Why a dried coffee drop leaves a ring: the edge is stuck and the rim dries first
30seconds
Spill coffee, let it dry, and you get a dark ring with a pale middle, not an even stain. The pigment started out spread through the whole drop.
The drop's edge gets stuck to the surface, and the thin rim evaporates fastest. Water flows outward to replace what is lost, dragging coffee particles to the edge and leaving them there. Drag the pinning slider to let the edge slide inward and watch the ring fail.
Deep dive · 2 min
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