Have you ever looked closely on a water droplet? The water molecules at the surface of the droplet like to stick to each other more then they like to stick to air. This forms surface tension: the surface of the water acts like a skin around the droplet. We had fun exploring surface tension at our Girl Scouts meeting today.
We started by placing water droplets on top of a penny with a small pippet. The water surface tension helps in maintaining a large drop of water on top of the penny.
Next, we compared a water droplet to an oil droplet. The water droplet is round and tall, in contrast, the oil droplet spread on the wax paper. Water has stornger surface tension compared to oil. However, detergent weakens the surface tension of water. We dipped a toothpick in liquid dish soap and touched the water droplet, which immediately spread on the wax paper like the oil droplet.
The detergent effect on surface tension can be used to power a boat. We created small paper boats and placed them in a water tray. Dripping water behind the boat had no effect, however a drop of soapy water launched the boat forward at full speed.
A similar effect can be seen by dipping a soapy finger into water cup with glitter, or black pepper. The glitter moves to the side of the cup really fast, like magic. I like to call this experiment the magic finger...
Finally we experimented with the surface tension of milk. We added a couple of drops of 3-4 different food colors into the middle of a plate with milk. Then we touched the middle of the plate with an earbud soaked in detergent. The detergent weakened the surface tension of the milk resulting in a beautiful array of colors on each plate.