Many kinds of stains are darker at the edges. It's because the capillary action slows down there. The area of the stain increases in proportion to r^2, and ther

Many kinds of stains are darker at the edges. It's because the capillary action slows down there. The area of the stain increases in proportion to r^2, and there is evaporation also. Thus the stain only spreads to a certain size. As the solvent thins out, it's not able to carry the pigment quickly, and so the pigment particles pack closer together. Pigment is still arriving from the center of the stain, but not moving farther out any more, so it has to accumulate.