The Coffee Ring Effect

Why do coffee stains always have a dark ring around the edge? It’s because the water’s surface is curved: it evaporates more quickly near the edges, causing it to flow outward from the middle, carrying coffee particles with it. We cited some early research demonstrating this effect in our 1997 watercolor paper, but now there’s video that actually shows the process happening at a microscopic scale. (Thanks Eric for the link!)

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