Scientists
are concerned about spilled coffee, because long hours of research
require coffee, and a spill can ruin your train of thought, if not your
valuable notes. Mechanical engineer Rouslan Krechetnikov of the the
University of California, Santa Barbara, and graduate student Hans Mayer
did some experiments after noticing coffee being sloshed at a fluid
dynamics conference (of all places).
Back at the lab, Krechetnikov and Mayer set up an
experiment: They asked a person to walk at different speeds along a
straight path with a filled coffee mug in hand. The volunteer did this
in one of two ways-either focusing on the coffee mug, or looking
straight ahead. A camera recorded the person’s motion and the mug’s
trajectory, while a tiny sensor on the mug recorded the instant of
spillage.
A fluid’s back-and-forth movement has a certain natural frequency,
and this is determined by the size of its container. In their paper
published last week in Physical Review E, Krechetnikov and Mayer show
that everyday mug sizes produce natural frequencies that just happen to
match those of a person’s leg movements during walking. This means that
walking alone, without any other interference, is tuned to drive coffee
to oscillate in a mug. But the researchers also found that even small
irregularities in a person’s walking are important: These amplify the
wilder oscillations, or sloshing, which bumps up the chance of a
spillage.
“This is a very cool study,” says Lei Ren, a specialist in the
biomechanics of walking at the University of Manchester in the United
Kingdom. “It reveals the sophisticated interplay between human body
dynamics and the fluid mechanics of spilling coffee.”
Their advice? Don’t walk too fast while carrying coffee, don’t fill
the cup to the brim, and watch what you’re doing.
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