What Is Surface Tension?

Have you ever conducted experiments with water? Certainly, you may consume numerous glasses of water daily. You also use it to cleanse your body during bathing or showering. But were you aware that it possesses some fascinating scientific properties?

For instance, were you aware that tiny insects, like water striders, are capable of walking on water? Were you aware that, with a gentle touch and steady hand, you can float a needle on the surface of water, despite its greater density compared to water?

Both floating needles and water striders walking on water can attribute their incredible feats to the same characteristic of water. What is this characteristic? It is surface tension, of course!

A scientist would explain that surface tension is a property of the surface of a liquid, such as water, that enables it to resist an external force. Surface tension results from cohesive forces between similar molecules. When measured, surface tension is expressed as force per unit length.

But what does this actually mean? Have you ever noticed how water tends to form droplets? When washing your family car, you can easily observe water droplets forming in various places. Instead of dispersing rapidly, water molecules tend to adhere to each other.

Why does this happen? Cohesive forces between liquid molecules make them more attracted to each other than to other types of molecules, such as those found in a car or a cup or the air. Consequently, molecules of a liquid tend to stick together, forming shapes with the least possible surface area. This is why water droplets and bubbles tend to assume spherical shapes.

As cohesive forces between liquid molecules cause them to tightly hold on to one another, something interesting occurs at the liquid’s surface. At the surface, liquid molecules can only bond with other molecules located beside and below them. Above them, there are only air molecules.

At the surface of a liquid, the cohesive forces between liquid molecules lead the surface molecules to adhere even more tightly to the molecules beside and below them, since there are no liquid molecules above them. This results in the phenomenon of surface tension, which creates a force similar to an elastic membrane on the liquid’s surface.

Because there are no liquid molecules pulling from above, the molecules at the liquid’s surface experience an inward or downward pull from the molecules below and beside them. This pull causes the surface molecules to contract and resist being stretched or broken. In other words, the surface molecules are under tension, which is why the term surface tension is used!

Thanks to surface tension, the surface of a liquid behaves like a thin, elastic membrane. This is why water striders can walk on water. It is also why you can float a needle on top of water. So why can’t you walk on water?

Sufficient force can overcome surface tension. Insects heavier than water striders and objects larger than needles can easily break the surface tension, allowing them to penetrate and separate the top layer of water molecules.

If you’re not careful, you may still experience the force of surface tension. When you jump into a pool, your hands create an opening in the water’s surface for your body to go through. However, if you belly flop, your body will encounter a much stronger force from the water’s surface tension, resulting in the painful stinging sensation you may have felt before!

Give it a Try

Make sure to try out the following activities with a friend or family member:

  • Do you have paper clips, a bowl of water, and a paper towel? That’s all you need to explore the concept of surface tension in the fascinating Floating Paper Clips experiment. Don’t forget to share what you learn with a friend or family member.
  • Want to experiment even more? Go online and watch Physics Girl perform Seven Surface Tension Experiments. Which ones can you try at home? Remember to ask an adult friend or family member for help before you begin. Have fun experimenting with surface tension!
  • One of the most enjoyable ways to experience surface tension is by creating giant bubbles! If you love playing with bubbles, now is your chance to do so while also demonstrating the principle of surface tension. You’ll need some bubble solution, which you can easily make at home. Check out these Bubble Solution recipes for ideas on how to make the biggest bubbles you’ve ever seen!

Sources of Wonder

  • http://weirdsciencekids.com/Definitionsurfacetension.html
  • http://kidsactivitiesblog.com/12902/what-is-surface-tension
  • http://water.usgs.gov/edu/surface-tension.html
  • https://en.wikipedia.org/wiki/Surface_tension

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