Put surface tension to the test in this surprising experiment.
You will need
- Baking dish
- Detergent
- Glycerine or glycerol
- Tablespoon
- Water
- 30-centimetre-long piece of craft wire
- Paper straw
Safety
Ask an adult to cut your 30-centimetre-long piece of craft wire. Be careful of the sharp ends of this wire.
What to do
Bend the 30-centimetre-long piece of craft wire to form a loop roughly 10 centimetres in diameter. This doesn’t need to be a perfect circle – in fact, making it a slightly elongated circle or even a rectangular shape works well for this demonstration.
Half fill a baking dish with cool tap water.
Add a tablespoon of glycerine and tablespoon of detergent to the water in the baking dish. Lightly stir it around to mix the glycerine and dishwashing liquid through the water without making too much foam.
Try making some bubbles using your wire loop. If the bubbles don’t form inside the loop, add a little more detergent and glycerine.
Lightly wet the paper straw in the bubble mixture.
Hold the wire loop with a bubble inside of it and place the paper straw across the centre. It should come into contact with the bubble without popping it.
Use a dry finger to pop one side of the bubble inside the wire loop. What happens to the straw?
What’s happening?
When one half of the bubble inside the wire loop pops, the bubble on the other side shrinks, dragging the paper straw across the wire loop. This tells us something amazing about bubbles!
Zoom in on a puddle of water, you’d see molecules acting a little like a large number of V-shaped magnets. Each ‘north’ pole of a molecular magnet loosely sticks to one of the ‘south’ poles of another, forming a big sticky mass of water. The molecules on the top of this puddle cling hard to one another, creating what is known as surface tension.
Mixing detergent and glycerine through the water is like inserting a small rubber band between each magnet, stretching the surface tension and allowing the water molecule ‘magnets’ to move about more.
Making a bubble stretches the surface between the sides of the wire loop until it is as tight as it goes without breaking. Adding the wet straw gives the water molecules on the bubble’s surface something loose to grab onto. When one side of the bubble pops, the stretched surface tension keeps pulling, dragging the straw with it.