Why is that? It’s because the edges of the spill are where the fluid is at its thinnest. A spill sitting on a flat surface is something like a pancake. Big in the middle and thin on the rim. As the thin edges evaporate, they are replenished by liquid and particles of coffee from the denser interior. That starts an outward push going in the puddle. The more it dries, the more particles get drawn out to the edges. When all of the liquid has evaporated we’re left with a ring of coffee particles making a stain.

这是为什么？这是因为泄漏的边缘流体在其最薄。泄漏坐在平坦的表面上是像煎饼的东西。在中，大轮辋上薄。由于薄边蒸发，它们是补充液体和颗粒密集的室内咖啡。这开始向外推，在水坑。它干得更会抽出更多的粒子边缘。所有的液体蒸发时，我们剩下的咖啡污渍颗粒环。