Matchstick Boats and Surface Tension

Build matchstick boats that will zip across a water surface, just powered by soap.

What you need

What to Do

Fill the bowl with water just a cm or so is fine

Gently float the matchstick on the water.

Place a tiny drop of washing up liquid near the matchstick, it is easiest to do this with another matchstick.

What may Happen

You should see that the matchstick zips away from the washing up liquid as soon as you drop the washing up liquid on the water. 

What is going on?

This all comes down to surface tension. If you imagine that you're a water molecule right in the centre of a tank of water, water molecules have an attractive force between them so you'd be being pulled in every direction by all the water molecules around you, all these forces would cancel out so you don't move.

Now if you imagine that you're a water molecule at the surface; you'll be being pulled in every direction from the side and from below but you're not being pulled from the top at all because you've just go air above you there. So what would be the effect on you? Well you might find that you're being squished down a bit and you end up with a more dense film of molecules on top of the water. So water is allways being pulled together, which is why water tends to clump together into droplets.

Water molecules are also attracted to other substances such as wood, glass or paper, so a matchstick for example will be pulled by the water wherever it is touching it.

 Washing up liquid is a surfactant, which is something that breaks down surface tension.  So by adding washing up liquid, one side of the matchstick has the attraction between the water molecules broken, whereas on the other side of the matchstick they're still attracting each other and the matchstick.

So what the matchstick feels is a pulling force from all the molecules on the clean water side, but on the other side there is virtually no surface tension. So rather than being repelled by the washing up liquid, it's actually being pulled from the other side across the bowl of water.

Written by Dave Ansell