Caught wind of this 2012 article while in discussion about the effect of coffee-slosh and why some people are better at walking with their coffee than others – explanation: plane of motion, pitching angle, gravity, equivalent acceleration…in short, waves, ripples, and other effects.
Each morning, blurry-eyed physicists try to solve a frustratingly complex mechanical problem: how to walk with a full cup of coffee, without letting it slosh over the sides. Writing in Physical Review E, Hans Mayer and Rouslan Krechetnikov at the University of California, Santa Barbara, report their study of the biomechanics of walking with coffee and the factors that lead to spills.
The sloshing of liquid in a cylindrical container, like a mug, is similar to the motion of a pendulum: the natural frequencies of oscillation depend on the liquid’s height and diameter (and, of course, gravity). In a typical mug, 7 cm in diameter and 10 cm tall, the lowest frequency oscillation of the coffee rocking back and forth in the cup is easily excited by walking at a normal pace.
In our busy lives, almost all of us have to walk with a cup of coffee. While often we spill the drink, this familiar phenomenon has never been explored systematically. Here we report on the results of an experimental study of the conditions under which coffee spills for various walking speeds and initial liquid levels in the cup. These observations are analyzed from the dynamical systems and fluid mechanics viewpoints as well as with the help of a model developed here. Particularities of the common cup sizes, the coffee properties, and the biomechanics of walking proved to be responsible for the spilling phenomenon. The studied problem represents an example of the interplay between the complex motion of a cup, due to the biomechanics of a walking individual, and the low-viscosity-liquid dynamics in it.
Read more: ‘Walking with coffee: Why does it spill?‘