Drip some drops of food coloring. If you’re using a number of food colorings, trickle them close together but far enough so they don’t mix.
Place a drop of liquid dish soap on one end of the cotton swab.
Now, gently place the soapy end of the cotton swab in the mix of milk and food colorings.
What’s the science behind this?
The liquid molecules on the surface of the milk (as on every other liquid) are strongly attracted to each other, thereby creating a phenomenon called surface tension - a thin, invisible “crust” on the liquid’s upper surface. The food coloring we’ve added sits on top of this crust.
Once we add dish soap to the milk, the dish soap spreads on top of the liquid to create a new crust. The droplets of food coloring are “pushed off” the droplets of dish soap and gradually a new, clear crust of dish soap spreads over the surface. This clear, translucent crust allows us to see the original color of the milk without the food colorings that have been pushed aside.
Every time the dish soap comes into contact with the milk, a colorful swirling motion is created. This happens because the dish soap bonds strongly with the fat molecules in the milk, and the fat begins to break up. This exciting color dance is visible until the dish soap covers the milk entirely, and all fat molecules have been dissolved.
Now try this!
Surface tension can also be seen like this:
Fill a small bowl or glass with water. Place a small square of paper towel on the surface of the water, and place a paperclip in the center of the paper square. Now, with a cotton swab, gently push the paper towel down into the water. Watch how the paperclip floats! The paperclip floats due to surface tension.
The small square of paper towel is there to help you place the paperclip on the surface of the water. Try repeating the experiment without it, and you’ll find that, in most cases, the previously easy task turns into quite a challenge. You’ll find that the paperclip sinks vertically in the water or that your fingers accidentally touch the water, thereby breaking the surface tension and sinking the paper clip.