Hello
Card
A drop of washing up liquid
Make a flat card boat with a v-shaped cut at the back.
Float the boat on some clean (not soapy) water, and then place a drop of washing up liquid in the V. You should see the boat moving.
Try the experiment again with the same water, does it work as well?
Try it with new water see what the difference is.
To try this experiment again you will have to use new water with no soap in it.
Try putting the drop of washing up liquid somewhere else and see what happens.
How does the soap power the boat?
If you float something that water sticks to (wets), like card, the surface tension will pull equally on all sides, a bit like an evenly matched tug of war, so nothing moves.
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Soaps and detergents break down the surface tension of water. They sit at the surface of the water between the water and the air. When you place a blob of soap or detergent on one side of the card there will be surface tension on one side and not the other, so the card gets pulled away from the soap.
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The soap breaks down the surface tension on this side, so the tug of war is no longer even and the card gets pulled across the water.
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