Ever wondered why it’s so hard to walk with a cup of coffee without spilling? It just so happens that the human stride has almost exactly the right frequency to drive the natural oscillations of coffee, when the fluid is in a typically sized coffee mug. New research shows that the properties of mugs, legs and liquid conspire to cause spills, most often at some point between your seventh and tenth step.
So says a pair of fluid physicists at the University of California at Santa Barbara (UCSB). They investigated the science of sloshing in a new study published in the journal Physical Review Letters E, and calculated the natural frequency at which coffee sloshes back and forth when held in mugs of a variety of sizes, from a dainty espresso cup to a cappuccino behemoth. They found that a normal human gait moves at nearly the same frequency, so each step amplifies the coffee’s heave-ho motion. Stumbling or changing pace — common occurrences when you’re low on caffeine — make matters worse by causing chaos in your cup, increasing the chance of a splash over the rim.
But now, there’s hope. By modeling the fluid and walking dynamics of the situation, and comparing the math with some real-world walking-with-coffee experiments, the UCSB scientists have uncovered a few tips for bleary-eyed coffee cup carriers. . .