We started off with a plate of milk and a small bowl with a dab of dish soap.
Then the kids dropped a few drops of food coloring in the milk. Nothing happened. The colors just stayed in one spot.
But then we dipped some cotton swabs in the dish soap and just gently touched the milk.
It was amazing to see all the colors just scatter!
A beautiful marbling effect formed. We took some white paper and pressed it gently on the milk to transfer the image.
What is happening?
The first concept to learn in this experiment is "Surface Tension":
The molecules in liquids like water and milk have cohesive forces that cause surface tension. When you use a magnifying glass to look at the surface at the edge of water or milk in a clear glass, you will notice that the very edge of the water or milk appears to rise up the side of the glass. The surface tension is actually pulling the water away from the glass inward toward the center of the surface.
In this experiment the food coloring just sits on the surface of the milk because it is less dense than milk. It floats on the surface and the colors do not mix because you did not stir the milk.
The soap contains chemicals called surfactants which reduces the surface tension of the milk. The surface of the milk outside the drop of soap is still higher, so it pulls the surface away from the area where the soap is, which causes the food coloring to stream away from the soap drop.
Another cause of the swirling.....
The soap also contains phosphates and various enzymes. Phosphates are compounds that have phosphorous and can dissolve calcium. The enzymes break down proteins and fats into smaller particles.
In this experiment the small molecules of soap attempt to surround and react with the fats, proteins and calcium in the milk.
The swirling effect in the milk occurs because the fat and other molecules in the milk are so spread out that the soap cannot easily surround it. As the enzymes start to react with the fat, protein, and calcium, the color pigments get pushed around, causing colorful moving patterns.
The surfactants in the soap reduce the surface tension of the milk which allows the food coloring to spread around. Then the enzymes in the soap start to react with the calcium, proteins and fats in the milk, which causes the color pigment from the food coloring to get pushed around.
It's not magic, it's chemistry!