of coffee and beet rings..... why ?

Question

ever notice that if you spill black coffee on a counter the out side of the ring is always more cocentrated than the interior.... same thing with beet juice...i always have wondered why ?

concentrated is correct spelling.. sorry

Answer 1

Coffee and juices are mixtures of water and small particles that give the mix flavor. When you spill a liquid on an irregular surface like a tabletop, it spreads out until some imperfection on the surface halts further expansion. The liquid then begins to evaporate at the puddle's thinnest point, the edge. You would think that the puddle would then contract, but the liquid can't overcome gravity, which made it spread out in the first place. Therefore, the solution and the particles flow from the center of the puddle to the edges to replenish what's evaporated. As more of the water evaporates, it deposits the dissolved solids on the edge of the puddle rather than the center. Some scientists at the University of Chicago figured this out and published a paper in the journal "Nature."

Answer 2

this is commonly seen with water-based liquids, even water itself.

it is caused by impurities that are not fully soluble in water "falling" out of solution by going to the surface of that liquid. since water has such a high surface tension, it makes a "bubble" shape. as these impurities "fall" down this bubble, they are concentrated at the edges of the bubble, making the "ring" once it dries that you routinely see.

i tried to make the explanation as simple as possible, i can explain it in more detail if you would like.

thanks for the excellent question.