Abstract
Inside collapsing bubbles in liquid water irradiated by ultrasound, water vapor is dissociated because the bubble temperature increases dramatically. As a result, OH radicals are created inside bubbles and dispersed in the liquid. As OH radicals are strong oxidant, solutes are easily oxidized. On the other hand, volatile solutes enter bubbles and many kinds of chemical reactions take place inside collapsing bubbles. For the industrial application of sonochemistry, a detailed understanding of cavitation bubbles is required. In the present study, computer simulations of bubble oscillations are performed in order to study a life of cavitation bubbles. According to the present computer simulations, bubbles are classified into five categories by their ambient radii. They are dissolving bubbles, stable sonoluminescing bubbles, unstable sonoluminescing bubbles, unstable bubbles, and "degas" bubbles. The range of the ambient radius for each category depends on the frequency and amplitude of ultrasound. A bubble emits light immediately after its appearance when the ambient radius is in a certain range, while the previous theory predicts that a bubble emits light only when it grows to the resonance size by rectified diffusion. Now we discuss the previous classification of bubbles; transient cavitation bubbles and stable cavitation bubbles. As is understood from the terminology, a "transient cavitation bubble" exsists only a few acoustic cycles or less. It expands to many times of its ambient size and collapses violently, often disintegrating into a mass of smaller bubbles. They are filled mainly with vapor. A "stable cavitation bubble" exists for a long period of time. It repeats expansion and collapse many times. They contain mainly a permanent gas. According to the present study, the previous classification of bubbles covers only a fraction of bubbles. "Transient cavitation bubbles" correspond to the unstable SL bubbles at high acoustic amplitude at low ultrasonic frequency in the present classification. "Stable cavitation bubbles" correspond to the stable SL bubbles.
