Repeat the above using rubbing alcohol in the cup instead of water. Compare the color patterns produced in this case with those produced with the corresponding pens using water. The experiment works even better with filter paper or chromatography paper, but these are more expensive. Black pens are recommended because they usually have many different dyes in their ink. Do not be disappointed if the marker does not produce a dramatic separation pattern.
Water rises up the coffee filter because of a phenomenon called capillary action.
It is the same action which a tree uses to soak water up its trunk and its branches. One can mimic a tree using a thick paper towel and rolling it as a trunk. The role of the water and the rubbing alcohol in this experiment is that of a carrying liquid, because these liquids carry the ink up the coffee filter. The technical term for this is an eluant. The size of the original dot on the paper has no effect on the separation pattern, although the separation is better defined if the dot is smaller.