What You Need:
- Cafeteria Tray (with raised edges)
- Plasticine modeling clay
- Liquid soap
- Food coloring
- Paper towels
- Bucket or basin for waste water
To Do and Observe:
1. Roll clay into long “worms” 1 to 2 centimeters in diameter.
2. Lay the worms out on the tray to produce a narrow valley about 3 to 4 centimeters wide that is closed at on end.
3. Push down on the worms so they form a tight seal with the bottom of the tray but don’t flatten. Clay should form walls.
4. Add water to the tray until it almost reaches the tops of the clay walls. Let the water settle before the next step.
5. Add a drop of food coloring to the tray near the open side of the clay channel. Drop the coloring from a height of about 5 centimeters so that some of the food coloring spreads out slightly on the surface while the rest sinks to the bottom.
6. Dip the toothpick in the liquid soap and touch the end of the toothpick to the water at the end of the canal opposite the dye. Observe what happens.
What's Going On:
When the food coloring is dropped into the water some of it spreads on the surface while the rest of it sinks to the bottom. When you add liquid soap to the water, the surface tension is weakened in one place and the water molecules on the surface immediately began spreading away from the site of the soap. The clay walls channel the flow in one direction. To make up for the water rushing away from the site where the soap was added, a second water current moving in the in the opposite direction forms along the bottom of the tray: a reverse current flows along the bottom to fill the space created when the water on the surface was driven away.
In this activity, shallow trays with raised edges should be used. Large Styrofoam food trays from a supermarket can also be used, but they should be the kind with a smooth surface not the rough texture. Light- colored trays make a better background for seeing surface tension effects.
Depending on the tray, water handling will be a bit of a problem. If you wish to repeat this activity then you must discard the water because of the liquid soap that was applied. It is recommended that you keep a bucket or large wastebasket near by because carrying shallow water-filled trays can be messy.
For Older Children
To extend this activity, once students understand the concepts they can use more clay and different food coloring colors to create a more complex path or patterns to the food coloring. They can also experiment with different materials and observe the results.