Do you have a kid who is interested in science? Here’s an easy experiment on capillary action that even younger students can do. All you need are 7 cups, water, paper towels, and food dye in three colors: red, yellow, and blue. Arrange the cups close together in a line. Fold a paper towel into eighths. Cut off the ends so it fits into two of the cups, like a bridge. Repeat 5 more times so there are enough paper towels to connect all the cups, but do not leave them in the cups yet.
Starting with the first cup, fill every other one with water. Put red dye in the first water-filled cup, yellow in the second, blue in the third, and red in the fourth. Stir the water to make sure the dye is spread evenly throughout the water. Put the paper towels in the cups and wait.
Eventually, the dyed water will spread into the empty cups through the paper towels via capillary action. The colors will make the secondary colors in the empty cups, resulting in a rainbow throughout the row. This is because the water yearns for a state of equilibrium. Capillary action means that the liquid will spread evenly throughout the absorbent material, which is why the water spreads into the empty cups.
Rainbow capillary action is a simple experiment that can be enjoyed by people of all ages. You can experiment further by using a stop watch to find out how long it takes for the capillary action to mix the different colors when you vary the length and thickness of the paper towels. You don’t need expensive supplies to do this and it can be done just about anywhere. The final result is quite satisfying. I enjoyed it and I hope you will too!