Why does coffee shoot out of the lid of your cup?

Scientists have worked out why coffee spits out of the lid of a disposable cup. Why does it happen so often, asks Harry Low

You're running late for work and you've purchased your coffee in a hurry. Just as you arrive at the office, a jet of hot liquid escapes from the tiny hole in the lid, leaving you with hot beverage residue on your clothes before the day has really started.

This is exactly what happened to Rob Kaczmarek after buying a cup of his favourite caffeinated drink. The marketing director at Convergent Science was intrigued by why the coffee shoots out so far and therefore set about modelling this, initially as a joke for those who enjoy a bit of computational fluid dynamics. It's the design of the lid that's the problem, he explains.

"It happens because of the sloshing of the coffee against the lid, which is kind of unique. At the end of the lid, the hole is right up above that. As the coffee sloshes against the end of the lid, that velocity is amplified and it splashes up through the actual hole."

Not all coffee cups are designed with a hole, of course. Some have lids with a tiny hole and others peel back to reveal a much larger gap, which offsets the shooting jets of hot liquid.

If the sides of the lid were more angled or rounded, this may not happen, says Cambridge University physicist Lisa Jardine-Wright, who has also experienced this problem. "I would think that the angle of the lid would approximately need to counter the angle at which you hold the cup."

The fullness of the cup is another factor, as is the speed at which you walk, she points out. A 2012 study entitled Walking With Coffee: Why Does it Spill? found that most spillages occurred between the seventh and tenth steps in any journey.

The same software is used for other areas of fluid dynamics. This could include predictions for how a flood or forest fire might spread. The technology is also used in Formula 1 and aerospace to improve the performance and efficiency of engines. The hope is this technology could help us solve simple problems like the one outlined here.

Until then, perhaps pack a flask.

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