Why 'foamy' latte doesn't spill like coffee

Scientists have found that just a few layers of bubbles can significantly dampen the sloshing motion of liquid.

Emilie Dressaire, an assistant professor of mechanical and aerospace engineering at the New York University Polytechnic School of Engineering, first thought about foam as a damping mechanism was 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.

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

The problems caused by energetic sloshing go beyond the annoyance of spilled beverages to questions of safety when transporting hazardous liquids like oil and liquefied gas in large tankers. 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 findings are reported in the journal Physics of Fluids.