Try this: Swirly bottle

Have you ever seen a toy that contained a liquid and whenever you shook it or moved it, you get a swirling pattern in the liquid? In this week's activity, we will learn how to make our own swirly liquid and find out a bit about how fluids behave.

You will need

• An empty bottle with a cap
• Water
• food colouring
• Liquid hand soap that contains "Glycol Stearate" (not "Glycol Distearate"). This can be a little hard to find. The only one I could find at my local supermarket was called "Soft As Soap".
• A drinking glass
• A teaspoon.

What to do

1. Pour the soap into the bottle until the bottle is about one quarter full.
2. Add a few drops of food colouring.
3. Fill the bottle with water, right up to the top.
4. Put the cap on the bottle and shake until the water and soap are mixed.

There are lots of things you can try doing with your swirly liquid.

• Try moving the bottle around slowly. The water layers should move smoothly over each other. This is called Laminar Flow.
• Move the bottle more qucklyquickly, the layers will start to mix and interact. This is called Turbulent flow.
• Pour some of the liquid into a glass. If you stir it, you will find when you stir it slowly the liquid will flow around the spoon, but if you stir it quickly it will form small whirlpools called vortices (just one is a vortex).
  

What's happening?

Glycol Stearate is the chemical that gives the liquid soap its pearly appearance. When we add it to the water, it helps show up the currents in the water. You will probably find that after a while the glycol stearate will settle to the bottom of the bottle, but if you shake the bottle it will spread it out again.

This liquid will last for years if you keep the bottle sealed, but it's probably a good idea to stick a "Do not drink" label on it if you're going to keep it. It tastes awful.

Applications

Scientists who study fluid dynamics have found that laminar flow, turbulent flow and vortices occur in the air as well. They affect how airplanes get their lift, how much fuel cars use, even how far a golf ball flies. For example, older cars often created large vortices in the air behind them, which slowed the cars down. Modern cars are designed to minimize these vortices, so they don't use a much fuel to go the same speed.