Next time you spill a little, let the coffee dry before wiping it up. You’ll notice that after it dries there is a darkish ring around the very edge of the spill, whereas the middle disappears almost entirely. What’s going on here? Robert Deegan and Sydney Nagel at the University of Chicago looked into this phenomenon, and they discovered something pretty interesting. The coffee puddle leaves a ring because it doesn’t simply dry where it sits. As the spill begins to dry, it actually creates its own outward motion, drawing the insides of the puddle toward the edges.

下一次你洒出一点咖啡，在擦掉前让咖啡变干。你会注意到，咖啡干后在溢出的边缘处有一圈浅黑的边，而中间的咖啡几乎全挥发了。这到底是怎么回事？芝加哥大学的罗Robert Deegan 和Sydney Nagel研究了这一现象，他们发现了一些非常有趣的事。咖啡滴不是简单地挥发掉，它实际上产生了自身的向外运动，将水滴中间的咖啡推向边缘。