Ultrasound induced cavitation and sonochemical effects

Abstract

The introduction of a strong acoustic field to an aqueous solution results in the generation of cavitation microbubbles. The non-linear motion of these microbubbles focuses energy from the macro-scale acoustic waves to the micro-scale vapor inside the bubbles. As a result, extremely high localized pressures on the order of hundreds of atmospheres and temperatures on the order of thousands of degrees Kelvin are generated. Under such extreme conditions molecular dissociation occurs and produces highly reactive free radicals. This phenomenon provides a means of "burning" substances in liquids and enhancing reactions that cannot be achieved by conventional means. Sonochemistry, the chemistry associated with this phenomenon, has found application in drug delivery, waste decomposition, water treatment, chemical reaction enhancement and numerous novel material processes. A theoretical framework that directly couples the dynamics of bubble motion and the associated kinetics of gas phase reactions is established for the first time in an attempt to understand the fundamental mechanisms of the sonochemical phenomenon. Several fundamental mechanisms, which are believed to be critical in understanding the unusual experimentally observed sonoluminescence and sonochemical behavior, are revealed. First, not all chemical reactions associated with bubble oscillation in a sound field have reached thermodynamic equilibrium. Second, chemical kinetics couples closely with the bubble motion and has significant impact on the dynamics of bubble motion when a bubble contains a combustible gas mixture. Third, the dissolved gases affect the activities of a sonochemical event through both thermal effect by changing the peak collapse temperatures in the bubble and chemical effect by directly participating in reactions. In addition, a laboratory scale sonochemical experiment is conducted to demonstrate the sonochemical effects as a result of ultrasonic irradiation in a Fricke solution. Effects of the dissolved gases on sonochemical activities are experimentally quantified and compared with the predicted results using the model developed in this thesis.