Convection Cells

Adapted from the book Exploratorium Snacks, available from the Exploratorium.

Create and observe the formation of convection cells in a solution of soap and water.

Materials

• 2 aluminum pie pans
• 1 cup of water
• 1 cup liquid hand soap that has glycol stearate, glycol disterate or glycerol stearate as one of the ingredients (this ingredient is essential!)
• 1 bottle of food coloring
• Measuring cup
• Small stirring implement
• Heat source (the best is an electric warming tray; or an electric heating pad covered with plastic, a hot water bottle, or a hot plate on a very low setting --DO NOT EXPERIMENT DIRECTLY ON A STOVE TOP)

Procedure:

1. Place one pie pan on top of the heat source (on the lowest setting).
2. Place the other pie pan nearby on an unheated surface.
3. Fill each pie pan with 1/2 cup of water.
4. Add 1/2 cup of liquid soap pouring very slowly. Try not to form bubbles.
5. Mix the soap gently into the water. Try not to form bubbles.
6. Place four drops of food coloring into each solution. DO NOT MIX THE FOOD COLORING INTO THE SOLUTION.
7. Observe and compare both solutions for at least three minutes.
8. After three or four minutes slowly and carefully remove the warm solution from the heat source. Observe it for an additional minute or two.

What to look for

• Is there movement in both the cool solution and the warm one?
• How would you describe the movement in the warm solution?Look closely at the location of the movement. What appears on the surface? What happens toward the perimeter? What happens to the warm solution when you remove it from the heat source?
• What patterns form?

What is happening

Not much in the cool pie pan! In the warmer one the soap solution at the bottom of the pan is heating up and becoming less dense. It rises in localized columns. When it reaches the surface, it cools, becomes heavier and sinks. The region where the solution rises and sinks is called a convection cell. When you remove the solution from the heat source the process slows and allows the convection cells to widen and gain definition.