Guinness bubbles sinking

Settled in 2013 by Eugene Benilov, Cathal Cummins, William Lee (University of Limerick).

Drinkers have long noticed that bubbles in a settling pint of Guinness appear to drift downward near the glass walls, seemingly defying buoyancy. An earlier 2008 study (Zhang & Xu) began analyzing the flow, and in 2013 Eugene Benilov, Cathal Cummins and William Lee at the University of Limerick gave a fuller quantitative account in the American Journal of Physics: because stout bubbles are unusually small and the traditional glass narrows toward its base, a large-scale circulating current forms that flows downward near the walls faster than the bubbles' own (slow) rise speed, dragging them visibly down, while bubbles in the interior still rise. Follow-up work (e.g., a 2018 AJP study on stout beers generally) refined the picture further, confirming glass shape as the key control parameter.

A pub argument about whether your eyes are playing tricks turns out to have a real, glass-shape-dependent fluid-dynamical answer.

Explanations that circulated

• That the sinking bubbles are purely an optical illusion (a genuinely common belief); high-speed imaging and the cited studies confirm the downward motion near the glass wall is real, driven by a circulating current.
• That any small bubble anywhere in the glass sinks; the effect specifically requires the glass to taper inward toward the base (as a classic pint/tulip stout glass does), which sets up a bulk circulation flowing down near the walls and up through the center.

References

• Why do bubbles in Guinness sink?, E. S. Benilov, C. P. Cummins, W. T. Lee, American Journal of Physics, 2013