Material Absorbency

Here’s a simple experiment inspired by the film, but simpler to repeat. Crucially, it doesn’t involve you getting drenched with a watering can. You can do that too, if you like, but don’t come running to us if your ‘waterproof’ jacket isn’t as waterproof as you thought.

What you need:

  1. A number of different materials, some waterproof, some very absorbent and some not absorbent at all, and all large enough to stretch over the top of a beaker or jam jar.
  2. A beaker or jam jar, and a large elastic band.
  3. A glass of water — you can add food colouring to make it easier to see, but only if you don’t mind staining the materials.
  4. A dropper or syringe.

What you do:

  1. Stretch one of the materials over the top of the jam jar or beaker, and hold it in place with the elastic band.
  2. Take some water in the dropper.
  3. With one person watching the inside of the jam jar, count as you place drops of water onto the top of the material. Do it slowly so the water has time to soak into the material. Stop adding water when the first drop falls from the material into the jam jar. Make a note of the material and how many drops of water it could absorb before the water finally fell through.
  4. You may find that some materials didn’t let any water through, and the water all sat on the top.

What’s going on?

Each water molecule is made up of an oxygen atom and two hydrogen atoms. These are bonded in such a way that the oxygen tends to attract the electrons in the water molecule, so the oxygen becomes slightly negatively charged. As a result, the two hydrogen atoms become slightly positively charged. If a water molecule finds itself on a surface with lots of molecules with negative charge, the positively charged hydrogen atoms in the water molecules bond weakly with the surface and the water stays in place.

Materials which absorb a lot of water will tend to have lots of places where the water molecules can form these bonds. If a material is designed to be absorbent it usually will have a very large surface area so there is even more places for the water to stick.

Waterproof materials will tend to have a tighter weave, so fewer places for water to bond. They can also be coated in special substances which are hydrophobic: they hate water and repel it, so water runs off.