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This Week's Experiment - #290 Coffee Rings
Before we hit the road, we always do some housecleaning, so that things will
be neat when we get back home. 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 week's 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.
From Robert Krampf's Science Education Company
PO Box 60982
Jacksonville, FL 32236-0982
904-388-6381
krampf@aol.com
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