Why Do You Spill Coffee While Walking and How You Can Stop It From Happening

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Scientists from the Department of Mechanical Engineering at the University of California, Santa Barbara, thought it would be a great idea to investigate why the hell we spill coffee every time we walk carrying cups. Don't you love science?

Here are their results and solutions.

Their experiment recorded volunteers walking with a gazillion combinations of different coffee cup types filled with different amounts of coffee. The volunteers walked looking at the cups, straight ahead or at the floor, while sensors monitored the liquid in the mugs.


The findings

Observing the videos, the scientists concluded that the coffee's natural frequency and the person's walk were the same, causing the coffee to oscillate. The oscillation would increase the more the person walked, with the spillage always ocurring between the seventh and tenth steps.

In other words: spillage is inevitable given a walking rhythm and a number of steps.


The solutions

But fear not, fellow coffee spillers, because you can take steps to avoid this from happening:

1. Don't fill the cups too much. Leave a gap large enough so the spilling step is beyond your walking distance.
2. Use a larger mug.
3. Walk slower!
4. Watch the mug, not the floor.


So, while the research paper—wrote by mechanical engineer Rouslan Krechetnikov and his graduate student Hans Mayer—starts like this:

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.


After reading their observations and recommendations, the truth is that the paper should have started like this:

Let's face it: Spilling coffee is inevitable and we are just stupid for not taking the obvious measures to stop it.


The research started when Krechetnikov realized that, while they can't say for sure if "coffee spilling has been detrimental to scientific research to any significant extent, it can certainly be disruptive for a train of thought."


Looking at my arm, burned by a cup of boiling hot coffee this weekend, I completely agree with this feeling. [Physical Review via Science Now]

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