When you pour yourself a nice pint of Guinness, there’s only one thing running through your mind, right? As the brew settles, why do the bubbles sink down instead of rising up?!
Okay, so the “Guinness cascade” may not have been your primary concern, but the gravity-defying bubbles did intrigue a few mathematicians at the University of Limerick, who explored how the shape of the Guinness glass affected the flow of bubbles in an article they posted at the pre-print arXiv.
Credit: Alexander & Zare
The Guinness cascade is not a new phenomenon, and a basic explanation already exists. All things being equal, the bubbles of gas in a liquid like soda or beer rise because gravity exerts more force on the denser liquid around them. But it turns out that where the bubbles are in the glass makes a big difference in their behavior. The bubbles near the walls of a container stick to the glass, which drags on them and slows their upward motion. The bubbles in the center of the cup, in contrast, can rise unimpeded. As they move, they exert a slight drag force on the surrounding liquid. This motion forms a column that circulates the beer in the center of the glass upward, while forcing the beer—and the bubbles—along the wall to sink down.
In fact, this effect happens in other liquids as well, but in a glass of Guinness, the cream-colored bubbles stand out particularly clearly against the dark drink.