Physics is a topic best not contemplated under the influence of alcohol. If the room looks like it’s spinning or a single image starts to look double, you can be sure that what you’re seeing is not the way the world really is. And yet even before your very first sip, there’s one screwy bit of physics that’s hard to deny: the bubbles in your glass of stout appear to be sinking.
Bubbles, of course, are supposed to move up, not down, for a very basic reason: the gas that fills them is lighter than the surrounding liquid. Just like a balloon filled with lightweight helium must rise in heavier oxygen-nitrogen air, so too must the swirl of bubbles in your beer move toward the top of the glass. Yet judging by what we see, a lot of them don’t. A new study in the American Journal of Physics, authored by William Lee, professor of industrial mathematics at the University of Huddersfield in West Yorkshire, England, explains why.
The falling-bubbles phenomenon is observed most conspicuously in stouts—strong, dark beers made with roasted barley or hops—and it’s the nature of their dissolved gasses that makes the difference. In lighter beers, the gas that creates the fizz and foam is entirely carbon dioxide. In stouts, it’s a mix of carbon dioxide and nitrogen.
Nitrogen dissolves in liquid less easily than CO2 does, which means that while the gas content of stouts is lower, the pressure is higher. Most important, the bubbles in a stout are smaller than those in a lighter beer—an average of a tenth of a millimeter across compared to a millimeter or more. Smaller bubbles are less buoyant due to the lower amount of gas they contain. But they still ought to move in only one direction, and that’s up.
Lee explains that in the early years of sinking-bubble research (yes, the investigations have been going on for a long time), there was some question as to whether the phenomenon was occurring at all, or, as he wrote in his paper, if it is merely “an optical (or alcohol induced) illusion.” In 2004, a Stanford University study, straightforwardly titled, “Do Bubbles in Guinness Go Down?” relied on videotape analyses to determine that that the bubbles under study were indeed moving south. Other research used computer models to confirm that finding, but they 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, any more than observing it in the real world does.”
In his new paper, Lee went old-school, eschewing computer models and relying on the modern-day equivalent of pencil-and-paper ciphering to develop a clear mathematical model for what is happening in the glass. His equations include multiple variables, including the volume of the bubbles relative to the volume of the liquid, the velocity of the bubbles, the velocity of the beer (which is not stationary) and the overall …read more
Source:: Time – Science