The story goes that Archimedes screamed “Eureka!” from the top of his lungs, streaming through the Grecian streets naked—as Grecian thinkers were known to do—when he finally solved the connundrum that he’d been toiling over.
Said connundrum was a commission from the king of Syracuse, Hiero II, to ensure that his crown was indeed constructed from pure gold, and not partially substituted with silver by the shady smith.
According to Vitruvius, the Roman architect responsible for most of our knowledge of Archimedes (and the inspiration for Da Vinci’s Vitruvian Man), Archimedes solved the problem while in the bath, explaining his nudist escapade.
It’s said that while entering the bath, Archimedes noticed the water level rising, and inferred that he could use this method to ascertain the volume of the crown by measureing the amount of liquid it displaced when it was submerged. Thus, knowing the weight of the crown and its volume, he could then calculate its density.
This discovery is what’s known as “Archimedes’ Principle”. Explicitly stated, the principle asserts that “the upward buoyant force that is exerted on a body immersed in a fluid, whether fully or partially, is equal to the weight of the fluid that the body displaces.”
The point of this anecdote, besides being mildly interesting, is to predicate the mystery that plagued physicists for decades.
Bubbles
Archimedes’ principle was pivoltal for the discovery of buoyancy, the phenomenon responsible—among many things—for why the bubbles of carbonated beverages float to the surface. However, there is one exception to this rule that has, until recently, gone unexplained.
Ever since Guinness introduced nitrogen to its formula in 1959, it has been observed that the bubbles of a freshly poured pint actually appear to sink, instead of float.
This phenomenon, colloguially termed the “Guinness cascade”, eluded scientific understanding until the inception of computer generated simulations. It seemed to counter our understanding of buoyancy; Archimedes’ Principle tells us that since air is less dense than Guinness, it should rise while the liquid should sink. How could the bubbles possibly sink when they are, in effect, pockets of air?
The answer is actually two-fold. You know how connoiseurs sound kind of pretentious when they talk about the different types of glasses (when to use a Glencairn or a tumbler, or when to chill or not chill)? Well, they actually have a point. The other answer actually involves the composition of Guinness itself.
For those unaware, the reason Guinness cans sound like they have a little marble inside of them rattling around, is because they kind of do! Though not a marble, each can of Guinness contains a little ball designed to release nitrogenated Guinness when the can is depressurized (opened). This is designed to preserve the creamy, foamy head signiture of a pint of Guinness.
The nitrogen bubbles are much smaller than their carbonated counterparts, which not only contributes to the creamy texture of the foam, but is actually one of the properties responsible for the cascade.
As seen in the above simulation, pouring a stout into a pint glass (one which is narrower at the base than at the top) forms flow circulations at the sides of the glass. As the beer at the sides falls, it actually pulls a lot of the lighter, i.e. nitrogen bubbles, down with it, generating the cascade and occluding the carbonated bubbles rising to the surface. Eureka!
I love me some Guinness, so it was fun finally going down the rabbit hole and learning about this neat little phenomenon.