Water Repellancy and Wicking

Put simply, these phenomenon depend on how water is attracted to a surface. It depends on water being repelled or attracted.

Water repellency occurs when the surface tension of the material is higher than water, often due to its chemical structure. If the surface tension is higher still, it will also repel oil. (Oil has a higher surface tension than water which is why the two don't mix.) Some plants, like the lotus, have a very water repellent chemistry on their leaves and this is sometimes called the 'lotus effect'.

You may recall from school, looking at test tubes with mercury in them. The top of the mercury (the meniscus) was higher than where it touched the glass. The glass repels mercury. When water is put in a glass it is attracted slightly up the walls and the meniscus is concave. In fact it wicks slightly.

Wicking is when a surface attracts a liquid, like a wick in an oil lamp. The liquid rises up through the small channels in the wick structure by capillary action. The capillaries are like extremely fine test tubes, and because they are so fine they have the effect of drawing up the liquid. This is how plants suck water up from the ground. The effect is amplified by evaporation of moisture from the top of the capillaries, through lighting the oil lamp or perhaps, in the case of plants, through sun and wind.

Most modern next to the skin outdoor clothing is wicking. If it were water repellent droplets of sweat would sit on the surface and drip uncomfortably down the inside.

Fur technology is water repellent, but other mechanisms come into play in bad weather:

• The fur is most dense on the inside and less dense on the outside, so water tends to drain outwards.
• The outer layer can become saturated because condensation occurs in it, it has become dirty or the water repellency has worn off - it can have a sentiment change from water hating (phobic) to water loving (philic). This has the effect of drawing water from the inner, into the outer material where it can drain away or be evaporated by the wind.

These mechanisms allow feather and fur technology to remove liquid water while preventing rain from getting in.