Theory of Sonoluminescence

The subject of this work is a theoretical explanation of Sonoluminescence: optical radiation from strongly modulated bubbles in liquids. The observed radiation is characterized by a high energy concentration and a spectrum which is black-body like and of very short duration, on the picosecond time scale. Different theoretical approaches to the problem have been explored. It is found that for most of the time the adiabatic approach (i.e., the assumption of spatial uniformity of the thermodynamic variables) is a very good approximation. But, shortly before the bubble reaches its minimum radius, the motion of the bubble wall becomes supersonic and shock wave(s) are produced. In this region a full solution of the gas-dynamic equations inside a modulated gaseous bubble coupled with the bubble wall dynamics is required in order to correctly understand the physical properties of the gas in the bubble. The propagation of the shock waves is followed and its consequences are compared with those of a self -similar behavior. The energy loss and change of equation of state of the gas resulting from the high temperatures and pressures in the collapsing bubble are incorporated. The effects of dissociation, ionization and radiation energy loss are included self-consistently in the treatment of the gas-dynamics equations. The radiation energy loss results in a sonoluminescent pulse whose properties are fully explored. We find that the inclusion of loss terms, especially due to radiation, is necessary to correctly describe the bubble dynamics and the resulting sonoluminescent pulse. A theoretical explanation of the pulse length and its spectral properties are presented and compared with experimental results. It is found that, for a variety of experimental conditions, the results of our theory are in very good agreement with experiment, although there remain aspects of the experiments that are not addressed by our theory.