Key concepts
Physics
Forces
Surface tension
Density
Buoyancy
Introduction
Have you ever seen a “water strider” (also called water bugs, pond skaters, water skippers, etcetera)? They are bugs that effortlessly hop around on the surface of ponds, lakes and rivers. How do they do it without sinking? Try this project to find out!
Background
If you glance at a water strider, at first you might think it’s floating in the water, just like a boat. If you look very closely, however, you’ll see it’s sitting on top of the water without actually breaking through the surface. How is that possible? It depends on a force called surface tension, which acts on the water strider’s legs when they touch the water. Surface tension is a pull at the water’s surface that occurs because its molecules are slightly attracted to one another. This property is responsible for many interesting phenomena, such as how bubbles form, how water makes droplets and how plants can suck water out of the ground. (See the "More to explore" section for a list of other fun projects involving surface tension.) In this case the surface tension creates a thin film, or skin, at the water’s surface that is difficult for very small, light objects to break through.
Every object is pulled downward by the force of its own weight. Objects sitting on water can remain above the surface if the upward pull of surface tension is enough to equal the weight. (You might ask, “Wait, how can surface tension pull something up?” or “Wouldn’t the water have to push the water strider up to support its weight?” Try the activity and then read the “Observations and results” section to find out.) Objects completely or partially submerged in water, such as boats, are pushed up by the buoyant force, which is equal to the weight of the water they displace. If the buoyant force is bigger than the object’s weight, it will float. Normally materials that are denser than water (they have more mass per unit volume), such as metals, will sink. Metal boats, however, can float because their hulls are shaped so they displace a lot of water (that is, there is a lot of empty air space inside the boat). In this project you will make a model water strider out of metal wire and see that you can use surface tension instead of buoyant force to make metal float.
Materials
- Thin wire (You can buy “magnet wire” or, with permission, cut apart an old cable such as a cell phone charger.)
- Wire strippers or scissors (only needed if you are cutting apart an old cable)
- Shallow tray or bowl
- Water
- An adult helper (if cutting apart an old cable)
Preparation
- If you are cutting apart an old cable, have an adult helper use wire strippers to strip off the insulation. The cable might have multiple smaller wires inside it, and those wires might also have insulation. Pull the wires apart and strip off any insulation until you have them down to the bare metal. If you don’t have wire strippers, you can ask an adult to carefully use scissors or a sharp knife to scrape off the insulation, but be careful not to cut through the whole wire.
- Fill a shallow tray or bowl with water.
Procedure
- Cut three pieces of wire, each about eight to 10 centimeters long.
- Tightly twist the wires together only in the middle to form your bug’s “body.” The untwisted ends of the wires should form your bug's six “legs,” with three on each side. Spread the legs out to evenly distribute the water strider’s weight.
- Curve each of the strider's legs into a long, shallow “U” shape. When you put the water strider down on a flat, solid surface, it should rest evenly on all six legs (that is, none of the legs should be sticking up in the air), with the body up off the ground. Adjust the legs if necessary.
- Gently place your water strider into the tray or bowl of water. What happens?
- If your strider sinks, try adjusting its legs. Remember to make sure they’re even so all six of them touch the water and the bug’s weight is evenly distributed. If one leg goes into the water before the others, all of the strider’s weight will be on that leg, and it will probably sink. If it still doesn’t float, try adjusting the legs’ shapes. You want as much wire to touch the water as possible, so make sure each has a long, very gentle curve and not any sharp bends.
- Once you get your wire bug to float, look very closely at where the legs touch the water. What do you see?
- Try shaking the bowl or tray to make waves—or sprinkling some water on top of your strider to simulate rain. What happens?
- Extra: Try making a water strider with different materials such as thicker wire or even paper clips. Does it still work?
- Extra: Try adding small weights to your strider (such as tiny bits of tape or paper). How much weight can you add before it sinks?
Observations and results
You might have had trouble getting your water strider to float at first. It’s important to distribute the weight evenly across all six legs, and you should have as much length of the wire touching the water surface as possible. This allows the maximum amount of surface tension to support the bug’s weight. If just one leg touches the water first or only a tiny part of each leg touches the water, then all of the bug’s weight is concentrated on a small amount of wire, and it will break through the surface.
Once your bug breaks through the surface, it quickly sinks because metal is denser than water. Waves and raindrops can easily break the surface tension and cause your bug to sink. How do real water striders stay afloat when real bodies of water are rarely perfectly still and flat? Their bodies are covered with tiny hairs that trap air bubbles, allowing them to quickly pop back to the surface after submersion.
If you looked very closely at where your bug’s legs touched the water, you should have observed they made small dents in the water’s surface. This is what enables surface tension to pull up on the legs. Imagine having a bunch of people hold a bedsheet by the edges and pull it tight. The sheet is flat and horizontal so at first it’s hard to imagine how it could pull something up. Now imagine tossing a ball into the middle of the sheet. The sheet will sag down slightly but the people pulling on the sheet (creating surface tension) will prevent the ball from sinking deeper into the sheet. Surface tension acts on the water strider’s legs in a similar manner.
Finally, if you tried using thicker wire or paper clips to make a strider, it might have been much more difficult—or even impossible! This occurs because surface tension has a relatively stronger effect on very tiny, lightweight objects. That’s why water striders are so small and why you can’t walk on the surface of water. Even if you try spreading out your arms and legs, you will still break through the surface.
More to explore
Measure Surface Tension with a Penny, from Scientific American
Make a Paper Fish Swim with Surface Tension, from Scientific American
Staining Science: Capillary Action of Dyed Water in Plants, from Scientific American
Science Activities for All Ages!, from Science Buddies
This activity brought to you in partnership with Science Buddies
