Physicists have figured out how exactly the characteristic sound produced when opening a bottle of beer with a toggle cap, which immediately follows the pop, is produced. It turns out that the primary source is the oscillating gas column in the bottle's neck. The scientists reached this conclusion after analyzing high-speed video and audio recordings of bottle uncorking. The authors also compared their experimental results with numerical simulations and a Helmholtz resonator model, achieving almost complete agreement with experiment. The study was published in Physics of Fluids.
To date, physicists have studied beer from a variety of unexpected angles: they've counted the number of bubbles in a bottle, examined the stability of foam when poured from the bottom up, and even examined a peanut dancing in a glass. The point is that, from a hydrodynamic perspective, beer is a very interesting liquid: it's under pressure in a bottle or can and also contains dissolved carbon dioxide.
For example, it's already known how gas escapes when a bottle is opened: when the pressure in the container exceeds the ambient pressure by 1.83 times, the escaping gas flow reaches the speed of sound, becoming an underexpanded jet stream. Although physicists have already modeled the uncorking of champagne and even observed Mach diamonds during their experiments, no one has studied the sound produced by opening a beer bottle with a pull-top closure (sometimes called a flip-top closure). This isn't the pop itself, but what follows—a characteristic tone with a frequency of around hundreds of hertz.
A team of physicists from Austria and Germany, led by Max Koch of the University of Göttingen, discovered that opening a flip-top beer bottle creates a standing wave in the neck of the vessel, reverberating for 70 milliseconds and imparting a characteristic tonality to the sound. To do this, the scientists used homemade ginger beer, a camera recording at 8,000 to 16,800 frames per second, and sound recording equipment that allowed them to obtain spectra with a resolution of 25 hertz after Fourier transformation.
Physicists analyzed the data obtained and divided the bottle's opening into four stages: the first is the release of gas with a shock wave, the reflection of this wave, and the condensation of water vapor; the second is the formation of a resonating gas-condensate column in the bottle's neck; the third is the release of dissolved carbon dioxide from the liquid; and the fourth is the splashing of the liquid due to the rising gas-liquid boundary.
During the second stage, the gas column in the bottleneck oscillated up and down with high amplitude for 70-100 milliseconds—this was the source of the main acoustic signal. The spectrum of the sound itself had a single strong peak at a frequency between 640 and 870 hertz (depending on the bottle's filling: from the smallest to the largest volume of liquid in the vessel), indicating a sinusoidal signal received by the microphone. However, the supersonic speed of the process remained questionable, as the condensate front emerged at a speed of 50-150 meters per second. This is less than the speed of sound in air (approximately 330 meters per second under normal conditions), but the study's authors noted that locally, the flows could still exceed the supersonic limit.
Scientists numerically simulated the process of bottle uncorking, assuming the escaping gas to be adiabatic, inviscid, and weightless. The physicists assumed the process to be adiabatic due to its short duration and also ignored the influence of gravity, as the Froude number in the experiment was approximately 635 units (consequently, the gravitational term in the Navier-Stokes equation was negligible). Ultimately, the simulations closely matched the experimental data. Furthermore, the authors of the article considered a Helmholtz resonator model, which also provided a good theoretical description of the observed resonance.
The scientists emphasized that their study was the first to disprove the widespread belief about the nature of the sound of champagne or beer being opened, but that several unanswered questions remained. For example, about what happens in the first milliseconds of opening a bottle, which could not be recorded with sufficient resolution by either a camera or a microphone.
Physicists love not only beer and champagne, but also other carbonated drinks: we previously wrote about how scientists blamed surfactants for bubble chains.
