How do some insects walk on water?

by Job

In a previous activity (Why do boats float? Why do balloons fly?) we have looked at why some objects can float in water. But some insects like the Pond Skater can walk on water, how is this possible?

Experiment time!

In the same way a Pond Skater can walk on water, you can make a paperclip float on top of water.

• Fill a shallow bowl with roughly 1 inch layer of water.
• Use tweezers to gently lower a paperclip into the water vertically: it should sink – so clearly the density of the paperclip is larger than that of water.
• Use tweezers to gently lower a similar paperclip into the water horizontally: it should float!

Why is this happening? This is because of a phenomenon called surface tension. The surface of any liquid behaves a bit like a "skin". If you press very gently, the skin will not break. We humans are too heavy to walk on water in this way, but paper clips and pond skaters can press on the water so gently that they can sit on top of it.

More Experiments!

Different liquids have different surface tension, which means that they can withstand a different amount of pressure on their surface before breaking.

• What happens if you try to add some washing-up liquid to the water where your paper clip is floating? (dip a cotton swab in some washing-up liquid and then gently touching the water surface)
• Try to add drops of different food colouring to the water, close to each other but not too close that they mix immediately, and then touch the cotton swab with the washing-up liquid in the middle of those three drops. See these links for the intended effect:
  • Colors on the mooooove! (from the Americal chemical Society)
  • Surface tension of water (from Science Sparks)

Exploiting surface tension, you can even create your very own soap powered boat!