Why a coffee is more likely to spill than a latte

Foam makes liquid more stable. That’s why black coffee is more likely to spill than a latte. CREDIT: Washington Post photo by Sarah L. Voisin.

It doesn’t take a team of physicists to figure out that foamy liquids are less splishy-splashy than those without bubbles, but one team of physicists did try to figure out why. Their work was reported Tuesday in the journal Physics of Fluids.

The study authors, who worked together as part of the complex fluid group at Princeton University, had noticed the phenomenon in the real world: Emilie Dressaire, who’s now an assistant professor at NYU Polytechnic School of Engineering, started thinking about the integrity of foamy liquids when a Starbucks employee told her she wouldn’t need a lid stopper to keep her latte from spilling. Her colleague Alban Sauret, now a researcher at the French National Center for Scientific Research, had recognized the same difference in foamy beers.

When carrying pints of Guinness, he’d learned as a student in Europe, sloshing was minimal. In both cases, it seemed the foamy layer on top of the liquid was to thank for the lack of spillage.

The experiment was what you’d expect: The team filled a glass container with water, dish soap, and glycerol, which is used to make liquids more viscous in the lab. Then they created uniform layers of tiny bubbles into the liquid.

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By rocking the containers back and forth using different motions, they were able to record the behavior of the liquids as they became foamier and foamier. They recorded the tiny waves produced by the shaking with a high-speed camera. Sure enough, their foam made the liquid more stable. Five layers of bubbles lowered the wave height by 10 times compared with plain liquid. But don’t go asking for a latte with extra, extra foam. More than five layers didn’t have much effect.

That’s probably because the foam is diffusing the energy of the slosh by way of friction against the glass container. Layers of foam on top don’t really move or come into contact with the container, so they’re not sharing the energy burden.

“The potential applications are much bigger than just beer,” Sauret said in a statement. The authors have hopes for industrial applications.

“This study demonstrates that a relatively thin layer of foam effectively damps sloshing,” they write in the study. “Our findings suggest that foam could be used in various industrial processes in which sloshing needs to be minimized,” particularly in the transportation of dangerous liquids, such as hazardous waste or oil.