How coffee and beer could make fuel transport safer

Emilie Dressaire 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. Later, when she 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.

The scientists took their observations from the cafe 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 a commercial dishwashing detergent.

The researchers experimented with two types of movements, either jolting the apparatus with a quick, side-to-side motion or rocking it steadily back and forth. They recorded the resulting waves with a high-speed camera. They found that just five layers of foam were enough to decrease the height of the waves by a factor of 10.

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

While coffee and beer drinkers will appreciate being able to keep their drinks in the cup, the researchers hope their findings could have much greater implications in fields such as the safer transport of liquefied gas in trucks and propellants in rocket engines.

Sloshing can exert considerable pressure forces on the walls of a tanker, which could cause a rupture or disrupt the motion of the vehicle, the researchers say.

The article, Damping of liquid sloshing by foams, has been published in the journal Physics of Fluids.