Lights, Camera, (Capillary) Action!
Summary
Two thin glass panes are squeezed together with a rubber band but separated on a single side by a paper. This paper clip spaces the glass panes so they are only touching on the opposide side. In the area that the two glass panes touch blue liquid from a pool underneath the panes is allowed to travel up and in-between the glass to the top of both panes. The liquid can travel up where the glass touches due to capillary action.
Introduction
How do trees suck water all the way up to their leaves? How do paper towels soak up a spill? Are these things related? Try this project to learn about capillary action, and repeat a classic demonstration from over 100 years ago!
Materials
- Two small picture frames with glass covers
- Paper clips (2)
- Rubber band
- Tray or plate
- Water
- Food coloring
- Dish towel
Prep Work
- Fill a tray or plate with a shallow layer of water and mix in a few drops of food coloring.
- Remove the glass plates from the picture frames.
Instructions
- Use the rubber band to hold the two glass plates together, with the two paper clips in between opposite edges as spacers.
- Dip one edge of the glass plates (a side without a paper clip) into the water. Hold the plates still and wait a few seconds.
What happens? - Separate the glass plates and wipe them dry with a towel.
- Use the rubber band to hold the plates together again, but this time only use one paper clip on one side (so at the opposite edge, the glass plates should be touching each other).
- Dip one edge of the glass plates (one of the sides adjacent to the side with the paper clip, not opposite it) into the water again. Hold the plates still and wait for a few seconds.
What happens this time?
Cleanup
What Happened?
When you put two paper clips between the glass plates as spacers, the gap between them was a constant width everywhere. When you dipped the edge of the glass plates into the water, the water was sucked up into the gap (and the food coloring made this easier to see). Since the gap was the same width everywhere, the water should have risen to about the same height along the length of the plates.
When you remove one of the paper clips, something interesting happens. The gap is now wider on one end, and gradually gets narrower until it disappears and the glass plates touch. This time, when you dip the plates in the water, the water goes up higher as the gap gets narrower. See Figure 9 in this reference—this experiment is over 100 years old! It demonstrates how something called capillary action can lift water up higher in narrower spaces. Read the Digging Deeper section to learn more about the science behind why this happens.
Digging Deeper
Have you ever looked closely at water in a drinking glass? You might notice that the surface of the water is not completely flat, but it forms a small lip, called a meniscus, that curls up around the edge of the glass. This occurs because of two forces: adhesion (the attraction between the water molecules and the glass) and cohesion (the attraction between water molecules and each other). Cohesion between water molecules gives rise to surface tension, or the way the surface of water acts like a stretchy "skin." Surface tension helps prevent raindrops from breaking apart and allows insects like water striders to walk along the surface of water without falling in. It also contributes to the water rising along the edge of the glass— as some water molecules are pulled up because of their attraction to the glass, they pull other water molecules on the surface along with them.
This phenomenon, called capillary action, allows water to be sucked up into small gaps, seemingly defying gravity. This might not seem like a big deal for a meniscus that's only a couple millimeters high in a glass of water. But what about a paper towel sucking water up a few inches, or a tree sucking water up tens or even hundreds of feet? How can they possibly get the water up so high? In this experiment, you demonstrated how the narrower the gap was, the higher the water would go. For a mathematical explanation, see this page.
Ask an Expert
For Further Exploration
- Try modifying the parameters of the experiment, for example by using larger or smaller plates of glass, using a thicker or thinner object as a spacer instead of a paper clip, using a different liquid, etc. How do the results change?
Related Resources
Activities
Links
- Blog Post: Colorful Walking Water Science Activity
