If you are not a fan of Stout beer, or staring at beer watching bubbles, this post is not for you. Feel free to move along, I won’t mind.
Aaah good, you are still here. As you might guess, I am a fan of both a good Stout and staring at a beer watching bubbles. The next time you are so fortunate as to be doing both things at the same time, notice that, when initially poured, the bubbles in the Guinness GO DOWN, they do not go up.
This is a very interesting phenomenon, and it usually gets more interesting as the night goes on. People, who are much smarter than me, have figured out why.
First up, we have the beer drinkers of Massachusetts Institute of Technology. Yes, MIT. Several mathematicians took a run at it, eventually determining that actual experimentation was needed.
Experiments are not usually the domain of mathematicians. But Benlivo and co demonstrate valour beyond the call of mathematical duty by actually performing the experiment in which they bravely pour Guinness into a cylinder.
https://www.technologyreview.com/2012/05/28/185947/irish-mathematicians-solve-the-guinness-sinking-bubble-problem/
Next up are the fine drinkers at Stanford. This is much less an article and more of a bulleted list of why, only instead of bullets, they use beer glasses.
https://web.stanford.edu/group/Zarelab/guinness/why.html
Third in line are the folks at British Royal Society of Chemistry. A bit more engaging, it has the phrase “we had a camera which uses 4500 frames a second and a zoom lens of times 10” in it. From careful reading, however, it appears that they use canned Guinness (“widget”). And while canned Guinness is better than no Guinness, it is disappointing that they evidently couldn’t find Guinness on tap in England.
https://www.rsc.org/AboutUs/News/PressReleases/2010/GuinnessBubbles.asp
Finally, the scholarly folks at Cornell University identified what I believe is the key part: the shape of the glass. Published as an actual scientific paper, it has formulas, flow simulations, and diagrams. Very impressive.
https://arxiv.org/abs/1205.5233
For those of you that are too drunk to follow any of those links, here is the short version.
- The glass matters.
- Bubbles generally go straight up.
- Since the glass slopes in, there are more bubbles going up in the middle than on the edges.
- Bubbles drag liquid with them.
- All this liquid being dragged up in the middle is more than being dragged up at the edges, so it wins and forces the liquid down in the edges.
- This forcing down at the edges is stronger than the bubbles going up, hence the bubbles are pushed down.
The next time you have a Guinness or some other heavy stout or porter in a proper beer glass, raise a toast to those academic institutions that contributed to this fine work.