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].



References:

1.  The article in question on arXiv:
http://arxiv.org/abs/1205.5233



3.  Bond Number:
http://en.wikipedia.org/wiki/E%C3%B6tv%C3%B6s_number

4.  About the research team:
http://www3.ul.ie/wlee/sinking_bubbles.html

5.  Boycott Effect:
http://www.nature.com/nature/journal/v104/n2621/abs/104532b0.html

7.  The tilted Guiness video:
http://www.youtube.com/watch?feature=player_embedded&v=2_V2MRNlZOs

8.  The AJP article
http://ajp.aapt.org/resource/1/ajpias/v81/i2/p88_s1

On waves in Guiness:
http://www.chem.ed.ac.uk/guinness/waves.pdf


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