为何普通咖啡比拿铁更易溢出杯子

Carrying a full cup of coffee from the kitchen to the dining room can be precarious for a sleepy-eyed caffeine addict who might accidentally send a wave of java sloshing over the rim. But add a bit of foam to the top and the trip becomes easier. Scientists have found that just a few layers of bubbles can significantly dampen the sloshing motion of liquid. The research, reported in the journal Physics of Fluids, from AIP Publishing, may have applications far beyond breakfast beverages, including the safer 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 and told she probably would not need a stopper to keep it from spilling. When Dressaire began working in the complex fluids group at Princeton University, she learned that her colleagues had noticed a similar phenomenon with a different foamy beverage: beer.

"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," said Alban Sauret, who is currently a researcher at the French National Center for Scientific Research (CNRS).

The scientists took their observations from the coffeehouse and the pub to the laboratory, where they built an apparatus to test the damping power of foam more systematically. They constructed a narrow rectangular container made of glass, which they filled with a solution of water, glycerol (a common substance that increases the fluid viscosity) and the commercial dishwashing detergent Dawn. By injecting air at a constant flow rate through a needle located at the bottom of the rectangular cell, the team created uniform layers of 3-millimeter-diameter bubbles. "The dishwashing foam is very stable, which allowed us to conduct the experiments without the bubbles disappearing," said François Boulogne, another member of the team.