I was preparing my latest favorite work-suitable drink a few minutes ago, and a drop of tea spilled over the side of the mug and ran down to the base. Naturally it immediately spread around the entire base, forming a ring on the desk. It was easily wiped up, but then I thought—why does it always spread around the entire base to form that unmistakable coffee ring?
It occurred to me that it might just be capillary action with the liquid flowing along the V-shaped channel formed by the table and the edge of the mug. Some googling did turn up the fact that ring-shaped coffee stains from single drops are caused by capillary flow: as the drop evaporates, it draws water from the inside.
But the instant ring from the mug? Either it’s something else, or it’s so obvious no one has throught it worth writing about.
Edit: *sigh* Read first, then post. I was just reminded that capillary action specifically refers to fluid moving against gravity. Any other thoughts?