Surface Tension

Contributed by Max Prichard

Introduction

• If you’ve ever seen water striders floating on water before, you’ve seen surface tension in action! Surface tension is a force in liquids (not just water) that wants to minimize surface area. Whenever you see little round droplets of water forming, what’s happening is the molecules on the surface are being pulled inwards by the molecules on the inside. However, the same is still true for a larger body of water with a surface, and it actually means that water almost has an invisible skin that light objects can’t penetrate. We’re going to explore this counterintuitive behavior in this experiment.

Materials

• A clear cup
• A paperclip
• Some tap water
• Some dish soap
• A piece of white paper
• A flashlight

Procedure

1. First, to see how surface tension works, fill up the clear cup with water. Place it on top of a sheet of white paper on a table.
2. If you drop a paperclip into the cup, it should sink immediately to the bottom.
3. Next, carefully lower a paperclip down until it meets the surface of the water. Make sure not to drop it!
4. The paperclip should float on the surface of the water, even though the other paperclip is floating. 
5. Take a flashlight, and shine it above the paperclip floating in the water. What does the shadow look like compared to the other (sunk) paperclip?
6. Try dipping your index finger in the water away from where the paperclip is floating. Does anything change?
7. Now, put a drop of dish soap on your index finger and rub it a bit with your thumb. Dip your index finger again in the glass. What happens to the paperclip now?
   

Physics Concepts and Questions

Surface tension is a phenomenon that arises in liquids that have an attractive force between the molecules. In a way, all the molecules are trying to be a close together to each other as possible. In doing so, the water molecules on the surface are being pulled tightly in, forming a skin, similar to stretching a sheet of plastic wrap over something. This sounds theoretical, but surface tension can manifest itself as a bona-fide force that can act on objects other than the water. This experiment demonstrates how things that are actually denser than water, like a metal paperclip, can use this surface tension to actually float! In the included video, we go over a (larger) analogous situation that actually involves very similar physics!

However, we saw that soap (a so-called ‘detergent’) actually broke the surface tension of the water, even without touching the paperclip. How do you think this works? 

So, we’ve seen that surface tension can make a paperclip float easily. But, if you think about it, there are many other things that rely on floating to work, like boats! Do boats float using surface tension? Should you be careful not to bring soap with you anytime you go on a boat??

Try the experiment with larger pieces of metal, like a dime. Can you get that to float? What is the largest object you can get to float that would usually sink?

Conclusions and Further Investigations

• We used some dish soap to demonstrate a substance that can break surface tension. But, can you find any other substances that do something similar? Try this experiment again with different materials you have around the house!
• Try making a little paper boat (make sure there are no holes!) and put it in the cup, and repeat the experiment. What happens this time?