Coffee Rings

I have been doing some house cleaning. In cleaning the counters, I noticed a coffee spill that had dried. It had formed a dark ring, with almost no coffee stain in the center. This reminded me of an article that I have read on the science behind coffee rings, giving me the idea for this experiment.

You will need:

• a light colored counter top
• a little bit of coffee or tea

Put a few drops of coffee onto the counter in a place where they will not be in the way. Wait about half an hour and then look at them carefully. What do you see? Each drop now has a dark ring around the edge.

Let the drops continue to dry, observing them now and then to see what is happening. Wait until the drops are completely dry. What do you notice? The original dark ring is still there and even darker. The area inside the ring has very little stain. Why?

You probably expected that the entire area would wind up with a fairly even stain, instead of the ring that you got. Lets think about what happened? The liquid part of the drop (water) began to evaporate. There was more of this evaporation at the edges than in the middle. Why? Think about the shape of the drop. In the center, the surface is flat. At the edge, the surface curves down to the counter. There is more surface area in the part of the drop at the edge than in the center. More surface area means more evaporation. The contact between the drop and the counter gives a place for the solid parts of the coffee to be deposited. This deposited coffee is what formed the original ring that you saw.

The ring also gives the liquid coffee a rough spot to stick to. Water is a very sticky substance, and it tends to cling to many things, including dried coffee. Because it sticks to this ring, the drop does not get smaller as the liquid evaporates. Instead of getting smaller, it gets shorter. Since the drop continues to evaporate faster at the edges, more and more of the coffee moves there, making the ring darker. At the end there is only a thin film of coffee left inside the ring, which produces very little stain in the center.

When you finish, be sure to clean the counter. Otherwise, we might both get into trouble for making a mess.

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