Key concepts
Physics
Water
Surface tension
Cohesion

Introduction
Have you ever noticed how hair floats freely when submerged in water, but when surrounded by air when wet it clings together? You can see this in animal fur as well: When wet dogs shake themselves their hair clings together in strands. Try this activity to see why wet hair is far less fluffy than dry hair!

Background
Water can also be called H2O because the smallest amount of water that is still water (called a water molecule) consists of two hydrogen (H) atoms and one oxygen (O) atom. Once the H2O molecule is broken, you no longer have water. This molecule is tiny—an average droplet of water (0.05 milliliter) contains about 1.5 sextillion or 1,500,000,000,000 billion molecules of H2O!

It sometimes looks like there is a thin film around a droplet of water that holds it together. This is a result of surface tension. H2O molecules attract one another and like to hold on closely. This behavior is called cohesion. The molecules can move around alongside one another but they stay connected. When an H2O molecule is in the middle of a droplet, it feels attracted to all of the surrounding H2O molecules. It is pulled in all directions equally, so there is no preferred direction to move. That is not the case, however, for an H2O molecule located on the edge of the droplet. This molecule feels attracted to those inside the droplet but none on the outside. It feels an inward attraction, thus only moves inward, holding the droplet together.

If you look at submerged hair and wet, nonsubmerged hair—or fur—you can see the effect of cohesion and surface tension. In this activity you will see this effect with paintbrushes.


Materials

  • Paintbrushes (Round ones work best; if available, select brushes with different thicknesses, types of bristles, etcetera.)
  • Bowl or sink filled with water
  • Towel

Preparation

  • Gather your materials in a work area that can get wet. 


Procedure

  • Start with a dry paintbrush. Observe its bundle of bristles. What shape is it? Is it fluffy or slick? Can you easily distinguish single bristles?
  • What is holding the bristles together? How easy is it to pull one to the side?
  • In a moment you will submerge the brush in water. Do you expect the bundle of bristles to look different? If so, how and why do you think this happens?
  • Submerge your brush and observe the bundle when surrounded by water. Did the shape change? Does it look fluffier or slicker? Can you distinguish single bristles as easily?
  • How do you think an individual bristle will respond if you try to pull it to the side? Why?
  • Get your hands in the water and feel how easily you can pull one bristle to the side.
  • Was this as predicted? If you had an explanation, would you like to change it?
  • What do you think the bundle of bristles will look like when you take it out of the water?
  • Remove your brush from the water and observe. What shape is the bundle of bristles? Is it fluffy or slick? Can you easily distinguish single bristles? Why do you think it looks this way when wet but not submerged?
  • Try to pull one bristle to the side. How easy is it?
  • You only tested one paintbrush. Maybe a different paintbrush will be different! Repeat the activity with another paintbrush. Does the same thing happen?
  • Extra: Pay attention to your own hair. How does it move when it is still dry? How does it move when submerged in the water? How does it move when you emerge from a bath? Why would this happen?
  • Extra: Observe the fur of animals when they are wet. Does it also cling together when wet and exposed to air?
  • Extra: Can you find other examples where the wet version of an item sticks together more than the dry version does? Do you think this can also be explained by surface tension?

Observations and results
The bundle of bristles was probably a little fluffy when it was dry or submerged in water. You could probably easily see the single bristles and pull one to the side. But you probably could not do this with a wet bundle of bristles once it was no longer submerged.

When pulling the bristles out of water, particles of water (H2O molecules) stick to them. That is what we call “wet.” Because water is a cohesive substance its particles like to be surrounded by other ones so they bundle up. Thanks to that behavior, they keep the individual wet bristles tightly packed. The water particles on the outside feel pulled toward water particles on the inside the bundle of bristles. This is referred to as surface tension, and it makes it harder to pull one bristle away.

When submerged in water the bristles are also wet, but the water particles around it can freely move in all directions. This allows the bristles to flutter out in any direction. A wet brush exposed to air dries as the water particles evaporate. Dry bristles can also freely move because they are submerged in air. This also happens with human hair or animal fur.

More to explore
Introduction to Surface Tension, from the Non-Newtonian Fluid Dynamics Research Group, Massachusetts Institute of Technology
Measure Surface Tension with a Penny, from Scientific American
Surface Tension Science: Build a Raft Powered by Soap, from Scientific American
Science Activities for All Ages!, from Science Buddies

This activity brought to you in partnership with Science Buddies

Science Buddies