Coffee-spilling explained

Regular readers of the blog might know that I like the following things:

  • learning how things work,
  • coffee, and
  • interesting, quirky science.

So imagine how excited I was when I saw a news release on a paper (Mayer & Krechetnikov 2012 Phys. Rev. E) that explains why we spill coffee as we walk. I have often been the bearer of a ‘spilluccino’ (my dad’s term, not mine!) back to my office – and always just thought I had an unusually bouncy or disruptive walking gait. Turns out, it’s not me, it’s physics!

The spillage of coffee while walking is related to the fluid mechanics of wine-swirling (which I discussed in an earlier post). In coffee, however, there are numerous factors to consider: the biomechanics of the walking gait (and how it differs from person to person), the size and motion of the cup, and the low-viscosity fluid dynamics of coffee. As in, if you walked with a cup full of toothpaste, its high viscosity would reduce its ability to slosh around in the cup as you walked.

The researchers found, via empirical testing of their hypothesis, that people tend to spill their coffee because they accelerate faster than the coffee can deal with. As we begin to walk away from the counter at the cafe, coffee in hand, we accelerate. This sets the coffee on an “initial sloshing amplitude”, which, as the wave hits the opposite side of the cup, pitches back and forth. As we begin to walk faster, the pitching is amplified and becomes unstable, which results in a messy swirling going on in the cup – and if there is insufficient space between the level of coffee and the lip of the cup, voila – a spilluccino. Recommendations for reducing spills included not filling cups to the brim, walking more carefully, having a cup with more flexible sides to suppress the movement and sloshing, or putting a series of baffles in the inner wall of the cup.

So maybe it was my unusually bouncy, rapid gait after all….

Final note: KeepCup is an Australian company that makes really cool reusable coffee cups. They’re environmentally friendly because they reduce waste from takeaway coffee cups and lids. Best of all, they have an excellent lid that, in my experience, keeps the sloshing coffee contained inside the cup as it is walked back to the office. Check them out: http://www.keepcup.com.au/ (I have no affiliation with KeepCup, I just like their cups).

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