Surface Tension

Author: Maddie Van Beek

What is surface tension? Have you ever worn a waterproof jacket and watched the water bead off of your sleeve? Or have you ever gone through a car wash and wondered why the water collects into little spherical blobs?

Image 1: Water beading on a leaf

Image 2: Liquid molecule forces

Have you ever filled a cup so full of water you thought it was going to overflow, but instead, it hovered just over the surface of the cup? Try it out! If you fill a glass with water slowly enough, you will see the dome-like shape of the water on top of the glass. 

In the image to the left, you can see that water molecules inside a droplet are attracted to each other in various directions, but water molecules on the surface are attracted to other molecules sideways and inwards. 

Surface tension is a “contractive tendency of the surface of a liquid that allows it to resist an external force” (Wikipedia). These forces are a result of the hydrogen bonds water molecules form with each other (see our answer to “Why does ice float?")

Put the definition into your own words! What is surface tension?

Below, check out a water strider using surface tension to "walk on water." The surface of the water is like a thin membrane that resists the force of objects such as the water strider.

Image 3: Water strider utilizing surface tension

Click for more info on surface tension!

Check for understanding! 

• What are some other examples of times you have seen surface tension in action? 

• Why do water molecules at the surface create stronger bonds? 

• What is the surface tension of water? Does it ever change, and why? 

• What is the difference between cohesion and adhesion? 

• When you add soap to water, what happens to the surface tension? Hypothesize why this is.

Image 4: A metal paperclip floating on the surface of water due to surface tension.

Click HERE to learn more. 

Follow up question: 

• Why does the addition of soap cause the spring to sink? 

Let’s try out a similar experiment! 


YOU WILL NEED:

• Cup
• Water
• Paperclip
• A square of toilet paper

Extension Materials

• Soap
• Baby powder

YOU WILL DO: 

1. Fill the cup with water.
2. Hypothesize: Does a paperclip float or sink? 
3. Place the paperclip on the surface of the water. What happens? Record. 
4. Remove the paperclip from the cup. 
5. Gently place the square (or small piece of the square) of toilet paper on the surface of the water. 
6. Gently place the paperclip on the surface of the toilet paper. 
7. Watch what happens and record observations. How is this different than the first time you put the paperclip in the cup? Why is this? 
8. You should see the toilet paper absorb water and sink, leaving the paperclip “floating” at the surface!
9. What happens when you bend the paperclip? Do different shapes make any difference? 

Extension

1. Try what Physics Girl did in her experiment! Put a bit of hand soap on your finger and touch the surface of the water. What happens? Why is this? 
2. Next, hypothesize how many paperclips the surface of your cup of water can hold! 
3. We know that soap makes the surface tension weaker, but can anything make the surface tension stronger? Take a guess!
4. Sprinkle baby powder on the surface of your water, and try the paperclip experiment again. How many paperclips “floated” this time? 
5. Try out other liquids such as soda, milk, vegetable oil, vinegar... do you obtain the same results, or are they different? 

Resources

http://en.wikipedia.org/wiki/Surface_tension

http://hyperphysics.phy-astr.gsu.edu/hbase/surten.html

http://www.sciencebob.com/experiments/paperclip.php

http://www.exploratorium.edu/ronh/bubbles/soap.html

​Image and video credits in order of appearance:

Apel, 2006. Water beading on a leaf. Uploaded from Wikimedia Commons on 8/8/2016. https://en.wikipedia.org/wiki/Surface_tension#/media/File:Dew_2.jpg File used in accordance with the Creative Commons Attribution 2.5 Generic license. No changes were made. 

Booyabazooka, 2008. Diagram of the forces on molecules of a liquid. Uploaded from Wikimedia Commons on 8/8/2016. https://upload.wikimedia.org/wikipedia/commons/thumb/f/f9/Wassermolek%C3%BCleInTr%C3%B6pfchen.svg/300px-Wassermolek%C3%BCleInTr%C3%B6pfchen.svg.png 
File released into the Public Domain. 

Vickers, 2008. Water strider. Uploaded from Wikimedia Commons on 8/8/2016. 
https://upload.wikimedia.org/wikipedia/commons/thumb/2/2c/Water_strider.jpg/1280px-Water_strider.jpg File released into the Public Domain. 
​
Billerbeck, 2010. Water strider or water bug.
​Uploaded from Youtube on 8/8/2016. https://youtu.be/b9nxUtoH7to

Physicsgirl, 2012. Amusing surface tension experiment. Uploaded from Youtube on 8/8/2016.
https://youtu.be/2F64qh9qPAI

Armin Kubelbeck, 2007. A metal paperclip floats on water. Uploaded from Wikimedia Commons on 8/8/2016. 
https://upload.wikimedia.org/wikipedia/commons/thumb/9/9c/Surface_Tension_01.jpg/1024px-Surface_Tension_01.jpg File used in accordance with the Creative Commons Attribution-Share Alike 3.0 Unported license. No changes were made.