Make a Milk Rainbow
IntroductionHave you ever seen a rainbow after a rainfall, or drawn a rainbow for fun? They can be fun to make using colorful markers or other craft supplies. But did you know you can actually make a simple rainbow using milk, liquid detergent (i.e., soap), and food coloring? How the rainbow is created by this mixture might surprise you! In this science activity, you will make your own milk rainbow and explore how detergent and surface tension are involved in its creation.
- Milk; 1%, 2% or whole. Avoid using non-fat milk.
- Red, yellow, green, and blue food coloring if you want to make a rainbow. Or substitute any colors you want.
- Liquid dish soap or liquid laundry detergent
- A plate or some other type of shallow dish, such as an aluminum pie tin. Make sure the bottom is flat and not wrinkled.
- Cotton swabs
- Paper towels are useful for cleaning up afterwards
- Because this activity involves dyed milk, it is best to do the activity outdoors or on a surface where it is easy to clean up food coloring and spilled milk. Doing this activity near a sink makes it easier to clean up at the end.
- 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 and 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. You can even try adding more soap to the end of the cotton swab.What happens? If you hold the swab in the milk long enough, do you no longer see the same effect?
When you touched the plain cotton swab (without soap) to the milk, you should not have seen any effect. However, when you touched the soap-covered end to the milk, you should have seen the food coloring quickly move away from the cotton swab. Putting the soap-covered cotton swab below the line of food coloring should have made the colors form a four-color rainbow as it all streaked and swirled up the plate, moving away from the soapy cotton swab.
This happens because the soap changes the surface tension of the milk, and this change in surface tension causes the food coloring to move. Specifically, the soap lowers the surface tension of the milk, and the food coloring moves towards an area in the milk with higher surface tension.
In a liquid, the molecules (small individual particles) are pulling and pushing away from each other, just like magnets both attract and repel each other. In the case of a water molecule surrounded on all sides by other water molecules, every pulling force is balanced by a pushing force. The overall effect is no change. But at the surface, where air and water meet, 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 tendency is called surface tension.
Some things affect surface tension. For example, some chemicals have a part that wants to interact with the water (called a hydrophilic, or polar, part), as well as a part that does not want to interact with the water (called a hydrophobic, or non-polar, part). When such a chemical is added to water, one part of it will try to get close to the water while the other part will push away from the water. This pulling and pushing separates the water molecules from each other. With the water molecules a further distance from each other they can not contract as much at the surface. Overall this means that the surface tension decreases. Chemicals that lower the surface tension of water are known as surfactants.
Detergent, like liquid dish soap, is mostly made of surfactants. Milk is mostly made up of water with some fat. The hydrophilic (water loving) part of the soap wants to interact with the water in the milk, while the hydrophobic part of the soap (the part that doesn't like water) wants to interact with the fat in the milk. Because of this, in this activity when the cotton swab with soap touches the milk, the soap separates the fat from the water. The fat dissolves when in contact with the soap (which is how soap cleans greasy, dirty dishes). Meanwhile, as the soap spreads out from the cotton swab, it decreases the milk's surface tension around it, and the higher surface tension that is further from this area actually pulls the milk (along with its food coloring) towards it. If enough soap is added, the soap and milk become evenly mixed and this is why the milk (and food coloring) no longer move when much more soap is added.
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For Further Exploration
- Try this activity with milks that have different amounts of fat (e.g., non-fat, 1%, 2%, whole, etc.). How does the result vary when using different types of milk?
- While you should have seen a simple, four-color rainbow in this activity, you could 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 when you make them on the plate. For example, you could try mixing one drop of red food coloring and one drop of yellow food coloring to make orange. Can you make a seven-color rainbow this way?
- This activity used milk, but you could try other liquids too, such as water, clear soda and melted butter. Can you get the same result using liquids other than milk?