While studying for his Ph.D. in the south of France, Alban Sauret was in a pub, doing some deep thinking about fluid dynamics. Namely, he noticed that his pint of beer spilled much less easily than a glass of water, and stayed relatively still when he jiggled it around.
"We noticed that when we were carrying a pint of Guinness…the sloshing almost didn't happen at all," said Sauret, now a scientist at the French National Center for Scientific Research.
Sauret followed up on the pressing scientific question, and now, thanks to him and some fluid physicists from Princeton University and the École Normale Superieure de Cachan in France, we finally know why beer sloshes less than water. It's the foam!
As it turns out, the frothy substance capping your favorite brew acts as a stopper. When a bartender is handing you a pint of beer, his movement creates tiny waves in your drink. If those wave get too big, you've got spillage on your hands. But foam in the beer helps to dampen those waves, essentially making them shorter, by "grabbing" on to the container's edges. And that's why, the scientists say, foamy drinks like beers and lattes might stand up better to motion than their non-foamy counterparts.
The researchers conducted their experiments using water laced with a bit of glycerol (a thickener) and detergent, which they put in a small container. (That's a big caveat of the experiement, as the dynamics might change in bigger containers, when the amount of foam that touches the edge is much smaller in relation to the overall foam content.) They built a contraption through which they could pump air into that mixture to create layers of tiny bubbles. Then they shook the container to generate waves and captured the motion with a high-speed camera. They found that the foam shortened the height—or amplitude—of the waves by as much as 90 percent.
So, you ask, what's the big deal with spilled beer? Well, transporting liquids like oil and liquified gases is a hugely important, high-stakes process. If these liquids slosh around too much while in their tanker trucks, they could destabilize the trucks or break the containers in which they're stored. Adding some foam to the mixture might help keep liquids in transit from moving around as much, and could save trucking companies a lot of money and potential hassle.
As for you and your bar habits—if can barely stand up straight, the fluid dynamics of foam probably won't help you much. In the sloshing-avoidance department, foam is a poor substitute for sobriety.
Daniela Hernandez is a senior writer at Fusion. She likes science, robots, pugs, and coffee.