Surface Tension

Here's what you'll need...
  • a bowl
  • water
  • paperclips
  • dish soap
  • toothpick

A few weeks backs Astro Amanda appeared on KTVI Fox 2 to explain the Cheerio's Effect. We've decided to take that experiment a step further to show you how to play with it at home.

The Cheerios Effect describes the curious phenomenon you can see each morning in your breakfast cereal. When you are nearly done with your cereal and have just a few pieces of cheerios left floating in your bowl, you may notice their tendency to aggregate or clump together.

The cereal bits do this because of several factors. One of which is surface tension. Molecules in fluids, like water or milk, have a mutual attraction to each other and are pulling on each other equally in all directions. Except, that is, for the molecules at the surface. Those top molecules are being pulled slightly inward by the force of all the molecules underneath them. This attraction leads to surface tension, a resistance to external forces.

When we drop the cheerios into a bowl of water they float on top because they are less dense than the water itself. But while floating on top they apply pressure to the water's surface. This results in a little dimple.

When we drop a second Cheerio in the water, it creates it's own little dimple. As the two cheerios float on the water's surface they will feel that the surface is tilted from their respective dimples and slide together.

But paperclips are more dense. They should sink in water. So how can they float? Because of that inward pull from the water molecules, it creates a kind of skin where the water and air meet. If we place a paperclip gently and evenly on this surface, we can float it.

To do this you can use another paperclip. Unfold the paperclip so that you have an L shape. Balance the paperclip you want to float on the horizontal part of the L and use the vertical leg of the L as a handle. Gently dip the L into the water. The balanced paperclip should float.

Floating the paperclips does take a steady hand so you may need a few practice tries.

The dish soap will disrupt the surface tension in the water causing the paperclips to either move quickly away from where the soap is added or sink into the water completely.

Will the paperclips float again after you've added dish soap? Try it and see.