The Pentagon Conducted Revolutionary Research on Why Your Coffee Spills While Walking

Your average master chief petty officer can probably hold a coffee mug on a rowboat in the middle of a hurricane without spilling a drop. Yet, apparently, some in the Pentagon questioned how others could spill coffee by simply walking. So they commissioned the best minds in the defense department to conduct a study and find out why.

At the low, low cost of only $170,000, the Defense Advanced Research Projects Agency (DARPA) studied the issue with researchers at the University of California, Santa Barbara (UCSB) to find out why an uncovered cup of coffee will spill while someone is walking with it. They found that any low-viscosity liquid can spill when subjected to human biomechanics.

In 2012, two fluid physicists at UCSB, H.C. Mayer and Rouslan Krechetnikov, conducted tests to observe the conditions under which coffee spills, with variations in walking speeds, cup sizes and liquid levels in those cups. They also examined the physical properties of the coffee and cups themselves.

While coffee may just appear to be dirty water, it actually acts very differently from water while it’s in a mug. The researchers noted the natural frequency in which coffee sloshes back and forth in a cup, caused by its viscosity, or the friction caused by the thickness of the coffee on a given surface. The frequency of the sloshing in the mug actually matches the natural human gait.

Each step taken while holding a cup of coffee amplifies the coffee’s motion, so sudden stops or stumbles greatly increase the odds of the liquid splashing over the rim. Mystery solved.

The researchers not only uncovered the mystery of why there are brown discolorations on your uniform and how easily they got there, they also offered some science-based solutions to keep your uniform clean or your Navy whites white.

1. Don’t walk fast.

This is a fool’s errand. One might think that moving faster than normal is going to help get them to where they’re going without spilling a drop, as if the coffee isn’t going to realize it’s being moved if they take it by surprise. Except the faster you walk, the more likely it is to reach the oscillation level that will spill the coffee. Instead, walk slower.

2. Don’t take your eye off the ball.

You came to the coffee mess for the coffee, so why would you be paying attention to anything else? Researchers counted the number of steps taken by subjects holding their coffee while having some watch their feet and others watch the cup. They say you have to watch that cup.

They found that watching the cup as they moved increased the average number of steps taken before a spill. One of the authors theorized that watching the cup allowed the holder to adjust their gait to slow the sloshing of the coffee.

3. If you have to move fast, start slow.

Chances are good no one is holding a coffee mug in the middle of a firefight, but there are a lot of reasons we might have to pick up our pace a little. Maybe the commander is walking toward your section. Finance personnel might suddenly remember they forgot to lose your travel voucher. Whatever the reason, if you’re suddenly in a hurry, starting off fast will cause a tidal wave in your mug.

4. Get an unusual cup.

This is America. You don’t have to conform to the standard coffee-mug design. Previous research done on liquid sloshing (for missiles) found that a different shape, a flexible container or a series of rings to act as baffles can suppress the natural coffee slosh and keep you from shelling out even more money at the base dry cleaner.

-- Blake Stilwell can be reached at blake.stilwell@military.com. He can also be found on Twitter @blakestilwell or on LinkedIn.

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