Foam on latte inhibits sloshing

Los Angeles Times : Black coffee carried in a cup is more likely to slosh over the edge than is a foamy latte. To look at the physics involved, Alban Sauret of the French National Center for Scientific Research and his colleagues filled a small rectangular container with water, glycerol, and Dawn dishwashing liquid. They built up several layers of bubbles by using a needle and a syringe pump. When the container was either jolted or set in a gentle, rocking motion, the researchers observed that the foam on top of the liquid “increases the damping coefficient and reduces the amplitude of the free-surface oscillations,” according to their paper published in the journal Physics of Fluids. The findings could have many large-scale, industrial applications, such as improving the transportation of oil and gas in tanker ships. The researchers note that their findings also apply to beer—a very foamy beer, like Guinness, is much less sloshy.