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The Bartender and the Barista: How Physics Makes Beer Easier to Carry than Coffee

Anyone who has ever carried a tray full of pint glasses without getting their feet wet knows that such a feat is hard work, but perhaps our sympathies should go out to the baristas in coffee shops instead. New research has concluded that carrying coffee without spilling is harder than beer since the foam on the surface of beer dampens sloshing.
Credit: Julius Schorzman via Wikimedia Commons

A team of physicists at Princeton and NYU Polytechnic School of Engineering set up an experiment which jolts three identical pint glasses carrying Guinness, Heineken, and black coffee, and measures the resulting oscillations.

This video is the team's entry to the annual APS Gallery of Fluid Motion competition and explains their whole analysis.

The pint of Guinness, which has the largest head of foam, sloshes the least, while the pint of coffee sloshes so much that it spills over the edge of the tall glass.

To test this more rigorously, the team lead by Howard Stone of Princeton studied the sloshing of liquid in a thin, clear container, with various amounts of foam added on top. Each trial showed the characteristic motion of damped harmonic oscillation, but the amount of time it took for the sloshing to stop depended upon the height of the foam on top.

Thicker foam results in decreased sloshing. Screenshot from the 'Why Beer Does Not Spill' video.

It turns out that foam thicker than about 5 layers of bubbles is enough to damp out nearly all of the motion at the top of the foam. The team was also able to mathematically relate the amount of damping to the height of the foam, as shown in the video.

These very practical results from a simple physics experiment could make things a lot easier for those in the industry of liquid transport. The researchers are presenting their work at next week's APS Division of Fluid Dynamics annual meeting.

But the barista at your local coffee shop may still face challenges — after all, not everyone wants to order a cappuccino. In that case, the coffee sloshing research which won a 2012 Ig Nobel prize might be helpful.

By Tamela Maciel, also known as "pendulum"


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