Physics is a better topic not contemplated under the influence of alcohol. If it seems that the room is spinning or if a single image begins to look double, you can be sure that what you are seeing is not how the world really is. And yet, even before his first sip, there is a part of physics that is hard to deny: the bubbles in his beer glass seem to sink.
Bubbles are supposed to move up, not down, for a very basic reason: the gas that fills them is lighter than the surrounding liquid. Just as a balloon filled with light helium must rise in a heavier air of oxygen and nitrogen, so should the whirlpool of bubbles in your beer move towards the top of the glass. However, judging by what we see, many of them do not. A new study in the American Journal of Physics, written by William Lee, professor of industrial mathematics at the University of Huddersfield in West Yorkshire, England, explains why.
The phenomenon of descending bubbles is most noticeable in stouts-strong, dark beers made with toasted barley or hops, and it is the nature of their dissolved gases that makes the difference. In lighter beers, the gas that creates steam and foam is completely carbon dioxide. In stouts, it is a mixture of carbon dioxide and nitrogen.
Nitrogen dissolves in the liquid less easily than CO2, which means that while the gas content of the stouts is lower, the pressure is higher. The most important thing is that the bubbles in a black beer are smaller than those in a lighter beer: an average of one tenth of a millimeter compared to a millimeter or more. The smaller bubbles are less buoyant due to the smaller amount of gas they contain. But they still have to move in one direction, and that's it.
Lee explains that in the early years of the sinking bubble investigation (yes, investigations have been ongoing for a long time), there was some question as to whether the phenomenon was happening at all, or, as he wrote in his article, if it is merely "an optical illusion (or induced by alcohol)". In 2004, a study by Stanford University, directly titled, "Do Bubbles in Guinness Go Down?" Was based on analysis of videotapes to determine that the bubbles under study were moving southward. Other research used computer models to confirm that finding, but offered little to explain why it was happening.
"The ability to reproduce a phenomenon in a simulation," Lee wrote, "does not always lead to a better understanding of that." phenomenon, rather than observing it in the real world. "
In his new job, Lee went to the old school, avoiding computer models and relying on the modern equivalent of pencil and paper cipher to develop a clear mathematical model of what is happening in the glass. they include multiple variables, including the volume of the bubbles in relation to the volume of the liquid, the velocity of the bubbles, the velocity of the beer (which is not stationary) and the general pressure throughout the bubble-beer-glass system [1
Once a beer is poured, Lee explains, the bubbles rise in a more or less straight, even when the tulip is constantly widening. That leaves a region of beer near the walls of the glass with a lower concentration of bubbles. The beer with less gas is more dense than the beer with more, and that region of beer sinks, taking with it the bubbles nearby, in relation to the glass. As the beer settles in the narrower bottom, the bubbles, which are now more concentrated, rise in mass and the cycle repeats itself.
"If you saw [stout] in a glass with sloping walls inward," Lee wrote in an email to TIME, "then things would turn upside down." As the bubbles rise, they pile up on the wall creating a lighter fluid and rich in bubbles that will rise upwards ".
The bubble-beer-glass system, like most dynamic systems, eventually runs out. If enough is left, the foam would disappear, the gas would dissipate and the thick would become still and flat. Of course, most of the stouts – or at least most of the good, cold, dark, rich and delicious stouts – do not stay for long. The human drinker intervenes, and then the human barman begins the process again.