What’s Really Behind Champagne’s ‘Pop’

Originally published on The Wall Street Journal

The cheerful pop of a Champagne cork often signals that a party is about to step up a notch, but for me, the taste and feel of bubbly are only part of the fun. Being a specialist in the physics of bubbles, I’ve often enthused at parties, to anyone who will listen, about a glass of sparkling wine as a small, portable bubble engine. Until recently, I hadn’t thought much about the pop that announces the arrival of the bubbles.

The pop itself, it turns out, also has a fascinating physics story to tell, once you know what to listen for.

A Champagne bottle is really a pressure vessel masquerading as a drinks container. The bottle itself is made of thick glass, the standard version weighing almost twice as much as a normal wine bottle (around 31 ounces instead of 18), while holding the same amount of liquid. The cork and the cage aren’t just the gatekeepers for the wine itself—they’re really there to guard high-pressure gas.

The wine has a huge amount of carbon dioxide dissolved in it (that’s where the bubbles come from), but the gas in the space just above the wine is also almost pure carbon dioxide. The pressure in a chilled bottle is around four times atmospheric pressure, and at room temperature, enough of the dissolved gas leaves the liquid to raise the pressure inside the bottle to eight atmospheres. When you remove the cage, you’re priming a cannon.

If you leave the bottle at this stage, the pressure will almost certainly push the cork free within a minute or so. This is the moment that’s worth your attention. As the cork flies free, at a speed of about 20 miles an hour, the compressed carbon dioxide whooshes out behind it. As this gas expands into the atmosphere, it shoves the surrounding air out of the way, giving away energy in the process.

The consequence is an astonishing drop in temperature, from 39 degrees Fahrenheit (for a chilled bottle) to minus 94 degrees. It’s called adiabatic expansion, and the bigger the pressure drop, the greater the cooling.

But this extreme cold only lasts for a few milliseconds. As the cold carbon dioxide from the neck of the Champagne bottle mixes with warmer air, water condenses out of the air to form tiny droplets, and these form the fog that drifts lazily out of the bottle after the pop.

So where does that pop come from?

Once the high-pressure gas has escaped, a partial vacuum is left behind in the neck of the bottle that sucks air back in. But too much gets sucked in, raising the pressure again and pushing some of that air back out. The air in the bottle neck oscillates in and out until it settles down, and this is what you’re hearing when your ears detect the “pop” noise—the oscillating gas sends a sound wave out into the air.

When I did some high-speed photography of this process in my lab recently, one thing stood out more than anything else: I could see the sound of the pop. Using a high-speed camera, I could see the fog moving, making the sound of the pop visible. The pop lasts only around 50 milliseconds, but it’s singing out the size of the bottle neck because that’s what the speed of the oscillation depends on. Slightly less Champagne in the bottle will make it ring with a lower note. The oscillation is far too fast to see, but each pop is a sure signal that it’s happened.

At New Year’s parties this year, I’ll certainly hear more in those pops than I ever have before. To the scientist in me, that seems to me to be the best way to start a new year. Happy 2018!