Don't forget to discard used milk.
• Milk (whole or 2%)
• Deep plate or bowl
• Food coloring (red, yellow, green, blue)
• Liquid detergent
• Cotton buds
Pour enough milk in the dinner plate to completely cover the bottom to the depth of about 1/4 inch. Allow the milk to settle.
Add one drop of each of the four colors of food coloring - red, yellow, blue, and green - to the milk. Keep the drops close enough together in the center of the plate of milk.
Find a clean cotton swab for the next part of the experiment. Predict what will happen when you touch the tip of the cotton swab to the center of the milk. It's important not to stir the mix. Just touch it with the tip of the cotton swab. Go ahead and try it. Did anything happen?
Now place a drop of liquid dish soap on the other end of the cotton swab. Place the soapy end of the cotton swab back in the middle of the milk and hold it there for 10 to 15 seconds.
Add another drop of soap to the tip of the cotton swab and try it again. Experiment with placing the cotton swab at different places in the milk.
Notice that the colors in the milk continue to move even when the cotton swab is removed.
What's the Science?
The secret of the bursting colors is the chemistry of that tiny drop of soap. Liquid detergent, because of its amphiphilicity (hydrophilic and hydrophobic ends) and surfactant (reduces surface tension) property interacts with the water and fat molecules in the milk. The soap's hydrophilic (water-loving) end dissolves in water, and its hydrophobic (water-fearing) end attaches to the fat molecules. This is when the fun begins.
The molecules of fat bend, roll, twist, and contort in all directions as the water-fearing end of the soap molecules race around to join up with the fat molecules while the water-loving end chases after the water molecules. During this process, the food coloring molecules are bumped and shoved everywhere. As the soap becomes evenly mixed with the milk, the action slows down and eventually stops.