Here is another really simple surface tension demonstration that I found out about from Ben Sill. It can be done on an overhead projector (OHP) or just with a streaming video camera. The projection technology is not central.
- A Ping-Pong ball
- A cup of water (or a round glass bowl if using an OHP).
- Some dish-soap
- Food coloring (optional)
- A student volunteer
- Fill the cup with water and let it settle so that it is not moving
- Ask the student to place the ping-pong ball in the center of the cup. This should be very difficult as it will quickly be drawn to the side of the cup.
- (optional flow visualization) Put a small drop of food coloring on the ball and place it drop down in the water. You should see some of the food coloring fall down (it is denser than water) and some get pushed out away from the ball near the surface (see analysis section below).
- Put a little of the dish soap on the surface of the water and then place the ball there. It will still eventually move off center but a lot more slowly.
Placing the Ping-Pong ball at the surface of the water creates curvature in the water surface all the way around the ball. The surface tension will act to minimize the total surface area. Unless the ball is placed exactly in the middle of the cup (impossible) there will be an asymmetry in the surface curvature that will draw the ball toward the side nearest to the ball, thus minimizing the total surface area.
When the soap is added the surface tension is reduced (soap is a surfactant) and the path to minimum surface area is less steep. As such, the ball will drift much more slowly such that it appears to be stable in the center of the cup. For a more formal discussion of surface tension see John Bush’s lecture notes here.
An index of all the demonstrations posted on this blog can be found here. Don’t forget to follow @nbkaye on twitter for updates to this blog. If you have a demonstration that you use in class that you would like to share on this blog please email me (firstname.lastname@example.org). I also welcome comments (through the comments section or via email) on improving the demonstrations.