On the origin of sonoluminescence and sonochemistry.

Recent experimental results on the origins of sonoluminescence and sonochemistry are reviewed and the conclusion reached that most observed effects originate from thermal processes associated with a localized hot-spot created by acoustic cavitation. Sonoluminescence is definitively due to chemiluminescence from species produced thermally during cavitational collapse and is not attributable to electric microdischarge. Homogenous sonochemistry follows the behaviour expected for high temperature thermal reactions. Ultrasonic irradiation of liquids containing solid powders dramatically increases their chemical reactivity and improves chemical yields for a wide range of synthetically useful heterogenous reactions. Shock waves generated from the cavitational hot-spot cause high velocity interparticle collisions in such slurries. Brittle solids are shock fragmented, which increases surface area. This increase in reactive surface provides for substantial increases in chemical reactivity. For malleable metal powders, these collisions are sufficiently violent to remove surface oxide coatings and to induce local melting at the site of impact for most metals.