For years, people have observed this phenomenon without giving it a second, or even a first, thought.
Hundreds of less-than-fastidious, coffee sipping scientists have probably glanced at the ring, mumbled something about surface tension and told their lab assistants to clean it up.
But it wasn’t until 1997 that six scientists at the University of Chicago pondered this earthshaking question and published their results in the prestigious international scientific journal Nature for the benefit of all mankind, or at least for those slobs among us who don’t wipe up their spills before they dry.
Here’s what they concluded after producing reams of mathematical calculations, undoubtedly supported by lots of caffeine.
When a coffee puddle finds itself on a flat, level surface, it tends to spread out in all directions. In any given direction, the liquid will stop spreading when it hits a barrier, any slight irregularity in the surface that it can’t cross, such as a microscopic ditch. Depending on where the barriers happen to be, the puddle will take on a certain shape: longer in this direction, shorter in that, like an amoeba.
As evaporation takes place, the puddle will start to dry first where it’s thinnest: at the edges. That would have the effect of making the puddle shrink, pulling its edges back, but it can’t do that because they’re stuck in the ditches. So as water evaporates from the edges, it has to be replenished from somewhere, and the only place it can come from is the interior of the puddle.
Thus, there’s a movement of water from the interior of the puddle to the edges, where it evaporates. That flow of water carries along with it the microscopic brown particles that give coffee its color. The brown particles then find themselves stranded at the edges when the puddle finally runs out of water.
First, clean your kitchen counter; no grease films allowed. If your countertop is light-colored, spill about a quarter-teaspoon (a milliliter) of coffee, black, no sugar, on it and let it dry overnight. You’ll see the brown ring. If your countertop is dark, the effect is much better if you use salt water. Dissolve about half a teaspoon (a few grams) of table salt in half a cup (250 milliliters) of water and make a few quarter-teaspoon (milliliter) puddles on the counter.
When they’re dry, you’ll see white rings of salt. The salt crystals are coarser than the coffee particles, so the rings will be more irregular.