You may not be able to blast a bottle of champagne off in the backyard, but it turns out that sparkling wine is its own kind of bottle rocket.
New high-speed videos reveal that the plume of carbon dioxide released from a popped bottle of bubbly can contain a Mach disk — a kind of visible shock wave typically seen in supersonic exhaust streams from jets and rockets. These shock waves appear when the pressure of the exhaust outflow is more than about five times as high as the surrounding air.
In champagne bottles stored at room temperature, carbon dioxide gas in the bottle’s neck is at least seven times as pressurized as ambient air. So when the bottle is uncorked, the gas that gushes out — at more than twice the speed of sound — forms a Mach disk in its plume. Within about a millisecond, the pressure inside the bottle’s throat is closer to that of the surrounding air, and the shock wave vanishes, researchers report September 20 in Science Advances.
“The discovery of these Mach disks was a complete surprise,” says Gérard Liger-Belair, a physicist at the University of Reims Champagne-Ardenne in France. The original intent of the study, he says, was to investigate how bottle temperature affects the appearance of a champagne plume.
In experiments with champagne stored at 20° and 30° Celsius, Liger-Belair’s team confirmed previous findings that bottle temperature influences plume hue: Warmer champagne puffed out white-gray plumes, and cooler bottles exhaled deep blue.
That’s because carbon dioxide is less soluble at higher temperatures, making the gas trapped inside a 30° bottle more pressurized. When the bottles are uncorked, gas in the 30° bottle undergoes a greater pressure drop, and therefore a bigger temperature drop, than CO2 freed from the 20° bottle.
“The lower the [final] temperature, the easier the transformation” of carbon dioxide gas into dry ice, Liger-Belair says. Gas from a 30° bottle forms large ice crystals that scatter all wavelengths of visible light, giving the plume its whitish hue. Meanwhile, gas from a 20° bottle forms smaller crystals that preferentially scatter shorter, bluer wavelengths of light — similar to the way that small atmospheric molecules paint the sky blue.