Do you know why the outer ring is dirty and the middle is clean?

In fact, it is not the outer ring that is dirty, but the whole water drop is dirty (marked)! ! ! The outermost circle is a boundary. I've thought about this question before, and I feel the same as you, but after I observed it, I found that the whole water drop was dirty ~ ~ ~ Maybe many friends didn't find it after reading my answer, but they still didn't believe it. If you don't believe me, you can try to buckle the water drops with your nails ~ you will buckle a whole drop of water one by one incompletely, not just the outer ring. Remember to look at the buckle with reflective effect from the side. Because the water drops are hemispherical when they reach the plane, the impurities in the water are concentrated outside the water drops under the action of gravity and pressure, so the outer ring is dirty and the middle is clean. Coffee ring effect refers to the phenomenon that when a drop of coffee or tea drops on the desktop, its particles will leave a dyed ring on the desktop, and the color of the ring is uneven, and the edge part is deeper than the middle. Physicists at the University of Pennsylvania have discovered the "coffee ring" effect, which is mainly due to the shape and flow direction of liquid stains.