Why coffee spills more easily than beer

Beer rarely spills when carrying glasses from the bar to the table, yet coffee does, noticed Alban Sauret at Princeton University (New Scientist, 29 November 2014, www.newscientist.com).

Actually Sauret did a study on which beverages were likely to spill the most to determine why it was so. He mixed water with glycerol and surfactants and blew bubbles to create stable foam on the top of the mixture. The mix allowed Sauret and his researchers to control the size of the bubbles and the thickness of the foam.

The researchers rigged up a tray that would hold several beverage cups and glasses. Then they covered several different liquids with different types of foam, and jolted the tray – to resemble people walking from the bar in a pub to a nearby table. Each episode was filmed to capture the sloshing motion and the videos were analyzed. The researchers noted how the velocity of individual bubbles related to the foam’s ability to dampen down the sloshing motion.

The findings were that just 0.3 centimetres of foam was enough to mitigate most of the liquid from sloshing over the side of the beverage container. With 3 centimetres of foam, the sloshing almost completely stopped.

Different beverages produced different results. Regular coffee without foam sloshed a lot. Café latte with foam sloshed less than regular coffee. Beer with a good head of foam rarely sloshed out of the glass – provided it wasn’t full to the brim. Hence researchers concluded that beer has its own anti-spill mechanism - which is good for beer drinkers in a busy pub.

Sauret thinks the study may have useful applications in keeping hazardous fluids from spilling.