Sonoluminescence is the emission of short (pico-second) of light from imploding bubbles in a liquid when excited by sound. It was discovered in 1934 during sonar experiments at the University of Cologne. Light emissions are the result of the ultra-high temperatures caused as collapsing bubbles generate an imploding shock wave that compresses and heats the gas into a plasma at the centre of the bubble.
Measurements suggest extremely high temperatures - estimated to be from 2,300 K to 20,000 K.
The mechanism of sonoluminescence is unknown - though there are several as-yet-unconfirmed theories.
The two most prominent theories are currently the Shock Wave Model and the Hot-Spot model.
[…] the shock wave model that a spherical shock wave converges at the bubble center where extremely high temperature plasma is formed. The other is the hot-spot model that nearly the whole bubble is heated by quasi-adiabatic compression, where ‘quasi-‘ means appreciable thermal conduction takes place between the heated bubble interior and the surrounding liquid.
Source : Multibubble Sonoluminescence from a Theoretical Perspective Molecules 2021, 26(15), 4624
Technical details of other theories are described at Wikipedia
Recent (2022) research from the University of Ottawa shows the statistical analysis of the photon emissions suggests that they are due to some kind of quantum level effects - though the exact nature of the process remains unclear. See Observation of Nonclassical Photon Statistics in Single-Bubble Sonoluminescence arXiv.org, 2022
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