Soap engine for a paper boat
Water has a lot of interesting properties. On of them is the water tension. This means that normally there is a surface force which origins from the fact that water molecules attract each other and try to form bonds with their neighboring molecules. In physics this is called cohesive attraction.
On the surface of the liquid you have less neighbors (there is nothing above) and therefore the bonds to the existing neighbors is stronger. This creates the surface tension. If you add a different liquid (like soap) on top of the water, you can reduce this surface tension. This is the reason why soap is so good in removing some kinds of stain and why you can use it for a pretty cool effect - as engine for a paper boat.
Materials you need for this experiment:
paper | To create your paper boat |
powder detergent | Powder detergent is needed as your "soap fuel". In powder form it works better than liquid soaps |
bowl with water | This will be your ocean for the boat |
scissors | To cut out the boat |
How to do the experiment:
 | First you need the paper boat. You can either print and cut out the paper boat on the left or create something similar by your own. However it is important that it is flat on the water (don't fold a classical paper boat) and has a compartment with a small exit opening for propulsion. |
| Carefully put this boat on the surface of the water. It is best if you position it somewhere near the edge of the water bowl, so that the boat can move forward without obstructions. |
 | Put a few floks of powder detergent in the opening of the boat. If you have only liquid soap, you can put a bit on a tweezer and touch with the soapy tweezer the open area of the boat (as shown in the image on the left). It is not advisable to pour the liquid soap directly in the tank of the boat, since you likely will get to much soap into the water which would destroy the surface tension everywhere and therefore makes the propulsion impossible. |
 | After your first "fueling" effort, the boat will swim forward for a while. When you then "refuel" it with additional soap, it will start again. |
If you have refueled your boat a couple of times, you will see, that the effect becomes weaker. The reason is that there is already a considerable amount of soap in the water which reduced the surface tension and therefore also reduced the effect. If you replace the water with fresh water (without soap), it will work again.
Try to experiment with different shapes of the boat and different shapes of your tank and propulsion opening. Which combinations work best and make your boat move faster? Have fun experimenting!