Science looks at why we spill our coffee

(EndPlay Staff Reports) - Why do we spill our coffee so easily?

Scientists have discovered that the sloshing of liquid is largely caused by how a person walks, which drives the coffee in a cup or mug to oscillate and eventually spill.

The paper published last week by University of California at Santa Barbara researchers found that common mug sizes produce natural frequencies, which just happen to match those of a person's leg movements during walking. Small irregularities in how a person walks can amplify the wilder sloshing of coffee, increasing the chance it will spill, reported the journal Science .

In other words, the properties of mugs, legs and liquid work together to cause spills. This happens most often between your seventh and 10th step, found lead author Rouslan Krechetnikov, a mechanical engineer at the UC Santa Barbara, and his graduate student, Hans Mayer.

You can reduce the chances of spilling your coffee by watching your cup, not your feet; by starting your walk slower; and leaving a decent amount of space between the top of your coffee and the lip of your mug, about one-eighth of the mug's diameter, said the researchers.

They also note that it is scientifically flawed to attempt to walk quickly with your cup, as if you are trying to reach your destination before the sloshing java waves reach a dangerous height.

Actually, the study said, the faster you walk, the closer your gait comes to the natural sloshing frequency of coffee, according to Discovery .

"This is a very cool study," Lei Ren, a specialist in the biomechanics of walking at the University of Manchester in the United Kingdom, said in Science.com. "It reveals the sophisticated interplay between human body dynamics and the fluid mechanics of spilling coffee."

The study was published in Physical Review E.

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