Science

You’ll tear your eyes out: a popped champagne cork ejects CO2 at supersonic speeds

You'll tear your eyes out: a popped champagne cork ejects CO2 at supersonic speeds
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The sound of a champagne cork turns out to have something in common with a rocket launcher, according to a recent post published in the journal Physics of Fluids. French and Indian scientists have used computer simulations to reveal in detail what happens in the microseconds after a bottle of champagne is uncorked. They discovered that in the first millisecond after the cork pops, the ejected gas forms different types of shock waves, even reaching supersonic speeds, before the fizz stabilizes and is ready to imbibe.

“Our paper reveals the beautiful and unexpected flow patterns that lurk under our noses every time a bottle of champagne is uncorked”, said co-author Gérard Liger-Belair from the University of Reims Champagne-Ardenne. “Who could have imagined the complex and aesthetic phenomena hidden behind such a common situation experienced by each of us?

Liger-Belair could imagine it, for his part. He studied the champagne physics for years and is the author of Market: the science of champagne. He gleaned a wealth of information about the underlying physics by subjecting champagne to laser tomography, infrared imaging, high-speed video imaging, and mathematical modeling, among other methods.

According to Liger-Belair, the effervescence of champagne comes from the nucleation of bubbles on the walls of the glass. Once they have detached from their nucleation sites, the bubbles grow larger as they rise to the surface of the liquid, bursting and collapsing on the surface. This reaction usually occurs within milliseconds and the distinctive crackling sound is emitted as the bubbles burst. When champagne bubbles burst, they produce droplets that release aromatic compounds that are believed to further enhance the flavor.

In addition, the size of the bubbles plays an essential role in a very good glass of champagne. Larger bubbles improve the release of aerosols into the air above the glass – approximately 1.7mm bubbles on the surface. And the champagne bubbles “ringing” at specific resonant frequencies, depending on their size. It is therefore possible to “hear” the size distribution of the bubbles when they rise to the surface of a glass of champagne.

Time sequence showing details of a cork expelled from a champagne bottleneck stored at 20° Celsius captured by high speed imagery.
Enlarge / Time sequence showing details of a cork expelled from a champagne bottleneck stored at 20° Celsius captured by high speed imagery.

Gerard Liger-Belair

as we have previously reported, champagne is usually made from grapes picked early in the season, when the fruit contains less sugar and higher acidity. The grapes are pressed and sealed in containers to ferment, like any other wine. CO2 is produced during fermentation, but it is allowed to escape because what you want at this stage is a base wine. Then there is a second fermentation, except this time, CO2 is trapped in the bottle, dissolving in the wine.

It is essential to find the right balance. You need about six atmospheres of pressure and 18 grams of sugar, with only 0.3 grams of yeast. Otherwise, the resulting champagne will either be too flat or too much pressure will cause the bottle to explode. You also need the right temperature, which influences the pressure inside the bottle. This high pressure CO2 is eventually released when the plug is popped, releasing a plume of gas mixed with water vapor that expands out of the bottleneck and into the surrounding air.

Previous experimental work by Liger-Belair and his colleagues used high-speed imaging to demonstrate that shock waves formed when a champagne cork popped. With the present study, “We wanted to better characterize the unexpected phenomenon of a supersonic flow that occurs when uncorking champagne bottles”, said co-author Robert Georges from the University of Rennes 1. “We hope that our simulations will offer interesting leads to researchers, and that they will be able to consider the typical bottle of champagne as a mini-laboratory.”

Based on these simulations, the team identified three distinct phases. Initially, when the bottle is uncorked, the gas mixture is partially blocked by the cork, so the ejecta cannot reach the speed of sound. As the plug breaks free, the gas can then escape radially and reach supersonic speeds, forming a succession of shock waves that balance its pressure.

These shock waves then combine to form telltale ring patterns known as shock diamonds (also called thrust diamonds or Mach diamonds after Ernst Mach, who first described them), commonly seen in rocket exhaust plumes. Finally, the ejecta slows down again to subsonic speeds when the pressure drops too low to maintain the required nozzle pressure ratio between the bottleneck and the edge of the plug.

The research is relevant to a wide range of applications involving supersonic flow, including ballistic missiles, wind turbines, underwater vehicles and of course, a rocket launcher. “The ground which moves away from the launcher by rising in the air then plays the role of the champagne cork on which the ejected gases come to impact”, explain the authors. “Similarly, combustion gases ejected from the barrel of a gun are thrown at supersonic speeds onto the bullet. The problems face the same physical phenomena and could be addressed with the same approach.”

DOI: Fluid Physics, 2022. 10.1063/5.0089774 (About DOIs).

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