The fizz-ics of champagne

Popping New Year champagne corks unleashed avalanches the world over.

An avalanche waiting to happen

As the last echoes of popping New Year champagne corks die away, physicists reveal what gives bubbly its fizz. Apparently the enticing sound of bubbles bursting in the foamy head of freshly poured champagne results from of a kind of miniature avalanche, say Nicolas Vanderwalle of the University of Liège in Belgium and his colleagues.

The fizzing sound is the sum of many tiny, unpredictable pops as individual bubbles burst at the surface of the foam, the researchers explain in the journal Physical Review Letters ¹.

If the bursting happened at a constant rate while the foam collapses, we would hear a uniform hiss like the 'static' from a radio. This kind of 'white noise' is produced in acoustic processes in which the sound frequency is essentially random while the amplitude stays constant.

Instead, champagne seems to crackle because the fizz from its foam is not like white noise at all. Vanderwalle and colleagues measured it with a microphone and find that it is much more 'spiky' because the bubbles don't burst independently, but affect one another.

Each pop lasts for just a thousandth of a second (a millisecond) or so. Some follow one after another in rapid succession, and their noise combines to generate a loud acoustic signal. Other pops happen on their own, not with a bang but a whimper.

Foams collapse as the liquid in the bubble walls drains downwards under the pull of gravity, making the walls ever thinner until they are too thin to survive. The researchers studied this process not with champagne (which has foam that collapses very quickly) but with soapy water, where the foam subsides slowly, typically over an hour or so.

They examined how the time between successive bubble bursts varied. Vanderwalle's group found that it obeyed a mathematical relationship called a power law, which means that the gap had no 'preferred' duration: it could be anything from just a few milliseconds to several seconds, and there's no way of predicting which.

Power laws are found in many natural systems that display 'avalanche' behaviour, such as earthquakes (which have no preferred magnitude), landslides or solar flares. They generally occur in systems in which interactions between the component parts are crucial to the behaviour of the whole. In landslides, for example, a tumbling pebble could knock into others and initiate a cascade, or it might disturb only a handful of others as it rolls down the slope.

Similarly, a bubble bursting at the surface of a foam forces other bubbles nearby to change shape, say the researchers. This can cause other bubbles to burst, which might result in an avalanche of pops. Or it might induce only a few 'sympathetic' bursts -- or none at all. The fizzing sound comes from this unpredictable, mutual dependence of one bubble on its neighbours.