Does Wet Hair Float or Cling?

Introduction

Have you ever noticed how hair moves freely when it is under the water, but clings together as soon as it emerges out of the water? Not only human hair does this; when wet dogs shake themselves after a swim, their hair clings together in strands. Try this activity to see why wet hair is far less fluffy than dry hair!

Materials

• Paintbrushes, the round ones work best. If available, select brushes with different thicknesses, types of bristles, etc.
• Bowl or sink filled with water
• Towel
• Area that can get wet
  
  
  

What Happened?

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. This is because the particles around it can freely move in all directions, allowing the bristles to flutter out in whatever way they feel like.

However, 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, water particles (H₂O molecules) stick to them. That is what we call "wet." As water particles like to be surrounded by other water particles, they bundle up. As such, the individual wet bristles remain tightly packed. The water particles on the outside feel pulled towards water particles inside the bundle of bristles. It makes it harder to pull one bristle away.

A wet brush exposed to air dries as the water particles evaporate, and dry bristles can again move freely because they are submerged in air.

Digging Deeper

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 ml) contains about 1.5 sextillion or 15,000,000,000,000 billion molecules of H₂O!

It sometimes looks like there is a thin film around a droplet of water that holds it together. This is because of surface tension. H₂O molecules attract each other and like to hold onto each other closely. This is called cohesion. The molecules can move around alongside each other, 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 into. That is not the case 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 wet non-submerged hair, you can see the effect of cohesion and surface tension. When pulling the bristles out of water, H₂O molecules stick to them. As water is a cohesive substance, these molecules like to stick together and pack closely. As such, the individual wet bristles remain tightly packed. The H₂O molecules on the outside feel pulled towards H₂O molecules inside the bundle of bristles. It makes it harder to pull one bristle away.

For Further Exploration

• Pay attention to people's hair in the swimming pool. How does it move when it is still dry? How does it move when submerged in the water? How does it move when the person emerges from the pool? Why would this happen?
• Observe the hair of animals when wet. Does it also cling together when wet and exposed to air?
• Find other examples where the wet version of an item sticks together more than the dry version. Do you think this can also be explained by cohesion and surface tension?

Related Resources

Project Ideas

Activities

Links

• Blog Post: Measuring Surface Tension with a Penny: Weekly Science Spotlight
• Blog Post: Dancing Water Droplets

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