Shock–induced collapse and luminescence by cavities

Recent observations of the collapse of single bubbles or clouds of cavities trapped within stationary acoustic fields have shown the production of light and these phenomena have been dubbed single– or multiple–bubble sonoluminescence. The experimental observation of the variations in the bubble radius with time, with simultaneous measurement of the light emission from the bubble, shows short pulses of light correlated closely with the point of minimum radius of the bubble. Results will be presented showing experimental observations of light produced from shock–collapsed cavities. The role of shock waves generated within the bubble by the motion of the walls will be discussed with application to the shock–wave hypothesis proposed as one of the mechanisms responsible for sonoluminescence. The likely asymmetry of the process, as recently proposed, and the consequent presence of a high–velocity jet will also be discussed. The effect of such ‘hot’ cavities within an explosive matrix is investigated and results are presented. The collapse of a single bubble shows that light emission from the explosive matrix is due to reaction caused by hydrodynamic heating by jet impact, not due to conduction from heated gas during compression. Similar results are seen in arrays of collapsing cavities. The effects of close, inert particles are discussed and an example is given.