Visualize the fluid motion of convection cells. This phenomenon occurs all around us but is usually unseen. For this activity, all you need is soapy water, a heat source, and some food coloring—the effect caused by the rising and sinking fluids is spectacular.
- Aluminum pie pan
- Hot plate or other heat source with variable settings
- Hot pads or oven mitts (not shown)
- Different colors of water-based food coloring
- A small cup of water (not shown)
- Liquid hand soap or shampoo with a pearlescent or metallic appearance (look for glycol stearate, glycol distearate, or glycerol stearate in the ingredients list)— try several varieties as some brands will work better than others
- Several coins of the same denomination (for example, 5 or 6 quarters)
- Safety goggles (not shown)
- Place your pie pan on top of a hot plate that is turned off and cool.
- Fill the pie pan about halfway with tap water. Squeeze in at least 2 tablespoons of liquid soap.
- Using your finger, gently stir together the soap and water, trying not to create bubbles.
- Add a few drops of food coloring to the mixture and stir gently until the color is well mixed. The food coloring will give your solution some contrast and will allow you to more easily see the convection currents and fluid flow.
- Let your soap solution settle in the pan so the surface is relatively still.
- Turn on your hot plate and set it to a low heat level.
Put on the safety goggles. Watch what happens to the liquid in the pan once you turn on the heat. You should start to notice swirling in the soapy water. Don't let the liquid get too hot! When overheated, the stearate molecules will break down, causing the pearlescent luster to vanish and your solution to turn clear.
Try different experiments! As your solution heats up (or cools down), investigate each of the following phenomena:
- Try blowing across the surface of the liquid.
- Put a piece of ice or another cool object in the solution.
- Try adding a few drops of different food coloring along the edges of the pan. What do you observe?
- Use the hot pads or oven mitts to remove the pan from the hot plate (be careful not to burn your fingers!) and place it on a flat, cool surface. Wait and watch what happens. How does the appearance of the liquid change? How long does it take for convection to totally stop?
- To reset your convection experiment, either let the liquid cool down completely or make a new batch.
If you have a few coins handy, wait until both the hot plate and pan are cool, then place the coins in different locations on the heating surface. Then place your pan on top of the coins. Heat will be conducted through the coins, localizing the heat source to a few discrete regions.
- Let the pan cool down and rearrange your coins. What do you notice about the patterns created on the surface of the liquid now?
In general, heating causes a fluid to expand in volume by reducing its density.
At the beginning of this experiment, because you thoroughly mixed the soapy liquid in the pan before heating it, the temperature and density of the soapy liquid is the same throughout. When the pan is heated, the liquid at the bottom of the pan heats up first, becoming less dense. This lighter liquid is pushed upward in localized columns by denser, cooler liquid. When this warmer, less-dense fluid reaches the surface, it cools, becomes heavier, and sinks.
The localized region where the fluid rises and sinks is called a convection cell. Adding food coloring highlights the movement of these convection currents.
When the pan is placed on the hot plate, convection cells rise through the liquid directly above the heating element (or coins, if you used them).
When the warm pan is then placed on a cool surface, the convection process slows. This causes the convection cells to widen and become extremely well defined. This slowing of the convection process allows the solution to calm down enough for darker, curved lines to appear. These dark lines show where the fluid of one or more convection cells is diving downward.
Convection happens in all types of fluids and locations. You can discover it in:
- Liquids: Convection cells occur in liquids great and small, from a cup of coffee to the oceans.
- Gases: The atmosphere and weather are affected by convection. Even the air surrounding your body convects.
- Plasma: At any given time, the surface of the sun has thousands of roiling convection cells as large as the entire earth.
- Inside our planet: The iron core of the earth churns because of convection, creating a protective magnetic field around the earth. The cause of earthquakes can be traced back to the slowly convecting mantle rock far beneath our feet.
Soap manufacturers often change their formulas. If you find a brand of hand soap or shampoo that works well for you, buy it in bulk so that you’ll have a supply for years to come.
If you’re working with younger kids or are reluctant to use an electric heating element, you can try this experiment using alternative heat sources such as a candle, a mug of hot water (see image below), or even the palm of your hand.
To avoid having to mix up fresh soap solution each time—and to minimize mess—you can mix a batch of solution in a culture bottle. These bottles are made of plastic and can’t be placed over direct heat, so you’ll need to try them on an alternative heat source as mentioned above. But the effect is still spectacular!