Summary
Introduction
Have you ever wondered how water droplets form on a window on a rainy day, why some bugs can mysteriously walk on water, or why you use soap to wash your hands? All of these things are related to the phenomenon called surface tension. In this activity you will learn how you can use surface tension to make a fish cut from a piece of paper seemingly swim around on its own!
Background
Whether riding in a car on a rainy day or accidentally spilling a glass of water on the kitchen counter, you have probably seen water droplets form. What holds these water droplets together, sometimes into nearly spherical shapes? Why don’t they spread out evenly or fall apart into even smaller drops? They are held together by something called surface tension. Water molecules are slightly attracted to each other. At the surface of a water droplet, the water molecules are more attracted to the other water molecules around them than they air to the air above them. This forms a thin “skin” of molecules that are strongly attracted to each other and difficult to break through.
So, sometimes high surface tension can be beneficial – like for the insects that walk on water. Other times we want to lower the surface tension of water, like when we are doing laundry or washing dishes. There are lots of tiny holes in between pieces of dirt or the fabric of clothing, and high surface tension can make it hard for water to squeeze into those holes. Adding soap or detergent decreases water’s surface tension, making it easier to penetrate all those little nooks and crannies. Do you think you can make use of this change in surface tension to make an object skim across the surface of the water? Try this project to find out!
Materials
- Shallow tray
- Tap water
- Piece of paper
- Medicine dropper
- Soap
Instructions
- Cut out a small “fish” shape from your piece of paper. Make sure the fish has a V-shaped notch in its tail.
- Fill your shallow tray partially with water.
- Carefully place the paper fish on the surface of the water at one end of the tray, with its tail facing outward. Make sure the fish floats and you do not push it under the water.
- Use the medicine dropper to carefully place a single drop of soap on the water in the V-shaped notch of the fish’s tail. What happens?
- Empty, rinse out, and dry the tray, cut out a new paper fish, and try again. Do you get the same result?
- Try re-using one of the paper fish with fresh water, re-using the tray of water with a new paper fish, or both. Can you get it to work a second time? Why or why not?
Extra: try cutting out and testing some different shapes. Do the shapes have to be “streamlined,” like a fish? What if the shape is symmetric, like a circle, or blocky like a square?
Extra: try the experiment with different liquids you have in your kitchen, like vegetable oil or juice. Do any liquids have the same effect as soap? Do any of them not work? Why do you think this happens?
Observations and Results
When you place the paper fish on the surface of the water and it floats, the surface tension of the water pulls at it from all sides. Since it is completely surrounded by water, surface tension pulls evenly in every direction, and it does not move. When you add a drop of soap to one side of the fish, the surface tension on that side suddenly becomes lower, so the fish is not pulled as hard in that direction, and it moves in the other direction. This is a great example of Newton’s first law of motion! The fish is initially at rest and the forces on it are balanced, so it remains at rest. When you add the soap, the forces on it become unbalanced, so it accelerates.
You will probably find that it is difficult to re-use a paper fish or a tray of water. Once the fish and water are “contaminated” with some soap as the soap spreads out, then the surface tension is lowered everywhere, not just on one side of the fish, so you can no longer make the fish move. However, you can probably find that the effect does work with many other shapes, even squares! You may also find that other liquids like oils (which have a lower surface tension than water) work just as well as soap, but liquids like juice (which is mostly water) do not.
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Cleanup
- Clean up any spilled liquids.
Additional Resources
- Surface Tension, Hyperphysics
- Build a Raft Powered by Surface Tension, Science Buddies
- Science Activities for All Ages!, from Science Buddies
