxyl radicals is important in the treatment of wastewater and drinking water. We use electron spin-resonance spectroscopy to demonstrate that when microbubbles of ozone in strongly acidic aqueous solution collapse, the ozone progressively decomposes and large quantities of hydroxyl radicals are generated. Moreover, the degradation of polyvinyl alcohol, which is ozone resistant, was also observed during the collapse of the microbubbles. These findings indicate that ozone microbubbles are potentially useful in future water-treatment applications.

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