Key concepts
Physics
Chemistry
Liquids
Molecules
Surface tension
Soap
 
Introduction
Have you ever made decorative rainbows for Saint Patrick's Day? They can be fun to make using colorful construction paper or other craft supplies. But did you know you can make a simple one with milk, liquid soap or detergent and food coloring? How the rainbow is created by this mixture might surprise you! In this science activity you'll make your own milk rainbow and explore how detergent and surface tension are involved in its creation.
 
Background
In a liquid, the molecules (small individual particles that can have positive and negative charges on their surfaces) can, just like magnets, attract and repel one another. In the case of a water molecule surrounded on all sides by other water molecules there is roughly the same amount of pulling and pushing force. The overall effect is no change. But at the surface, where the water molecules are exposed to air on one side, the water molecules experience more pulls downward, toward the other water molecules below them, than upward toward the air. This causes the water at the surface to contract, minimizing its surface area. This phenomenon is called surface tension.
 
Certain compounds affect water's surface tension: Some are are hydrophilic (or polar), which are attracted to water. Others, however, are hydrophobic (or nonpolar) and repel water. Some compounds have both properties. When such a compound is added to water, the hydrophilic end will try to get close to the water molecules whereas the hydrophobic end will push away from them. This pulling and pushing separates the water molecules from one another and consequently decreases surface tension. Compounds that lower water’s surface tension are known as surfactants.
 
Materials

  • Milk; 1 percent, 2 percent or whole

  • Plate or some other type of shallow dish, such as an aluminum pie tin (Make sure the bottom is flat and not wrinkled.)

  • Red, yellow, green and blue food coloring. Other or fewer colors could also be used to make something other than a four-color rainbow.

  • Cotton swab

  • Liquid dish soap or liquid laundry detergent

  • Paper towels  (useful for cleaning up afterward)

 
Preparation

  • Because this activity involves dyed milk, it is best to do the activity outdoors or on a surface that can safely have dye spilled on it, in case an accident occurs. Doing this activity near a sink or hose makes it easier to clean up at the end.

 
Procedure

  • Slowly pour enough milk into the plate so that the bottom is completely covered. Allow the milk to settle for a moment.

  • Add one drop of each color of food coloring to the milk, forming a horizontal line of drops near the bottom of the plate. To make a simple, four-color rainbow, add the colors in the following order, going from left to right: red, yellow, green then blue. How do you think the drops will become a rainbow?

  • Touch one end of the cotton swab right below the middle of the line of drops, on the plain milk. What happens?

  • Take the other end of the cotton swab and rub some liquid dish soap on it. Touch the soap-covered end to the same area as before, right below the middle of the line of drops, on the plain milk. What happens this time?

  • Try touching a few other areas of the milk with the soap-covered cotton swab end. What happens as you continue to touch the milk in other places?

  • Hold the soap-covered cotton swab end in one spot for a few seconds. What happens? If you hold the swab in the milk long enough, do you not longer see the same effect?

  • Extra: Try this activity with milks that have different amounts of fat (for example, nonfat, 1 percent, 2 percent, whole, etcetera). How do the results vary when using different types of milk?

  • Extra: You should have seen a simple, four-color rainbow in this activity but you can also try to make a more complex, seven-color rainbow. To do this you will need to mix together some food colorings to make orange, indigo and violet, and then add them to the line of drops on the plate. For example, you could try mixing one drop of red and one drop of yellow food coloring to make orange. Can you make a seven-color rainbow this way?

  • Extra: This activity used milk, but you could try other liquids, too, such as water, clear soda and melted butter. Did you get the same result using liquids other than milk?

 
Observations and results
Did you see the food coloring move away from the soap-covered cotton swab when you touched it to the milk, making a four-color rainbow streak across the plate?
 
When you touched the plain cotton swab (without soap) to the milk, you should not have seen any effect. When you touched the soap-covered end to the milk, however, you should have seen the food coloring quickly move away from the cotton swab. Because a line of food coloring drops was made in the milk, putting the cotton swab below the middle of the line should have made the food coloring form a four-color rainbow as it all streaked and swirled up the plate, moving away from the cotton swab.
 
Detergent, such as liquid dish soap, is mostly surfactants. These can lower the surface tension of water as well as milk, which contains water and molecules of fat. Surfactants have a hydrophilic part that wants to interact with the water and a hydrophobic part that wants to interact with the fat molecules. Because of this, when the cotton swab with soap touched the milk, the soap separated the fat from the water in the milk, dissolving the fat (which is how soap cleans greasy, dirty dishes). This also decreased the milk's surface tension. As the soap spread out from the cotton swab, it decreased the milk's surface tension around it, and the higher surface tension surrounding this area pulled the milk (along with its food coloring) toward it. If enough soap is added, however, the soap and milk become evenly mixed and the milk (and food coloring) no longer move when more soap is added.
 
Cleanup
Do not drink the dyed milk because it has soap in it. Instead, carefully pour the dyed milk down directly down the sink drain, trying not to splash the sink itself. (You should also run water as you pour.) If you need to move the plate to a sink to do this, it is easiest to add a few paper towels to the plate to soak up some of the dyed milk (and help prevent spills) before attempting to move the plate.
 
More to explore
Introduction to Surface Tension, from Massachusetts Institute of Technology
Laundry Science 101: Surfactants, from Eco Nuts
Magic Milk (pdf), from Sarah Mater and Lisa Hurlbut, Nipissing University
Fun, Science Activities for You and Your Family, from Science Buddies
Build a Raft Powered by Surface Tension, from Science Buddies

This activity brought to you in partnership with Science Buddies