Old News, Still Fizzy
The joy of finishing a year and starting a new one was once symbolized by the taking down of the old wall calendar and the hanging of the new one. There still are the walls, but calendars have a new home on our computers, phones, and watches. Luckily one other tradition that marks this completion of yet another collective ellipse around the sun is still alive and well. We will still hear the pop and the fizz at many a celebration in these coming days. Pop, we will deal with another time. Let’s talk about the fizz today.
To get the fizz, we need bubbles. Turns out bubble growth and then eruption is central to not only the fizziness of our beloved beverages but also to other important phenomena such as volcanic eruptions and natural geysers. The eruption rate of bubbles depends on the kind of gas present in a liquid and its concentration. For example, the initial gas concentration makes the eruption rate of bubbles in champagne two orders of magnitude higher than that of carbon dioxide-based beer (Zhang and Xu, 2008). In contrast, bubble growth is much slower in a beverage like Guinness where the gas is a 1:3 mix of carbon dioxide and nitrogen. This also results in smaller bubbles that can be entrained resulting in the sinking bubbles in a pint of Guinness. Peer at them, next time you have one in front of you. Fair warning, though, the perceived direction of bubble travel may be dependent on the number of pints consumed before initiating the observational experiment.
Okay, so we have bubbles and they are erupting as the liquid in their walls get pulled down by gravity. Each eruption makes a tiny sound. String a bunch, a whole bunch, of these together and you have the fizz of a freshly poured carbonated drink. There is one last subtlety though that gives the sound its familiar texture. If the rate of eruptions were constant, that is, if a bubble erupted on the clock every millisecond or so, the fizz would sound more like static and less like it does. Here we are rescued by the influence of one bubble eruption on other bubbles in its proximity. As a bubble erupts, it distorts the surface of the liquid around it also changing the shape of nearby bubbles. This increases the probability of the next one erupting. However, because the proximity and density of bubbles is almost random, the rate of eruptions is not constant. That is, one eruption might start an avalanche, but another one may just be a solitary event. This fluctuation of eruption rate is what makes the fizz. Without it, life might have been just a tad bit more predictable and boring. Cheers!
Vanderwalle N, faLentz JF, Dorbolo S, Brisbois F. (2001) Avalanches of popping bubbles in collapsing films. Physical Review Letters 86:179–182.
Zhang Y, Xu Z. (2008) “Fizzics” of Bubble Growth in Beer and Champagne. Elements 4(1):47–49.