I came across a paper that looked at the vexing question of why we so often spill coffee when walking with a cup of it, and what might be done to prevent it. Of course, coffee shops give us lids but quite often we use actual mugs that do not come with lids. The research was stimulated when a physicist at a conference watched his colleagues walk gingerly while holding cups of coffee and started wondering about it.
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.
So how does one avoid a spill? Krechetnikov and Mayer’s answers may not come as a big surprise. Starting your walk slower—that is, accelerating less—will help. So will leaving a decent gap between the top of the coffee and the mug’s rim; this should be at least one-eighth of the mug’s diameter-for a normal mug, about a centimeter should do it. But the researchers’ “take home” advice is to look at what you’re doing—so long as your mug isn’t filled too high, a watched mug almost guarantees a clean run.
Who says that physics research does not produce useful results?