Understanding Sonoluminescence

Sonoluminescence is the transformation of sound into light. In practice this phenomenon usually occurs as a result of the expansion and collapse of gas filled bubbles in a fluid driven with a powerful sound field. The most commonly accepted causal model is that the emission of light occurs after the heating of the contents of the bubble up to temperatures exceeding 10000K at the end of the collapse phase. We present data showing that, in the case of a sonoluminescing bubble driven at 23.4 kHz, the emission of light occurs during the bubble collapse, but typically 50 to 100 ns before the minimum bubble radius -- a time period when the conditions in the bubble are expected to be moderate and near room temperature and when the velocity of the collapsing bubble wall is below the speed of sound. These results imply that the cause of light emission during sonoluminescence may not be a violent heating like when a hammer strikes an anvil. Rather, we consider that the path of irreversible phase transformations occuring through the course of expansion and collapse might mediate a transfer of latent heat into an excited condensate. As the condensate discharges, high temperatures arise and a continuous bremsstrahlung spectrum results. We also discuss related phenomena.