The mystery at the heart of the perfect pint

:HERE'S SOMETHING to contemplate as you wait for a pint to settle. Why do Guinness bubbles sink? Research from the University of Limerick (UL) is helping to answer the question – and it seems that the shape of the glass is a key factor.

“The phenomenon of sinking bubbles has intrigued us since we regularly drink Guinness on a Friday evening after maths seminars,” says researcher Dr William Lee from the UL Mathematics Applications Consortium for Science and Industry.

“Although we were aware of previous work in the area my colleague Prof Eugene Benilov pointed out there was a gap in the theory. While it was understood that the sinking bubbles were due to the small size of the bubbles and a circulation in the glass, there was no explanation of the origin of the current.”

To fill in the theoretical gaps, the researchers used computational fluid dynamics and insight from a field trip to the pub with a measuring cylinder to find out the density of a stout beer. "One of our colleagues tilted the cylinder when it was full of settling stout and we saw that the bubbles went down on the lower face and up on the higher face," says Lee. "That told us that the slope of the glass was the key factor." The findings, in a paper by Lee, Benilov and Cathal Cummins at arXiv.org, point to the typical narrow-ended pint-glass shape as having an important role to play in the sinking-bubbles phenomenon.

Lee says he would be interested in seeing whether it is possible to redesign a pint glass to reduce the settling time. There could be wider implications for this. "More generally, a lot of industrial processes involve bubbly flows so the research may find applications there." So while waiting for your pint to settle, check out this website for more details: ul.ie/wlee/sinking_bubbles.html.