Have you ever woken up early in the morning and gone outside to find little beads of water perched on the tips of blades of grass? Why doesn’t the water fall off? You might also think of the water strider insect, which seemingly skates atop the surface of the water with ease. These instances are possible due to surface tension, which is the tendency for the surface of a liquid to be strong. What changes occur when the surface tensions of two different liquids are combined? We will seek out the answer to this tense question in the following experiment.
Much of what we now know about surface tension was discovered by the scientist, Agnes Pockels, who was an early pioneer of “surface science.” She did experiments in the sink of her own kitchen and made many important observations that influenced how we study liquids. Surface tension is not only important for many engineering processes - it is also important to the health of the planet, as it allows for the formation of bubbles, which help distribute organic material throughout the layers of the ocean. Today we’re going to do an experiment that will help us better understand how surface tension can be altered with soap and water.
The boat should zoom forward in the water. This is because liquids always move from low surface tension to high surface tension; this is called the Marangoni effect. Soap has a much lower surface tension than water, so it pushes out from the notch in the back of the boat. This force pushes the boat forward, similar to a rocket pushing fuel out of the tank to propel itself upwards.
Don’t forget to clean up when you are done. The water can be poured down the drain, and the bowl can be rinsed and put away. You can throw away the boat and cotton swab or toothpicks. Put any other materials back where you found them.
This experiment uses a very small boat of a very particular shape. What might happen with a different size or different shape? Can the boat carry any kind of a load? Can you put weight on it and have it still move? How much? Instead of putting weight on the boat, is there a way to turn it into a tug boat, pull something behind it, and still have it work? How does making any of these changes affect the performance of the boat?
This experiment was selected for Science at Home because it teaches NGSS Disciplinary Core Ideas, which have broad importance within or across multiple science or engineering disciplines.
Learn more about how this experiment is based in NGSS Disciplinary Core Ideas.