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Best Kind of Fluid Dynamics: Beer

As many of our readers of legal age know, bubbles in beer do some weird stuff.  In fact, they were the inspiration for the first bubble chamber.  Personally, I have always been stumped by the bubbles sinking in Guinness.  This favorite Irish stout can be mesmerizing, with the bubbles slowly falling down the outside of the glass.  Researchers at the University of Limerick may have found the answer.  Its not the beer that's special, its the shape of the glass in which it is usually served.  I know that most labs do experiments with Guinness after hours, this is the first time I've heard of it ending in a paper.

Naturally, bubbles go up. It is the way of the universe. Air is less dense than water so it should always go up.  However, in Guinness, the beer on the edge of the glass is flowing down faster than the bubbles are rising.  They appear to fall but are really being pushed down by the beer.  This has been well understood for a while (physicists love their beer) but the question of why the liquid flows downward has not been answered till now.  A team of mathematicians from Limerick University studied the way beer moves  in glasses and found that Guinness can thank its distinctive glass for the bubbles' odd behavior.  

If you are drinking in a reputable bar you will be served your tasty Guinness in a distinctively shaped pint glass (not a Red Solo Cup).  Pint glasses are bigger at the top than they are at the bottom and this is what causes the beer to flow.  As bubbles move they drag a bit of the beer with them, there is a frictional force between beer and bubble.  So, in a region where a lot of bubbles are rising, the beer is rising too.  But if there is another region where fewer bubbles are doing their thing, the beer will begin to fall.  A pint glass creates this situation.

In a normal pint glass, the top is bigger than the bottom.  As the bubbles rise, the ones near the angled edge of the glass spread out and the bubble density decreases.  But, the density of bubbles near the center stays the same.  Because of this bubble discrepancy, the fluid near the edge of the pint glass falls while the  beer in the middle rises.

In this video you can clearly see the currents set up by the bubbles.  They burst out from the middle of the glass and fall down.  But, if poured into a less dignified glass, this beer would be nothing special.  Ok, not true, it would still be tasty, just not a scientifically interesting.

I've always loved the science of Guinness.  Now I'm going to go do some experiments.  Just like everyone else that's read this paper.



    even if it was just an experiment.


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