William Lee

• Theoretical Mechanics
• Engineering Mathematics
• Scientific Computation

• Sinking bubbles
• Bubble nucleation

See also bubble nucleation in stout beers: link.

One of the mysteries of stout beers is the sinking bubbles observed while a recently poured pint is settling. Our research, arXiv:1205.5233, completes the explanation of this phenomenon. We show that in fact stout beer bubbles may sink or rise: which they do depends entirely on the shape of the glass!

One of the distinctive features of stout beers is the sinking bubbles observed while the head forms in a recently poured pint (Wikipedia, video). Previous experimental and simulation work has demonstrated that this is due to a downwards current close to the edge of the pint glass (the current and bubbles move up in the centre of the glass). The small size of the bubbles found in stout beer allows them to be dragged downwards. Our research explains why this current forms and the role played by the shape of the glass in promoting it. This resolves the final mystery associated with this puzzling phenomenon.

(a,b,c,d) Glass slopes inwards: sinking bubbles. (a) Buoyant bubbles begin to rise. (b) This leaves denser, bubble free, beer at the wall. (c) The heavier beer flows downwards under gravity. (d) Bubbles are dragged downwards by the current. (e,f,g,h) Glass slopes outwards: rising bubbles. (e) Buoyant bubbles rise. (f) Bubbles accumulate at the wall, reducing the density of the beer. (g) The lighter beer rises due to its buoyancy. (h) Bubbles are observed to rise. Larger image.

Simulation of bubbly flow in a pint of stout beer. Bluish colours indicate low bubble concentration and reddish colours indicate high bubble concentration. Note that a region of low bubble concentration forms at the wall of the glass.

A neat way to confirm our theory is to pour stout beer into a tilted measuring cylinder. Sinking bubbles can be seen on the lower surface and rising bubbles on the upper surface.

The team

	The research was led by Prof. Eugene Benilov from the Department of Mathematics and Statistics at the University of Limerick.
	Cathal Cummins is a graduate student at the University of Limerick. Cathal worked on this project while a student on the University of Limerick's taught MSc in Mathematical Modelling.
	William Lee is a junior lecturer in applied mathematics at the University of Limerick and a member of the MACSI research group.

In the media

• Discoblog, Discover Magazine. Why do bubbles in Guinness sink?.
• Physics Central. Best Kind of Fluid Dynamics: Beer.
• Johnathan Gray, Evening Herald, p. 25 (30.5.2012). Mystery of Guinness bubbles is solved.
• Claire O'Connell, Irish Times, p. 12, (31.5.2012). The mystery at the heart of the perfect pint.
• Michael Harper, RedOrbit.com. Fun With Beer And Mathematics.
• Phys.org. Irish mathematicians explain why Guinness bubbles sink.
• Jenny McCudden, Irish sun (30.5.2012) p 14. Guinness fizz riddle gets sunk.
• Jason Palmer, BBC news online. Falling stout bubbles explained.
• MIT Technology Review. Irish Mathematicians Solve The Guinness Sinking Bubble Problem.
• World Science. Cold case solved? Study probes riddle of sinking beer bubbles.

Previous research

• Experimental research on sinking bubbles was carried out by R. J. Zare (Stanford) and A. J. Alexander (Edinburgh). BBC news, Guardian.
• Computational fluid dynamics simulations of sinking bubbles were carried out by C. Fletcher and colleagues (New South Wales). ScienceNOW, Fluent News (pdf, see page 13).
• The effect responsible for the sinking bubbles is known as the Boycott effect and was first observed in the sedimentation of red blood cells. (Video, glitter.)

Acknowledgements

We would like to thank Science Foundation Ireland, SFI for making this research possible by RFP Grant 11/RFP.1/MTH3281 and Mathematics Initiative Grant 06/MI/005 (MACSI).