Revealed: Why foamy beer doesn't spill over

Love to have a cup of coffee or that pint of foamy beer but do not want it to spill over as you move -- new research may have just revealed how the sloshing can be stopped as you walk along with your mug.

Researchers from the Princeton University have found that just a few layers of bubbles can significantly dampen the sloshing motion of liquid.

The research -- that borrows from simple principles of physics -- has applications far beyond breakfast beverages and could mean safer transportation of liquefied gas in trucks and propellants in rocket engines.

"While I was studying for my PhD in the south of France, we were in a pub and we noticed that when we were carrying a pint of Guinness beer which is a very foamy beer, the sloshing almost did not happen at all," said Alban Sauret, 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 bubbles three-millimetre in diameter.

"The dishwashing foam is very stable which allowed us to conduct the experiments without the bubbles disappearing," said Francois Boulogne, another member of the team.

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 found that just five layers of foam were enough to decrease the height of the waves by a factor of 10.

The foam dissipated the energy of the sloshing liquid through friction with the sides of the container.

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

The 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 concluded in a paper that appeared in the journal Physics of Fluids.