Key concepts
Physics
Energy
Liquids
Molecules
Surface tension

Introduction
Have you ever wondered why a water strider bug can walk on water? Would you believe it is based on the same reasons soap can clean your dishes? In fact, if you look around you carefully, you can find dozens of interesting phenomena that depend on the surface tension of water. In this science activity you will make a little raft that is actually powered by surface tension—and use your vessel to investigate how surface tension works!

Background
If you've ever blown up different types of balloons, you know it's pretty easy to blow up a balloon made of thin, soft, stretchy rubber. That is because the balloon offers little resistance to becoming stretched out as it's blown up. But for a balloon made of thicker or stiffer rubber, more energy is required to inflate it. The balloon is more resistant to stretching, and can be a model for surface tension. How "stretchy" the balloon material is determines how much resistance (or, in this case, surface tension) must be overcome by your breath 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 constantly pushing and pulling against one another with tiny electric charges, similar to how magnets can attract and repel one another depending on their charges. When a water molecule is surrounded by other water molecules, pulling forces are balanced by pushing forces, so it's not pulled in one direction more than any other. But at the surface, where air and water meet, that's not true; water molecules there get pulled down, toward other water molecules, more than up, toward the air. This causes the surface tension of water.

Materials

• Scissors
• 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 sheet of packaging.)
• Utility or EXACTO knife and an adult helper to use it
• 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.)
• Toothpick
• Large basin, sink or bathtub
• Liquid dish soap or liquid laundry detergent
• Water
• Medicine dropper (optional)
• Ruler (optional)

Preparation
• With an adult's help, cut the Styrofoam piece into a small rectangular shape, such as one about three inches by two inches or four inches by three inches. This will be your raft. If you want, you can make it a different shape than a rectangle, so long as it is symmetrical, not longer than about six inches and has a flat area on one side that's at least two inches long.
• On one of the short ends of the raft (or the flat area that is at least two inches long), about half an inch in from the end of the raft, take a utility knife and carefully cut out a square about one inch by one inch in size. Center the square along that end. This will be the back of the raft.
• Again, with an adult's help cut a small square 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, whereas the sponge will rest in the raft's square hole. Run the toothpick closer to the top side of the sponge so that when you place it in the Styrofoam hole, the bottom of the sponge piece is at least as low as the bottom of the Styrofoam raft. Why do you think it's important that the sponge will touch the water?
• Now, with an adult's help cut out the small remaining strip of Styrofoam between the square and that end of the raft, but make the cutout not as wide as the sponge piece. For example, if your sponge piece is one inch on each side, only about one-half inch or three-quarter 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.) Your raft is now ready to sail!

Procedure
• Fill a large basin, sink or bathtub with water. Make sure there's enough room for your raft to move around.
• 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 not moving, 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 out of it and refill the basin. Test the raft again, as you did before.
• Knowing that detergents decrease water’s surface tension, can you use this fact to explain why the raft moves this way?
• Extra: You could repeat this activity but test substances other than detergent to see whether they affect the surface tension. For example, you could try different soaps, toothpaste, vegetable oil, table salt, etcetera. Do other substances affect surface tension, and the motion of the raft, in the same way that the liquid dish soap does?
• Extra: 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 heavier or lighter affect how the it travels using surface tension?
• Extra: Think about how you could change the design of your raft to improve it. Can you alter its design so that it goes faster or straighter?

Observations and results
Did the raft move forward, away from the end where the sponge piece was placed?

You should have seen that when you added a drop or two of detergent on the sponge piece of the raft, the raft should have quickly moved away from the side where the detergent was added, propelling it forward through the water.

When detergent is added to water, it decreases the surface tension of the water. Compounds that lower water’s surface tension are called surfactants, which work by separating the water molecules from one another. 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 surface tension. How do you think a surfactant such as dish soap help to clean dirty dishes?

More to explore
Introduction to Surface Tension, from Massachusetts Institute of Technology
Sticky Water, from Exploratorium
Fun, Science Activities for You and Your Family, from Science Buddies