Why You're More Likely to Spill Your Cup of Coffee Than a Latte

Most people who make an early morning commute probably know that they're more likely to spill their coffee than their latte. But why does that happen? Scientists have taken a closer look at the physics involved and have found that it all has to do with foam.

"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, one of the researchers, in a news release.

Intrigued by this phenomenon, the researchers took their observations to the lab. They created a device to test the damping power of foam more systematically. They constructed a narrow, rectangular container made of glass which they then filled with a solution of water, glycerol and dishwashing detergent.

By injecting air at a constant flow rate through a needle located at the bottom of the container, they created uniform layers of 3-millimeter-diameter bubbles. The scientists then experimented with jolting the apparatus with a quick, side-to-side motion or rocking it steadily back and forth. Then, they recorded the resulting waves.

In the end, the researchers found that just give layers of foam were enough to decrease the height of the waves by a factor of ten. They believe that the foam dissipates the energy of the sloshing liquid through friction with the sides of the container; the bubbles that do not make contact with the walls of the container do not contribute much to dampening.

The findings don't just have implications for whether you choose to get coffee or a latte, though. It could also have implications for the transport of hazardous liquids like oils and liquefied gas is large tankers.

The findings are published in the journal Physics of Fluids.

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