Last updated December 4, 2018 at 10:13 am
Popping a cork at a big celebration sets off a gloriously complex train of events. Phil Dooley explains.
Opening a bottle of champagne not only signifies the start of a celebration, but also uncorks a swathe of sophisticated physics phenomena that contribute to the special appeal of bubbly.
Just as the pop of a cork marks a change in mood, it also marks a sudden change for the champagne. Pressure that has been building for months during the fermentation process is quickly released and suddenly things are out of equilibrium. What makes champagne fun are the dynamic processes that bring the system back into balance – pops, bubbles and fizz.
But where did the pressure come from in the first place? The answer is micro-farts. The yeast introduced by the winemaker feeds on sugar in the wine, using its energy to power its life, and then ejects its waste: carbon dioxide.
In each bottle of sparkling wine, yeast microbes produce more than 10 grams of this gas. That equates, in the confines of the sealed bottle, to a pressure about three times that found inside a car tyre. Under these conditions most of the carbon dioxide dissolves in the wine.
As the cork is loosened the pressure of the gas in the bottle pushes it out. Cork speeds can reach more than 50 kilometres per hour, says Gérard Liger-Belair from University of Reims Champagne-Ardenne in France.
Behind the cork pop
Liger-Belair has made a career of studying the physics of bubbly; for him a glass of champagne is “a fantastic playground”, although he insists he does not drink his experimental samples.
Instead, he enjoys a glass of champagne with laser tomography, infrared imaging, high-speed cameras and mathematical models.
His measurements show the speed of the cork depends on the temperature of the wine. The carbon dioxide is much less soluble at higher temperatures, which leads to a higher pressure inside the bottle and thus a faster launch speed.
At the perfect drinking temperature of eight to 10 degrees Celsius the corks pops out at around 40 kilometres per hour. His experiments extend only to 20 degrees Celsius, at which the speed is in the low fifties. Presumably, champagne warmer than that is inconceivable in France.
In the moments after the it pops, fleeting wisps of fog appear – another dynamic phenomenon. The sudden five-fold drop in the pressure of the gas in the neck of the bottle causes a temperature decrease of around 80 degrees Celsius. As it momentarily dips below minus-70, traces of water and alcohol in the gas condense into an evanescent mist that quickly evaporates as the gas approaches room temperature.
Next the fizz begins, as the carbon dioxide dissolved in the wine starts to escape. If one were to let the contents of the bottle go absolutely flat, it would take more than 10 hours and involve the release of more than six litres of gas.
A bad pour can let the fizz out too quickly, says Liger-Belair. He recommends a gentle stream into a tilted glass to preserve bubbles. Pouring into the middle of a vertical flute will stir up the wine and release too much carbon dioxide immediately.
The secret of the foam
Even with such precautions, there is an initial rush of bubbles. University of Tokyo physicist Hiroshi Watanabe was part of a team that used supercomputers to model how quickly bubbles of different sizes form in liquids, and how the different sizes interact.
“After many bubbles appear at the moment of uncorking a champagne [bottle], the population of bubbles starts to decrease,” Watanabe said in an interview with Smithsonian.com.
“Larger bubbles become larger by eating smaller bubbles, and finally only one bubble will survive.”
Bubbles are a vital part of the taste of champagne, say researchers from the Sorbonne University in Paris, France. As they burst, they throw droplets of wine into the air above the surface and enhance the drinking experience.
The scientists identified two mechanisms that produce droplets. First, as the surface of the bubble ruptures, it throws up dollops 50 times smaller than the radius of a hair. Then as the rounded bubble shape collapses, it sends up a jet of up to 10 slightly larger droplets.
“The tiny droplets ejected during bursting are crucial for champagne tasting as their evaporation highly contribute to the diffusion of wine aroma in air,” said Elisabeth Ghabache and her colleagues in a paper in the journal Physics of Fluids.
There’s been much debate about how champagne should be served. Some insist on a narrow flute, while others prefer a wide coupe.
Liger-Belair does not recommend the coupe, despite its claim to fame as being modelled on the left breast of French queen Marie Antoinette (or Napoleon’s wife Josephine, or model Kate Moss).
His gas chromatography and infrared images showed that flutes funnelled the aroma-carrying carbon dioxide more effectively into the headspace above the glass where the drinker can inhale it. But carbon dioxide’s acidic nature is actually an irritant if the concentration is too high, so Liger-Belair recommends the middle ground, a tulip shaped wine glass.
Exactly how the flavours interact with the nose and taste buds is a very individual thing. So the scientific thing to do – even if you don’t have infrared cameras and a gas chromatography set up – is to emulate Liger-Belair at your next celebration and perform your own experiments, one glass at a time.