You know how you spill coffee, you don’t just get a uniform stain? Instead, you get a stain with a dark ring around the edge. In the image to the left, you can see this in the coffee “blotches” (the round rings are caused by the coffee that was stuck to the bottom of the round mug).
It turns out that, as of 10 years ago, this was a question that wasn’t actually well-understood scientifically. So Sid Nagel set out to figure it out. Sid Nagel’s a condensed matter physicist who is famous for setting his mind to variety of interesting fluid problems. In particular, he studies the interesting properties of granular materials (like, why does sand sometimes flow like a fluid and sometimes jam-up like a solid?). I’ll have to write a post just on that sometime in the future!
Anyway, back to coffee rings. The puddle of spilled coffee starts to evaporate. But something interesting happens as it evaporates, Nagel found. If you could look at the coffee particles under a microscope as it dries, you’d see the particles flowing from the center out to the edge. Why’s that? It turns out that the edge can’t “retreat” as it dries… the edge of the drop is stuck where it first falls, for the most part. Here’s why. Say you spilled the coffee on some paper. The paper is a bit rough. The edge of the drop gets “pinned” in place by that roughness. So that means that the drop gets flatter as it dries (rather than shrinking in width).
So as the edge loses liquid by evaporation, liquid from the center of the drop has to flow outward to replenish it. So the coffee particles constantly flow out towards the edge of the drop, the water dries out, and more coffee particles flow to the edge. So when the entire drop is dried out, you have a dark ring around the edge! This isn’t particular to coffee — any dark liquid will do.
Nagel started this research by noticing the interesting behavior of the coffee stains on his countertops!
One neat way to show that this explanation works is to try to keep the drop from evaporating at the edge. You can do this by covering the whole drop except for a little hole at the center. If you do this… no ring! Try it.