Cracking the Code of the "Ploop": How Physicists Unravel the Mystery of Swing-Top Bottles
- by Martin Scheufens
- 2 Min
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Call it quirky, but Max Koch and his team at the University of Göttingen have taken on a seemingly mundane task: studying the science behind the sound made when a swing-top bottle is opened. And their findings have made their way into the prestigious journal "Physics of Fluids" under the special issue "kitchen flows."
At first glance, this experiment might not seem very high-brow. But the research group "Ultrasound and Cavitation" at the Third Physical-Biological Institute has gained valuable insights.
To conduct their experiments, they needed a high-speed camera, a microphone, and plenty of swing-top bottles. The bottles needed to contain carbon dioxide and be under significant overpressure, up to five bar. When opened, the pressurized carbon dioxide expands rapidly. The group discovered that the sound is not a single shockwave, but rather a rapid "ah." Their high-speed video recordings captured condensation within the bottle neck that oscillated in a standing wave. These recordings, along with high-resolution audio recordings and flow simulations, confirmed that this wave is the source of the "ah" sound.
As Loud as a Hummingbird's Wingbeat
During the "ploop," the gas in the bottle neck oscillates at a frequency of about 800 Hertz. "The frequency of the bang is much lower than the resonance that occurs when you blow across the full bottle like a whistle," says Koch. "This is due to the sudden expansion of the carbon dioxide-air mixture in the bottle, as well as the strong cooling to about minus 50 degrees Celsius, which reduces the speed of sound." When gas expands, it gets cold. This effect is far more pronounced when opening champagne bottles.
When opening the bottle, the pressure above the liquid drops suddenly, causing the previously dissolved gas in the liquid to form bubbles, and the liquid level rises. The jerk that usually accompanies the opening of a bottle, as well as the top striking the bottle neck from the outside, also contribute to this foaming effect.
Koch's experiment was not the first attempt to unravel the mystery of the "ploop." Back in 2007, the state of Schleswig-Holstein invested 1.5 million euros to develop an optimal new closure for a well-known beer brand based in the region. The use of public funds at the time became a political issue.
Max Koch's project, however, didn't come with a hefty price tag. For his "fun project," the hobby brewer used 0.33-liter bottles of homemade ginger beer. The recipe can be found in the study.
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- Swing-Top Bottles
- Physics
- Sound Wave
Enrichment Insights:
- While the experiment mainly focuses on swing-top bottles, the same principles can apply to opening twist-off cap bottles, albeit to a lesser extent.
- The research also shows that the "ah" sound is not unique to the brand of beer tested; this phenomenon occurs universally in swing-top bottles.
- Besides the "ah" sound, hydrodynamic instabilities can sometimes be observed when opening swing-top bottles, which can lead to a more dramatic sound and splash effect.
- The study highlights the complex interplay of physics, chemistry, and acoustics involved in the simple act of opening a swing-top bottle.
- During their experiments, Max Koch and his team at the University of Göttingen utilized a microphone to record the sound wave produced when a swing-top bottle is opened.
- To study the "Ploop" sound, they conducted experiments involving carbon dioxide under overpressure, similar to the gas in a beer bottle, using a high-speed camera and a variety of swing-top bottles.
- The discoveries made by Koch's team were published in the prestigious journal "Physics of Fluids," shedding new light on the complex sound wave phenomena associated with the opening of swing-top bottles, akin to the sound of a hummingbird's wingbeat.
