Sunday, February 10, 2013

Guinness and the Physics of the Bubbly Flow

This month's lead paper in the American Journal of Physics[8] regards a subject that is dear to most grad students hearts, beer.  A team of scientists Limerick, Ireland report on research they did to determine why the bubbles near the wall of a pint glass of Guinness flow down instead of up.  The preprint[1] of the article can be found on arXiv.  The article inoduces fluid mechanics of the physics of the 'bubbly flow'.  In addition to the bubbly flow, the article introduces the 'anti-pint' (picture 1).  One wonders if the  phrase was coined while the research team[4] performed the experimental portion of their work, measuring the settling time of a Guinness Stout.

If you have a fear of fluid mechanics like I did until today, then this article is an excellent gentle introduction to the subject.  In terms of their relation to your Guinness, the article introduces the Reynolds number[2], the ratio of inertial to viscous forces in a fluid flow, and the Bond number[3], a parameter that defines the weight (pun intended), of gravity on the shape of a bubble, and Stokes Formula[6] to determine the drag force on bubbles in the beer.

The article surmises that the downward flow of bubbles is due to the Boycott Effect[5] discovered by Dr. Boycott in 1920.  If you've ever wondered why the doctor in medical dramas holds the test tube of blood at an angle, there's actually a reason.  The tilt influences the flow of sediment, (blood corpuscles in this case), and the settling time is reduced.  An example of this is shown in the video of Guinness in a tube below[7].


1.  The article in question on arXiv:

3.  Bond Number:

4.  About the research team:

5.  Boycott Effect:

7.  The tilted Guiness video:

8.  The AJP article

On waves in Guiness:

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