We’ve all tried it at one time or another. We accidentally brewed coffee into a mug when we needed the coffee to go, we want to dump two cups into a carafe, our mug has a crack in it so we need to transfer, etc – and every single time, we spill.
Precious, precious coffee is lost, slipping down the outside of the mug and puddling on the counter where it must be soaked up by a rag or a paper towel and not into my bloodstream.
Why is it so hard? Why can’t we learn our lesson???
Science, at least, has the answer to the first question.
It’s because of the Coanda effect – named after Henri Coanda, who tried (and mostly failed) to build a jet-powered aircraft in 1910. But while he didn’t really get off the ground, he did contribute to the body of knowledge on how airplane wings produce lift.
The Coanda effect is this: on a curved surface, a moving stream of fluid will create internal pressure that keeps it moving along that surface.
In aeronautics, this means an airplane wing can generate lift using air as a “fluid.” Faster moving molecules have lower pressure than lower moving molecules, so a jet of air is basically a low-pressure stream surrounded by high-pressure streams – which means the stream of “fluid” stays stuck to the surface, even when it curves.
In the arena of coffee, it means your precious, caffeinated liquid is also experiencing pressure from the ambient air, and as it flows along the surface of your mug, the pressure keeps it stuck, even as if curves around the lip.
The Coanda effect isn’t infinite, though. You can pour the coffee over the lip – it’s just still trying to stay “stuck” while you do it.
That’s why it’s much easier to pour freely out of something like a wine glass, or a spout, or anything with a sharp enough curve to release the liquid on the first attempt.
So, there you go.
And now you have someone to curse at the next time you forget about physics and try to split your already-poured coffee with your significant other.
Just one more reason to keep it all to yourself.