Science Spotlight

Surface Tension

Take a moment to imagine a warm summer Vermont day – they’re coming soon! You have the luxury of jumping into a pool to cool off. As you hit the surface it feels kind of hard but you splash in and sink right down. Sometimes if you jump just right, the splash is minimal, but other times you create a wave of water! This is all because of surface tension. Surface tension is caused by the molecules in the liquid being attracted to one another. In most of the liquid, molecules are being pulled up, down, and both sides. This neutralizes the forces. The surface of a liquid has molecules pulling in many directions, but because they are at the top there is no force pushing them down. This causes the molecules to pull in on themselves, creating internal pressure and forcing the surface of the liquid to contract. When you jump into a pool, you are breaking up the tension of the molecules. That is why you feel an initial impact. Try this experiment below to see surface tension in action!

Materials: Plate, milk, food coloring, dish soap, cotton swab.

Directions: Pour some milk onto the surface of the plate. Add some drops of food coloring to the center. Take a cotton swab and put a dab of dish soap on the swab. Stick the swab in the middle of a drop of food coloring. What happens?

How It Works: Surface tension is caused by the force of each of the molecules in the milk pushing against each other. When the dish soap is added, it binds with the fat molecules and reduces the surface tension of the milk. The molecules away from the cotton swab have higher surface tension and pull the milk and the food coloring towards them. This is how you created beautiful art!

Want more? Research it! Does this experiment work with other liquids? Why?