The pop of a champagne cork appears to have something in common with a rocket launcher, according to a recent paper published in the journal Physics of Fluids. Scientists from France and India used computer simulations to reveal in detail what happens in the microseconds after uncorking a bottle of champagne. They found that in the first millisecond after the cork pops, the ejected gas forms different types of shock waves — even reaching supersonic speeds — before the bubbles settle and are ready to record.
“Our paper unravels the unexpected and beautiful flow patterns hidden right under our noses every time a bottle of bubbly is uncorked,” said co-author Gérard Liger-Belair of the University of Reims Champagne-Ardenne. “Who could have imagined the complex and aesthetic phenomena behind such a common situation experienced by any of us?”
Liger-Belair, for example, could imagine. He has been studying the physics of champagne for years and is the author of: Uncorked: the science of champagne† He has gained countless insights into the underlying physics by subjecting champagne to laser tomography, infrared imaging, high-speed video imaging and mathematical modeling, among others.
According to Liger-Belair, the effervescence of champagne is caused by the formation of bubbles on the glass walls. Once they break free from their germination sites, the bubbles grow as they rise to the liquid surface, bursting and collapsing at the surface. This reaction usually takes place within a few milliseconds and the distinctive crackling sound is emitted when the bubbles rupture. When the bubbles in champagne burst, they produce droplets that release aromatic compounds that are believed to further enhance the flavor.
The size of the bubbles also plays a crucial role in a really good glass of champagne. Larger bubbles enhance the release of aerosols into the air above the glass – bubbles about 1.7mm across the surface. And the bubbles in champagne “ring” at specific resonant frequencies, depending on their size. So it is possible to “hear” the size distribution of bubbles as they rise to the surface in a glass of champagne.
As we reported earlier, champagne is usually made from grapes picked early in the season, when there is less sugar in the fruit and higher acid levels. The grapes are pressed and sealed in containers to ferment just like any other wine. CO2 is produced during fermentation, but it is allowed to escape because you want a base wine at this stage. Then there is a second fermentation, except this time, the CO2 is trapped in the bottle and dissolves in the wine.
Finding just the right balance is critical. 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 affects the pressure in the bottle. The high pressure CO2 is eventually released when the cork is popped, releasing a plume of gas mixed with water vapor that expands from the bottleneck and into the ambient air.
Previous experimental work by Liger-Belair and his colleagues used high-speed imaging to show that shock waves were formed when a champagne cork was popped. With the current study, “We wanted to better characterize the unexpected phenomenon of a supersonic flow that takes place during the uncorking of champagne bottles,” said study co-author Robert Georges of the University of Rennes 1. “We hope our simulations will provide some interesting clues.” to researchers, and they might think of the typical bottle of champagne as a mini-laboratory.”
Based on those simulations, the team identified three distinct phases. Initially, when the bottle is uncorked, the gas mixture is partially blocked by the cork, preventing the ejecta from reaching the speed of sound. If the cork is released, the gas can escape radially and reach supersonic speeds, creating a succession of shock waves that balance the pressure.
Those shock waves then combine to form telltale ring patterns known as shock diamonds (also known as barrage diamonds or Mach diamonds after Ernst Mach, who first described them), usually observed in rocket exhaust plumes. Finally, the ejecta slows back to subsonic speeds when the pressure gets too low to maintain the required nozzle pressure ratio between the bottleneck and the rim of the cork.
The research is relevant to a wide variety of supersonic flow applications, including ballistic missiles, wind turbines, underwater vehicles – and of course a rocket launcher. “The ground moving away from the launcher as it rises into the air then plays the part of the champagne cork on which the expelled gases hit,” the authors explained. “In the same way, combustion gases ejected from the barrel of a gun are thrown onto the bullet at supersonic velocities. The problems involve the same physical phenomena and can be treated with the same approach.”
DOI: Physics of Fluids, 2022. 10.1063/5.0089774 (About DOIs).