What Is Sonoluminescence?

Fill a round glass flask with water and play a loud tone into it that is too high for most people to hear. Add one small air bubble and turn off the light. In the middle of the flask, a tiny point of light appears and keeps glowing. The light comes from a bubble much thinner than a hair. How can sound turn into light?

What is sonoluminescence?

Sonoluminescence is light produced by sound. The name combines the Latin sonus, “sound”, with “luminescence”, the giving off of light. A strong sound wave in a liquid makes a gas bubble grow and then collapse. During the collapse, the gas inside gets so hot that it gives off a short flash of light.

There are two kinds. In multi-bubble sonoluminescence, very strong sound creates clouds of many bubbles that form and collapse at random. Together they make a faint, flickering glow. In single-bubble sonoluminescence, one bubble is held in place by the sound wave and flashes once in every cycle of the sound. The single bubble is much easier to study, so most of what we know comes from it.

What happens to one bubble in one cycle of sound?

Let us follow the single bubble from the opening scene. Small loudspeakers glued to the flask make the water vibrate about 26,500 times per second. So one cycle of the sound lasts about 38 millionths of a second. In each cycle, the pressure in the water rises and falls by about 1.3 times normal air pressure. Here is what happens during one cycle.

1. The bubble rests. At the start, the bubble has a radius of about 5 micrometres. A micrometre is a thousandth of a millimetre.
2. The pressure drops. In the low-pressure half of the cycle, the water pulls on the bubble. The bubble grows to about ten times its starting radius, around 50 micrometres. The gas inside spreads out and becomes very thin.
3. The pressure rises. Now the water pushes inward. The thin gas cannot push back, so the bubble wall rushes inward faster and faster.
4. The gas is crushed. Within a tiny fraction of the cycle, the bubble shrinks to less than one micrometre. The gas inside is squeezed into a tiny space.
5. The gas heats up and flashes. Squeezing a gas heats it. Because the collapse is so fast, the heat has no time to escape into the water. The gas reaches several thousand degrees, probably more than 10,000, and gives off a flash of light. The flash lasts between about 50 and 300 picoseconds. A picosecond is a millionth of a millionth of a second.
6. The bubble recovers. The bubble bounces back a few times, settles at its resting size and waits for the next cycle. Then everything happens again, about 26,500 times per second, with the regularity of a very good clock.

You may know the heating effect from a bicycle pump. When you pump a tyre quickly, the pump gets warm, because squeezing air heats it. The comparison stops working at one point. The pump squeezes the air slowly and only a little, so most of the heat leaks away. The bubble squeezes its gas far more and far faster, so the heat stays concentrated.

The result is an enormous concentration of energy. The sound spreads its energy through the whole flask. The collapse gathers part of it into a space about the size of a bacterium, for a moment shorter than a billionth of a second. Physicists estimate that the energy density rises by about twelve orders of magnitude. That means a factor of about a million million.

Does this happen outside the laboratory?

It does, and one example comes from an animal. Snapping shrimp, also called pistol shrimp, are small shrimp that live in warm, shallow seas. One of their claws snaps shut so fast that it shoots out a jet of water. The pressure in the jet drops so low that a bubble forms. When the bubble collapses, it makes a loud crack that stuns small prey.

In 2001, the physicists Detlef Lohse and Michel Versluis and the biologist Barbara Schmitz showed that this collapse also makes a flash of light. The flash is too faint for human eyes, so they measured it with a very sensitive light detector. Their results showed that the gas in the shrimp’s bubble reaches at least 5,000 degrees. The researchers jokingly called the effect “shrimpoluminescence”.

The same kind of bubble collapse happens wherever pressure in a liquid drops very low and then rises again. Engineers call it cavitation. It happens near fast ship propellers and in pumps, where collapsing bubbles slowly eat holes into the metal.

What sonoluminescence teaches you

The main lesson is that a large effect does not always need a large cause. A modest amount of energy can do surprising things when it is focused into a tiny space and a tiny moment. When something has a stronger effect than you expected, ask whether the energy or effort behind it was focused rather than large.

The idea appears in several places.

• Cleaning and chemistry. Ultrasonic cleaning baths for glasses and jewellery work through collapsing bubbles. Chemists use the same bubbles as tiny, very hot reaction chambers to drive chemical reactions.
• Damage in machines. Ship propellers and pumps are damaged because cavitation focuses energy on small spots of metal, again and again.
• Solving messy problems. Multi-bubble sonoluminescence was known for more than fifty years, but it was too chaotic to study well. Real progress came only when researchers learned to trap one stable bubble. The same approach helps with your own messy problems. Maybe a computer program crashes only sometimes, or a headache comes and goes. Try to create one clean, repeatable case before you look for the cause.

A second lesson is about knowledge itself. Scientists can produce single-bubble sonoluminescence on demand and predict when a bubble will glow. They still cannot fully describe what happens at its centre. Being able to make something happen is not the same as understanding it completely.

What do physicists still not know?

The basic picture is accepted. In a review from 2002, Michael Brenner, Sascha Hilgenfeldt and Detlef Lohse concluded that the evidence favours heating by compression. The hot gas becomes partly ionized, which means that some atoms lose electrons. The free electrons and charged atoms then give off the light. Several details remain open.

The first open question is the temperature at the centre. The light from a single bubble in water has a smooth spectrum without clear lines, so every temperature estimate depends on a model. In 2005, David Flannigan and Kenneth Suslick used sulfuric acid instead of water and argon gas in the bubble. Their bubbles were far brighter and showed clear lines, and they found temperatures of about 15,000 degrees or more. They also found signs of a plasma, a hot gas of charged particles. How hot and dense the core of a bubble becomes is still not known.

The second question concerns a common misunderstanding. Headlines like to say that the bubble is “hotter than the Sun”. The surface of the Sun is about 5,500 degrees Celsius, so the claim can be true for the gas inside. But the hot region is tiny and lasts for a moment. The flash carries only a tiny part of the energy the sound puts in, and the water around the bubble hardly warms up.

The third question is nuclear fusion. In 2002, a research team reported signs of fusion in collapsing bubbles in the journal Science. Other researchers did not confirm the result. In 2008, a panel at Purdue University found the lead researcher guilty of misconduct in his follow-up work. Today, fusion in bubbles is not an accepted result.

Learn more

Sonoluminescence has no single discoverer. In 1934, two German researchers, H. Frenzel and H. Schultes, put a photographic plate into a water bath filled with strong ultrasound. The plate darkened, and they explained this with light coming from the sound field. For more than fifty years after that, only the flickering glow of many bubbles was known. In 1990, Felipe Gaitan, a doctoral student of the physicist Lawrence Crum at the University of Mississippi, reported how to trap one stable, glowing bubble. Theorists such as Detlef Lohse and chemists such as Kenneth Suslick then worked out much of what happens inside it.

If you enjoy puzzles in which a small point of light reveals something about physics, read Why Is the Night Sky Dark? Olbers’ Paradox, Explained. It shows how the darkness between the stars tells us that the universe has an age. For the question of why the laws of physics allow such effects at all, continue with Why Does the Universe Seem Fine-Tuned for Life?