Coffee spilling while walking

Settled in 2012 by Hans Mayer, Rouslan Krechetnikov (UC Santa Barbara).

Nearly everyone who has carried a full mug of coffee across a room has spilled it, but the phenomenon was not studied rigorously until Hans Mayer and Rouslan Krechetnikov at UC Santa Barbara did so, publishing in Physical Review E in 2012 (work that also won an Ig Nobel Prize). By combining biomechanics of walking, cup geometry and sloshing dynamics, they showed spilling typically results from a resonant coupling: the natural frequency of the coffee's surface oscillation is close to the frequency of a walker's arm/torso motion, so small perturbations grow over about 10 steps until the liquid crests the rim. Their model also suggested practical countermeasures, like watching the cup or walking backward, that damp the resonance.

A universal minor domestic disaster turns out to be a textbook case of mechanical resonance between your gait and your coffee.

Explanations that circulated

• That spilling is simply a matter of how full the cup is or how fast one walks in isolation; the 2012 study showed it is specifically a resonance problem, where the natural sloshing frequency of the coffee (set by cup size/liquid depth) can lock onto the frequency of the walker's own hand and gait oscillations, amplifying sloshing over a handful of steps.

References

• Walking with coffee: Why does it spill?, H. C. Mayer, R. Krechetnikov, Physical Review E, 2012