My daughter is currently obsessed with rainbows, so STEM rainbow activities are extremely exciting for her. Therefore, she was super happy to learn that we are going to grow a pretty rainbow on a piece of paper towel!
Capillary action allows the colors of the rainbow to form a rainbow right before your eyes. Let’s explore this science phenomenon that looks almost like magic!
How to Make a Rainbow on Paper Towel
- Fold the paper towel in half.
- Cut off about 1/3 of the paper towel. Save it for later.
- Draw the rainbow colors on one end of the paper towel in rectangular blocks. Make sure to go over the colors a few times with the markers so there is enough dye to travel up the paper towel.
- Repeat the same on the other end. Make sure the colors line up on both ends.
- Pour water into the two glasses until they are about 3/4 full.
- Place the two ends of the paper towels into the cups. 1/2 of the rainbow blocks should be in the water. Do not fully submerge the entire colored portion of the paper towel in the water.
- Watch the colors travel up the paper towel!
How long it takes for the color to travel up the paper towel will depend on how much dye you used and the absorbancy of the paper towel. We used Bounty and the colors climbed up the paper towel right away. However, it took about 5-10 more minutes until all the white space disappeared.
If you don’t cut the paper towel, it’s possible that the colors don’t travel all the way to meet in the middle. That would be a bummer to have a broken rainbow!
Paper towels are in short supply these days, so let’s not waste the ⅓ that you cut off. You can also color it with rainbow rectangular blocks just like you did with the other section, but just on one end. Then simply dip the end of the paper towel in the water while holding it vertically and watch the colors climb upward!
Another thing we noticed was that the dye from the markers didn’t spread and disperse right away in water. The colors almost looked like the northern lights in the water – how pretty!
Science Behind the Paper Towel Rainbow
Similar to what we saw in the rainbow walking water experiment, the capillary action is what causes the water to move up the paper towel.
Water adheres to the walls of the small vessels in the paper towel, causing an upward force on the liquid at the edge. In simpler terms, capillary action allows liquid to move through or along the surface of another material (the wall) in spite of other forces such as gravity.
Capillary action happens all around us and helps us with everyday life. For example, when you dry yourself with a towel after a shower, the towel absorbs the water from your body by using capillary action.
Plants and trees use capillary action to live. Water is absorbed by the roots, and capillary action allows the water to travel up the plant. I used this to explain to my kids why when we water plants, we should water the water at the base near the roots instead of sprinkling water at the leaves.
Final Thoughts on the Growing Rainbow on Paper Towel
Demonstrating capillary action with markers and paper towels is not only fun, but also very easy to do. It’s something to keep in your back pocket for days when your kids are driving you crazy complaining they are bored.
For more rainbow activities, check out: