Why a cappuccino or latte is less likely to spill than a straight black

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A latte or a cappuccino is less likely to spill if the cup is bumped, threatening to send a coffee wave sloshing over the rim.

Scientists have found that a few layers of bubbles significantly dampens the splashing motion of liquid.

The research, reported in the journal Physics of Fluids, may have applications far beyond breakfast, including the transport of liquefied gas in trucks and propellants in rocket engines.

Emilie Dressaire, now an assistant professor of mechanical and aerospace engineering at the New York University Polytechnic School of Engineering, remembers first thinking about foam as a damping mechanism when she was handed a latte at Starbucks.

When Dressaire began working in the complex fluids group at Princeton University she learned that her colleagues had noticed a similar phenomenon with beer, which is also known for its foamy head.

Alban Sauret, now a researcher at the French National Center for Scientific Research (CNRS), said: “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.”

The scientists took their observations to the laboratory where they built an apparatus to test the damping power of foam.

They found that just five layers of foam are enough to decrease the height of the waves from a pump to the cup by a factor of ten.

The team believes that the foam dissipates the energy of the sloshing liquid through friction with the sides of the container.

More than five layers of bubbles did not add much additional damping, because the top layers of foam didn’t really move.

The team also found that bubbles that do not make contact with the walls of the container do not contribute much added damping.

The authors hope their research on foam may one day lead to cheap and easy ways to transport large amounts of fluids with minimal sloshing.

“The potential applications are much bigger than just beer,” Sauret said.

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