Bubbles in Beer
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Have you ever been in the pub getting pissed and trying to be intellectual ? What subjects did you discuss? How the fly lands upside down on the ceiling ? The reason ants can't read ? Why do the bubbles in beer form from no where? And why do they get larger and more spaced farther apart as they rise? Well I can't answer the former two, but the latter pair is what this article is about.
It has all to do with the interplay among gases, liquids, solids, temperature, pressure and gravity.
It seems that beer bubbles begin as tiny clusters of molecules that grow on rough scratches on the glass. The bubbles form from carbon dioxide, which is dissolved in the beer.
An old and outdated theory was that the bubbles double in size as they rise because of a change in hydrostatic pressure. This would require a pressure difference of two atmospheres from the bottom to the top of the glass - a rise of nearly 9 meters! They don't even make a Western Bitter that big.
It is now proven that the bubbles act as nucleation centres for themselves, accumulating more carbon dioxide as they rise through the beer. As the bubbles get bigger, their buoyancy becomes greater relative to the beer. A simple application of Archimedes' principle:
Fbubble = Vbubble (Pbeer - Pbubble) x g.
(F is the force on the bubble; V is its volume, P is density, g is the gravitational constant [9.81 m/s/s] )
Drag on the surface holds the bubble back from rising, but drag increases less rapidly than buoyancy. So, the bigger the bubble, the faster it rises. Thus, higher bubbles race away from lower ones in the stream, spacing bubbles farther apart at the top of the glass.
The fluid dynamics of this process turn out to be much more complicated than they seem. For example, there is no simple method to predict the drag on a particle moving in a viscous medium, and when the two theories were tested, the results did not match either one.
Surfactants - the slippery substances that give staying power to the head of foam at the top of the glass - affect the ascent of the bubbles. If the bubbles rise in a tall enough glass, even their shape changes, from a sphere to an ellipse that oscillates and travels in a zigzag path.
These findings could also apply to other carbonated beverages such as mineral water or champagne - though a study of the latter will not be taken up because of the high price, since only out of work scientists are doing such useless studies such as professor Richard Zare and post-doctoral researcher Neil Shafer, the two responsible for this amazing breakthrough in beer technology. Cheers!
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