How It Works: Bubble Power

When we read that physicists were dropping speakers into water to try and kick start cold fusion reactions, which could supply endless clean energy, we thought that they must have gone barmy. That's because we'd never heard of sonoluminescence, the mysterious phenomenon where sound makes bubbles all-powerful. B. James McCullum and Stuart Smith get to grips with some weird science.

We all know the legend of the opera diva hitting a high note and shattering glass, but could she also make the glass burst into flames or even cause two hydrogen nuclei in the contents of the glass to combine, providing enough energy to power a small country? It might not be as farfetched as it sounds.

Back in the 1930's, a pair of scientists found out that sending ultrasonic waves into a liquid could expose photographic plates – sound energy was being turned into light. The phenomenon known as sonoluminescence was born.

Although no one is quite sure how sonoluminescence actually works, everyone agrees that when an ultrasonic generator (read really powerful speaker) is put in liquid, it can create tiny glowing bubbles.

Acoustic cavitation in diagram form

Everybody also agrees that sound waves advance as patterns of positive and negative pressure. So the traditional wisdom about sonoluminescence has it that, when the negative pressure portion of a sound wave passes through a bubble in a liquid, it causes that bubble to swell in size by several orders of magnitude. When the subsequent positive pressure portion of the sound wave approaches, that large bubble is forced to shrink rapidly by a process called acoustic cavitation. In this theory, the collapse results in all the energy that went in to the expansion of the bubble being compressed in to a very small area. In other words, a proportionately huge amount of energy gets focused into a very tiny package. As a result the small amount of gas trapped inside the bubble is heated to absurdly hot temperatures, causing it to glow.

$A diagram of how fractoluminescence might work$

Fractoluminescence in diagram form

Not all scientists buy this explanation though – some physicists who study bubbles (and yes there are such things) believe that a bubble has to remain perfectly mathematically spherical in order for acoustic cavitation to occur, and they argue that nature just doesn't work that way. One of the more prominent competing theories states that super-fast jets of liquid shoot across the bubble and actually fracture the water molecules on the other side (yes, you can break a molecule like this). The break causes a release of energy and hence light. You can witness a similar effect from breaking or even simply rubbing any number of crystals – in this case it is called triboluminescence (rubbing) or fractoluminescence (breaking). It results from charges within molecule being forced apart, and then combining back together, which ionizes the air and causes a flash.

Some researchers have even speculated that sonoluminescence may be a manifestation of the proposed infinite energy of a vacuum, commonly referred to in modern science fiction as zero point energy. If this is the case, it may be due to an exotic phenomenon called the Casimir Effect. According to this theory, substances in a vacuum moving closer together eventually get attracted to each other because there is "more of a vacuum" between them then elsewhere in the vacuum. The resulting motion may result in the change of "vacuum" or "zero point" energy to real energy and hence the spark. Since, in this scenario, you’re producing something (energy) from nothing (a vacuum), theoretically the amount of energy we could produce from a system like this is limitless. (Click for more information on the Casimir effect.)

The real reason why anybody cares about lights created from sound, however, is because some have speculated that if the acoustic cavitation mechanism holds true, then temperatures inside the bubble may reach 2 million kelvins – hot enough to produce a cold fusion reaction which could provide enough environmentally safe energy to power everything for, well, ever. Unfortunately, it appears that the real temperature may be closer to 10,000 degrees, which is only as hot as the surface of the sun. Whilst not hot enough to spark cold fusion, it would still make a hell of a grill.

So can you yell at water and make is light up? Not exactly. In fact, the authors submerged a 12 megahertz medical ultrasound probe in a coffee pot of water this morning and got nothing. It takes a bit of effort. On the other hand, fractoluminescence, which is also fairly cool, is easy to demonstrate. A Wint O Green flavor Lifesaver brand candy is the ideal equipment for this experiment, but sugar cubes or certain types of quartz will work as well. After retiring to a dark room, take a pair of pliers (or even a hammer) and crush the candy. You should see flashes of a bluish light. The benefit to the lifesaver over other crystals is that it also contains oil of wintergreen, which will fluoresce, accentuating the effect. (Shown on YouTube.)

Get more from James and Stuart or find out how some other things work:

- Whizz Bang - How fireworks work
- Bloody ice cream - Why we get brain freeze
- Little stunners - What happens inside an electric eel
- Ah what's the point - How marijuana works

Image: Peter Rol

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