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Bubble Tricks

In this activity students use what they know about bubbles to do some impressive tricks. By experimenting with bubbles, students learn about surface tension, elasticity, minimal surface structures, and how to blow really, really big bubbles!

Bubbles fascinate both children and adults with their beautiful shapes and colours. Such simple ingredients—soap and water—create mesmerizing examples of both geometry and chemistry. By experimenting with bubbles, students learn about surface tension, elasticity, minimal surface structures, and how to blow really, really big bubbles!

Bubble Structure
Bubbles are soap films wrapped around air. Soap films are made from soap and water. The soap film looks like a sandwich with soap as the bread on the outside and water as the filling on the inside.

Soap molecules have two ends, a hydrophobic end and a hydrophilic end. The hydrophobic end avoids water and attaches to oil. The hydrophilic end avoids oil and attaches to water. Soap thus helps attach oily dirt to water so the dirt can be washed away from clothes and dishes.

Surface Tension & Elasticity
Surface tension is the force between liquid molecules at the surface of the liquid (in this case water). The water molecules cling to each other, which causes the surface of the water to behave like an elastic sheet. Elastic substances tend to return to their original shape when they’re stretched.

Water doesn’t make stable, free-floating bubbles all by itself. If you blow a bubble in water, it pops quickly because the surface tension of the water is relatively high and the water is not very stretchy. Adding soap decreases the surface tension so that the water can stay stretched around the bubble.

Even as the bubble is blown up larger or is stretched in various ways, soap stabilizes bubbles by an action known as the Marangoni effect. In places where the bubble is stretched, the surface concentration of soap decreases, which causes the surface tension to increase. This area of increased surface tension springs back to its original form and prevents the bubbles from stretching to the point of popping. The result is spherical bubbles with a uniform surface tension.

Soap also reduces water evaporation so the bubbles last longer, although this effect is relatively small.

Minimal Surface Shapes
When you play with bubbles, you’ll notice that they come in predictable shapes. That’s because deforming a bubble takes energy. The bubble tends to spring back to the shape that is stretched as little as possible – the minimum surface. Floating around in the air, a bubble will become a sphere, because a sphere has the least surface area for a given volume of air. Another way to think about this is that the bubble is stretched most evenly as a sphere (as compared to an egg shape, or a cube). A shape with corners or edges means that the soap is stretched unevenly.

Objectives

  • Use their knowledge of soap films and minimal surface structures to make bubbles of various sizes, shapes, and arrangements.

Materials

  • Per class:
    All Purpose Bubble Solution
    towel or paper towels for cleanup

  • Per student:
    1 small bubble wand
    1 big bubble wand
    1 straw
    1 yogurt container lid

Key Questions

  • What is different about the surface that two bubbles share versus the rest of their surface area?
  • How many different methods can you use to achieve the same final structure?

What To Do

Finger bubbles

  1. Use your hands as a bubble wand. Make a circle with your thumb and index finger, and dunk it into bubble solution.
  2. Try to blow a bubble.

Make a bubble dome

  1. Pour a little bubble solution into a yogurt lid.
  2. Wet one end of a straw, hold it against the wet bubble lid and blow a sitting bubble.
  3. Try to make a bubble dome inside your bubble dome.

Make a bubble caterpillar

  1. Dip a straw and a wand into the bubble solution.
  2. Place the end of the wet straw just below the wand and lightly blow to form a bubble about 5 cm in diameter.
  3. Stick it to the underside of the bubble wand. Use a light touch and quickly take the straw away for best results.
  4. Blow another bubble with the straw. Place the straw lightly beneath the last bubble in the chain and quickly take the straw away.
  5. Continue to add bubbles to make a caterpillar. Be careful—too many bubbles will make the caterpillar fall!

Bubble in a bubble

  1. Make a bubble the size of a volleyball with a big bubble wand and catch it so it hangs underneath the wand.
  2. Wet the end of a straw with bubble solution, blow out the excess from the inside, poke it into the bubble and blow a small bubble.
  3. Pull the straw out quickly and the small bubble should remain in the big bubble.
  4. Try making a bubble in a bubble without using a straw, once you have mastered the above technique. Make a volleyball-sized bubble in the air and once it is close to your mouth, blow sharply with a “puh” sound. With some practice, you can push the surface of the bubble into forming a new bubble.

Extensions

  • Can you devise a way to pierce a bubble with a straw or a pin without popping it?
  • Hint: dip the pin in soap!

Other Resources

Science World | YouTube | Bubbles

Exploratorium | Soap bubbles

David A. Katz | The Chemistry (and a little bit of physics) of Soap Bubbles