Beer – Science Everyday

Bubbles and Science Rabbit Holes

After reading Rajini Rao’s champagne science post, I figured I should join in. Except most of you know I drink beer and not much wine or champagne. Also, since the champagne post is related to New Years celebration, I’ll mention that craft beer in the USA, in the Midwest in particular, is improving every year. Here’s to more craft beer in the Midwest.

Rajini’s post reminded me of two beer-bubble related posts. One is why do bubbles sink when you pour a glass of Guinness stout. The other is what causes a “beer volcano”.

Part I: Sinking Bubbles

Guinness stout and a few other beers, e.g., Boddingtons Pub Ale, have nitrogen gas in them, in addition to carbon dioxide. It creates a creamy head and takes away some of the bitterness (from the hops) that some people don’t like. Some of you may know that Guinness and Boddingtons in cans, have a widget, which is a plastic device with pressurized nitrogen. It helps recreate the pub experience of a draught beer. Did I mention rabbit holes? Back to sinking bubbles. Physicists at the University of Limerick in Ireland published a beautiful paper based on simulations and experiments, explaining why bubbles sink instead of rise in a pint of Guinness.

Fluid Dynamics

Benilov et al, explain that the small (nitrogen and carbon dioxide) bubbles sink along the surface of the glass while the bubbles in the center of the glass rise. Using the Bond number, Stokes formula, Reynolds number, and void fraction, they simulated bubbly flow for various shapes of glasses. The Bond number helps determine the shape of the gas bubbles. The Stokes formula is used to estimate velocity. The Reynolds number is used to determine whether you have laminar or turbulent flow and the void fraction estimates the amount of volume that the gas contributes to. It turns out that the shape of the glass matters. If the glass has a wide opening that narrows towards the bottom, the bubbles will sink. If the opening is smaller at the top, like a beer bottle, the bubbles won’t sink.

All of this lead me down memory lane (which seems fitting at the end of the year) as Reynolds numbers and Stokes equations factored heavily in my undergrad studies as a chemical engineer. Again, speaking of rabbit holes, I was fascinated to find that the Navier-Stokes equations are part of the Clay Mathematics Institute seven most important open problems. From the Wiki: The Navier–Stokes equations are also of great interest in a purely mathematical sense. Somewhat surprisingly, given their wide range of practical uses, it has not yet been proven that in three dimensions solutions always exist (existence), or that if they do exist, then they do not contain any singularity (they are smooth). These are called the Navier–Stokes existence and smoothness problems.

Part II: Beer Volcano

Some of you may know the prank where you smack your mate’s open beer bottle with a strong hit and the beer comes gushing out. Now with good craft beer, that would be sacrilegious so I won’t be surprised if my beer pals are not aware of such shenanigans. Again, fluid dynamics tells us that it is due to a mushroom cloud, not unlike that of an atomic bomb.

Rodriguez-Rodriguez et al explain that a compression wave is generated by the sudden vertical impact on the beer bottle and that waves bounce back and forth until damped out. The free surface at the small bottle opening is key to forming the train of expansion/compression waves that drive the bubble implosions. The rapid collapse of the bubbles is called cavitation, which is a serious problem with pump failures.

I better wrap up this post before I go down another rabbit hole. So cheers and a happy New Year to you.

Links to references and more info:

Champagne Science via Rajini Rao

https://plus.google.com/u/0/+RajiniRao/posts/WNRS3NVBHsr

Chemistry of Champagne via Compound Interest

https://plus.google.com/109532635324178687034/posts/acpSfMU9uUb

Sinking Guinness Bubbles via Bryan Jones

https://plus.google.com/+BryanJones1/posts/ZonTTDKWTuy

Why do bubbles in Guinness sink?

http://arxiv.org/abs/1205.5233

Beer, mushroom clouds, science, what more can you ask for?

https://plus.google.com/+ChadHaney/posts/BZvh3hUpByB