Why does spilled coffee leave a ring?

I’m not talking about coffee you somehow manage to get stuck on the bottom of your cup, but that coffee in itself dries weirdly. The outlines of spilled coffee are always darker than the inner edges or its center. Surprisingly, how this happens or why has been left unanswered by science for years until recently.  What they discovered is how to control the physics behind coffee rings, which is the angle coffee droplets are placed on a surface, to the point they could stop coffee rings entirely.

Coffee particles seen microscopically appear to bundle at the edges of where it is spilled.

Why so much research in a seemingly pointless phenomenon? Well, because science. Nothing is ever totally pointless in science and can always have profound consequences; afterall, penicillin, was seemingly discovered by accident. Anyways, the science behind coffee rings would supposedly help in the field of blood diagnostics, blood diseases and anemia.

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