Sonoluminescence happens when small gas bubbles are frozen in liquid solutions at ultrasonic frequencies. They are held in place through acoustic waves. When the state (solid/liquid) that the bubbles reside in is broken, or the bubbles collapse, light is produced. The light is produced by thermal energy from the bubble collapse.

You can commonly see this effect in mentos when they are crushed: https://www.youtube.com/watch?v=tW8q_JfmcbU&t=126s However, this is called Triboluminescence.

Sonoluminescence is very interesting. Mainly because many properties about it are unknown. Even though we know that the light is produced by thermal energy, we still don’t know how the entire mechanism works.

There are some theories about Bremsstrahlung radiation, the argon rectification hypothesis, and hotspots that could possibly cause the thermal energy that creates the light. There are other theories that suggest temperature differences in a solution could also cause Sonoluminescence. This is still being studied however, and scientists are using spectral analysis to prove it.

Some other theories involve collision-induced radiation and corona discharges, nonclassical light, proton tunneling,and electrodynamic jets.

Sonoluminescence was first discovered at The University of Cologne in 1934 through sonar research. Two scientists, Hermann Frenzel and H. Schultes were experimenting with placing ultrasound transducers in tanks of photographic developer fluid. Seeking to speed up the process of photo development. However, they soon noticed bubbles appearing on the film, and saw the fluid was emitting light when the ultrasound was turned on. They didn’t have the equipment at the time to study the bubbles, but the bubbles in their experiment are called multi-bubble sonoluminescence (MBSL).

In the 1960s, more experiments on Sonoluminescence were conducted. Peter Jarman from the Imperial College of London proposed that these brief flashes of light were thermal in origin and could arise from micro shocks while the cavities from the bubbles are collapsing.

In 1989 experimental devices started to be used to track how this effect happens. Particularly for stable single-bubble sonoluminescence (SBSL). This effect happens when a single bubble is trapped in an acoustic wave. While it is trapped in this wave it emits a pulse of light every time it is compressed by the wave. Studying the acoustics the bubbles are trapped in allowed for a more comprehensive study of sonoluminescence. Especially for SBSL, as the scientists were only observing one single bubble.

It was also observed that under certain conditions the bubble in SBSL had an internal temperature that could be hot enough to melt steel. These observations were also noted in 2012 in similar experiments. In 2012, scientists were able to collapse a bubble and note that it contained a temperature of 12,000 Kelvins. However, most have temperatures that range from 2300-5100 Kelvins. The temperature of the bubble is very dependent on the gas that forms the bubble, and the composition of the state it is suspended in.
Scientists often use spectral measurements to calculate the temperature.

Sonoluminescence happens very fast. So it is not very stable. However, it can be made so in laboratory settings. When it is in a stable condition , the bubble can be made to collapse over and over again. It will also emit light everytime it collapses. On average, the light that flashes from the bubbles lasts between 35 and a few hundred picoseconds. The average size of the bubbles are around 1 micrometer in diameter depending on the state it is suspended in.

SBSL can have very stable periods and positions when it comes to emitting light. The frequency of the flashes can be more stable than the acoustic waves creating them. However, geometry wise the bubble has many instabilities. Because it is being crushed over and over again.

Depending on the gas that is creating the bubbles in Sonoluminescence, it can result in a brighter burst of light. Gasses such as helium, argon, and xenon can be responsible for this.

In 1937, researchers started looking into why these bubbles emit light.They could just collapse or naturally release their gas overtime, however they don’t. They first suggested that the light from these bubbles were caused by charge separation in the bubble’s cavity. For example, one wall of the bubble might have a negative charge, and the other a positive. When they collide, they emit light. However, there is some evidence to suggest that this is not the case.

If this charge separation was to happen, then a breakdown of the charges should happen during the expansion phase of the bubble. But instead the bubble collapses in an asymmetric way, and this breakdown stage does not occur.