Soaking up water

These pictures show what happens when a droplet of water lands on an absorbent material. You can see holes in the absorbent materials. These allow the material to absorb the water. The moment after the drop lands it begins to flow into the holes. A few moments later it has spread out through the holes and has been completely absorbed.

Do all materials absorb the same amount of water?
No. It depends on the size and number of holes they have in them. Some materials do not have any holes and so do not absorb any water. A sponge is designed to have holes to hold water. A sponge will hold many times its own weight of water.

What materials can people use to keep them dry?
Leaves and stems are covered with a thin layer of natural wax. Wax is a natural waterproofing material. Thatch is an example of this use for keeping water out of houses.
Animal skins are also naturally waterproof and many groups of people have used them for keeping both dry and warm. < Artificial materials are, however, better in most cases and these are now used worldwide.


If a material does not absorb water is it waterproof?
It may be waterproof or it may be water-resisting. A waterproof material may not have any holes in it so there is no way that water can enter it. Plastic and rubber are examples of waterproof materials without holes. A water-resisting material has holes in it but it is coated with a material which stops the water passing into the holes. It normally only lets water through if it is touched. An umbrella is like this.
How is water stopped from getting into a water-resisting material?
The coating on the material repels water. It leaves the water sitting as large drops on the surface. The coating can be made of wax, oil or a plastic called silicone. Umbrellas are made of fibres coated with water-repelling silicone. Leather is a natural material which is water-resisting and contains natural oils, which is why it is used for shoes and boots.


Why are water-resisting materials more comfortable to wear than completely waterproof materials?
Waterproof materials do not have any holes so nothing can pass through them. Wellingtons are made of rubber. This is a waterproof material. It keeps the water away from your feet. If you wear Wellingtons for a long time you may notice that your feet get damp. This happens because your feet sweat. When they do this the sweat evaporates and forms water vapour. This cannot pass through the wall of your Wellington and so it condenses and makes your feet damp. If you wear a jacket made of rubber it will soon become uncomfortable because your body sweat will have nowhere to go.
When you wear shoes you do not find that your feet get damp because the upper parts of shoes are made from materials which are water-resisting. These have small holes which can let water vapour pass through so your feet stay dry. Other outdoor clothes are made of water-resisting materials so you do not become uncomfortably hot and damp. Sweat passes through them as water vapour.
Why are raincoats called macs?

They are named after Charles Macintosh. He invented a cloth made from rubber and wool cloth in 1823. Macintosh took wool cloth, painted one side of it with liquid rubber, and placed another thickness of wool cloth on top. This sandwich of fabrics was waterproof but the rubber did not show. This material was used to make very successful raincoats that we simply call 'macs'.

Birds fly in wet weather how are they waterproof?
The bird's feathers keep them water proof. Each feather has a central shaft with many side branches which zip together. As a bird goes about its daily life, some of the side branches become unzipped. This makes them less water-resistant so a bird spends part of its day zipping up part on its feathers. This process is called preening. Birds which spend their time swimming on water use oil to spread on their feathers as they preen.
What is a damp-proof course?
This is the line of black material that you see running along a wall about 14 centimetres or two bricks above the surface of the ground. Bricks absorb water from the ground. In time the water would reach the inside of a house and this would make the building damp.
The damp-proof course is made from a material which is waterproof. Water cannot get through it, so the building above the damp proof course is protected from the water in the ground.