Thursday, September 19, 2019
Physics of Beer :: physics alcohol beverage
Carbonated beverages contain dissolved carbon-dioxide gas. The way to get gas to dissolve in liquid is to pressurize the mixture, meaning that the pressure inside a beer can is greater than the pressure outside the can. This is why you see little bubbles spray out when you open a beer can. Breaking the seal depressurizes the mixture, causing the gas to come out of solution, starting with the gas closest to the top (that's where the pressure decrease starts). Because gas is lighter than liquid, as soon as it comes out of solution, it rises to escape the can. When it escapes, it carries with it a small amount of liquid from the very top of the can because that liquid is blocking its path. When beer is agitated (shaken), cavities form small bubbles, which then collect as foam on the surface. If you then open the bottle, and explosively decompress the beer, the foam eruption is the work Boyle's law and inflation by diffusion. On the other hand, if the shaken beer is allowed to sit for a while and "decompress," the gass in the bubbles will be driven back into solution by the bubble skin tension making it safe to open. If you are impatient you can tap on the can. When you tap on the can, you knock bubbles off the bottom and sides of the can, at which point they rise to the top (because gas is lighter than liquid) and there is only a small amount of liquid blocking their escape when you open the can. As a matter of fact most bubbles do move upwards. The bubbles in the center of the glass, free from the effects of the wall, move upwards most quickly and drag liquid with them. But the liquid moving up in the center of the glass, having nowhere else to go, must eventually turn towards the walls and start to move downward. The liquid moving downward near the walls tries to drag down bubbles with it. Larger bubbles have sufficient buoyancy to resist but smaller bubbles (less than 0.05 mm) are continuously dragged to the bottom of the glass. A stout like Guinness has a creamier, longer lasting head than a canned lager beer. In addition, Guinness is less fizzy than a regular lager beer. Guinness is canned with a mixture of carbon dioxide and nitrogen. Nitrogen is not absorbed into the beer nearly as well as carbon dioxide, so even though a can of Guinness may be at the same pressure as a can of lager, it contains less CO2 (and is therefore less fizzy) because the nitrogen makes up some of the pressure.