"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." And so begins the abstract for a paper entitled, "Walking with Coffee: Why Does It Spill?" in which researchers in the Department of Mechanical Engineering at the University of California, Santa Barbara set out to determine why, exactly, coffee tends to splash out of your cup no matter how gingerly it's held as you walk from kitchen to table. The answer, researchers discovered, lies in how well you take your coffee in stride.
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As ScienceNow explains, "A fluid's back-and-forth movement has a certain natural frequency, and this is determined by the size of its container." And to determine the frequency of coffee as it swishes in a typical mug, researchers asked test subjects to walk with a cup of coffee, sometimes while staring straight ahead and other times while looking at the mug. Cameras and sensors were set up to keep track of the subjects as they walked, and the coffee as it spilled.
With this grand experiment, they were able to pinpoint the precise fluid dynamics behind the coffee spill: "Particularities of the common cup sizes, the coffee properties, and the biomechanics of walking proved to be responsible for the spilling phenomenon." Or, as ScienceNow explains in layman's terms, "Everyday mug sizes produce natural frequencies that just happen to match those of a person's leg movements during walking." And these frequencies increase with just the smallest amount of "noise," like uneven steps or subtle movements in how the cup is held. These amplifications build and build, until the inevitable happens, and you've got coffee on your neatly pressed sleeve just as you walk into your first meeting of the day.
To minimize spillage, the team essentially suggests that you walk softly and carry a big mug, which you may already have figured out through your own personal failed experiments in trying to run with a hot cup of coffee in hand. Indeed, walking too quickly results in liquid chaos and, thus, a tendency to splash, so walk slowly. And when pouring the coffee, leave a generous space between the coffee and the rim to ensure a safe splash zone within the mug.
The study was conducted mostly for personal satisfaction; as one of the engineers told ABC News, "We just wanted to satisfy our curiosity and, given the results, to share what we learned with the scientific community through peer-reviewed literature." Accordingly, their study was published in the April issue of Physical Review E, listed in the Table of Contents as an "Interdisciplinary Physics" article.