Build a Bathtub Toy Raft Powered by Surface Tension
IntroductionHave you ever wondered why a water strider can walk on water? Or how detergent can clean your dishes? If you look around you carefully, you can find dozens of similarly interesting phenomena that are all linked to the surface tension of water. In this science activity, you will make a little toy raft that is actually powered by surface tension, and use your vessel to investigate how surface tension works!
- A small, flat piece of Styrofoam, at least about three inches by three inches. For example, this could be part of a take-out container or a meat packaging tray.
- A toothpick
- Kitchen sponge. Use either a fresh sponge or rinse the sponge thoroughly with water to make sure it does not have any soap in it.
- Liquid dish soap or liquid laundry detergent
- Large basin, sink, or bathtub
- Utility knife or X-ACTO knife
- Optional: Medicine dropper and ruler
- Cut the Styrofoam piece to be a small rectangular shape, such as about three inches by two inches, or four inches by three inches. This will be your raft.
- On one of the short ends of the raft, about half an inch in from the end of the raft, take a utility knife and carefully cut out a square that is about one inch by one inch in size. Center the square along the end. This will be the back of the raft.
- Cut a small square out of the kitchen sponge, about one inch by one inch in size. This sponge piece should fit in the square you cut out of the raft, so adjust the size of the sponge piece if needed.How do you think the sponge will power your raft?
- Run a toothpick horizontally through the sponge piece. The toothpick will rest on the Styrofoam raft, while the sponge will rest in the raft's square hole. Run the toothpick closer to the top side of sponge so that when you place it in the Styrofoam hole, the bottom of the sponge piece is as low or lower than the bottom of the Styrofoam raft. Why do you think it is important that the sponge will touch the water?
- Now cut out the small remaining strip of Styrofoam between the square and that end of the raft, but make the cutout narrower than the sponge piece. For example, if your sponge piece is one inch on each side, only about ½ inch or ¾ inch of the Styrofoam strip should be cut away. This is because when you place your raft in the water, you will want the sponge piece to have some open water behind it, but part of the Styrofoam strip needs to still be there to prevent the sponge from floating away.
- Place the toothpick and sponge on top of the Styrofoam. Your toy raft is now ready to launch!
- Fill a large basin, sink, or bathtub with water. Make sure there is enough room for your raft to move around in.
- Once the water has settled, put the raft onto the water surface and let it float. Make sure the sponge piece stays in the raft's square cutout.
- Once the raft is still, carefully put one or two drops of liquid dish soap or laundry detergent on the sponge piece. Make sure none of the detergent accidentally drops into the water. If needed, you could use a medicine dropper to better aim the drops.What happens to the raft when detergent is placed on the sponge? How does it move?
- If you want to observe the motion again, empty the basin, rinse the raft (especially the sponge) to get all of the detergent off of it, and re-fill the basin. Test the raft again, as you did before.Does it always move in the same way? How do you think the movement is related to surface tension?
What Happened?You should have seen that when you added a drop or two of detergent on the sponge piece of the toy raft, the raft should have quickly moved away from the side where the detergent was added, propelling the raft forward through the water. This is because detergent changes the surface tension of water and this change in surface tension causes the raft to move. Specifically, the detergent lowers the surface tension of the water, and the raft moves towards an area with higher surface tension.
If you have ever blown up a balloon, you know it is pretty easy to blow it up when it is made of soft, stretchy rubber. That is because the balloon offers little resistance to becoming stretched out as it is blown up. But for a balloon made of thicker or stiffer rubber, more energy is required to blow it up. The balloon is more resistant to stretching. The balloon can be a model for surface tension. How "stretchy" the balloon material is determines how much resistance (surface tension) must be overcome by your blowing to inflate it. Surface tension is defined as the energy needed to increase a surface area by a certain amount.
Liquids also have surface tension. The liquid's molecules (small particles) are pushing and pulling against each other, similar to how magnets both attract and repel each other. When a water molecule is surrounded by other water molecules, pulling forces are balanced by pushing forces, so it is not pulled in one direction more than any other. But at the surface, where air and water meet, that is not true; water molecules there get pulled down, towards other water molecules, more than up, towards the air. This is the surface tension of water.
When detergent is added to water, it decreases the surface tension of the water. Compounds that lower the surface tension of water are called surfactants. Surfactants lower the surface tension by separating the water molecules from each other. In this activity, the surface tension is lowered in the area where the surfactant is added, and the higher surface tension in front of the raft pulls the raft forward. As the detergent spreads through the water, it decreases the surface tension throughout the water, and the raft stops moving eventually because there is no longer a difference in the water's surface tension behind and in front of the toy raft.
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For Further Exploration
- You could repeat this activity but test substances other than detergent to see whether they affect surface tension of water. For example, you could try different soaps, toothpaste, vegetable oil, table salt, etc. Do other substances affect surface tension, and the motion of the raft, in the same way that the liquid dish soap does?
- You could try changing the size or shape of the raft and repeat the experiment. How does using a raft that is a different shape, or a heavier or lighter raft, affect how the raft travels using surface tension?
- Think about how you could change the design of your raft to improve it. Can you alter the raft's design so that it goes faster or straighter?