Well, now, finally
some research with some truly practical benefits:
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.
Their experiment involved having a subject carry a full coffee mug while walking in a straight line at different speeds. One set of movements had the walker concentrate on the coffee mug, while another had the walker look straight ahead. The sloshing of the coffee has a natural frequency, which is a function of the size of the mug.
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.
How to avoid a spill? Well, alas, most of us have found through years of trial and error (read: stained clothes and carpets and burned hands) the same things the researchers found: don’t fill the cup to the very brim, walk slower, don’t accelerate abruptly, and always keep your eye on the cup. Or use a cup with a lid. (Note: this latter idea does not always work, especially with ill-fitting lids, as the inside of my car can attest...) But:
the researchers' mathematical model will enable scientists to investigate different cup designs without actually making them. Engineers already know of slosh-control techniques: Tanker trucks contain inner ridges, or baffles, to damp the gasoline's motion, for instance, because too much sloshing could make a truck overturn.
Hopefully someone will tackle one other crucial coffee-related question: why does an overturned coffee cup seem to spill immeasurably more actual liquid than the cup could possibly hold—especially if one’s computer keyboard is nearby?
