The Physics of Spilled Coffee

Posted in : Mechanical Engineering Physics Sociology

Scientists face many obstacles on the path to greater knowledge. But new research suggests how to avoid one of the more common pitfalls: spilled coffee.

“I cannot say for sure if coffee spilling has been detrimental to scientific research to any significant extent,” says study author Rouslan Krechetnikov, a mechanical engineer at the University of California, Santa Barbara. “But it can certainly be disruptive for a train of thought.”

Krechetnikov and his graduate student Hans Mayer decided to investigate coffee spilling at a fluid dynamics conference last year when they watched overburdened participants trying to carry their drinks to and fro. They quickly realized that the physics wasn’t simple. Aside from the mechanics of human walking, which depends on a person’s age, health, and gender, there is the highly involved science of liquid sloshing, which depends on a complex interplay of accelerations, torques, and forces.

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.

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