If you’ve ever walked with beverages in your hand, you probably know that coffee tens to spill easily, while beer doesn’t. Emilie Dressaire, an assistant professor of mechanical and aerospace engineering at the New York University Polytechnic School of Engineering, believes the secret lies in the foam.

Coffee easily spills due to lack of foam. Image via Glamour.

She came up with the idea to use foam as a liquid dampener when she was served a latte at Starbucks and told she probably would not need a stopper to keep it from spilling. This seemed pretty strange, as she realized that coffee really spills a lot, but latte doesn’t.

“What we observe in our cups of coffee, this happens every time you’re carrying liquids in a container that’s partially filled,” study co-author Emilie Dressaire, an assistant professor of mechanical and aerospace engineering at New York University, told ZME Science.

For her research partner, Alban Sauret, the realization came a bit differently.

“While I was studying for my Ph.D. in the south of France, we were in a pub, and we noticed that when we were carrying a pint of Guinness, which is a very foamy beer, the sloshing almost didn’t happen at all,” he said.

So scientists took their observations from the cafe and the pub to the lab; they built a device to sistematically the damping power of foam. They found that just five layers of foam were enough to decrease the height of the waves by a factor of 10. The foam was also able to dissipate the energy of the sloshing liquid through friction.

Of course, this research isn’t about coffee and beer – the findings could be applied in many areas, including cargo transportation and hydronautics.

“When you think about your cup of coffee, you can see that the motion can get pretty violent,” Dressaire said. “Imagine the same thing but at a much larger scale … you’re going to generate forces against the walls of the container that are going to be really high every time the ship hits a wave. So the motion of the liquid inside the ship can lead to structural damage and it can also can disturb the motion of the ship itself.”

In the meantime, if you’re clumsy and want to drink a beer, you might want to stick to a Guinness – since that’s one of the foamiest bears out there, it’s the least likely to spill over.

“This study has also made me choose my beers differently,” she said.

Read the full article here: Damping of liquid sloshing by foams.