Is water actually wet? The answer to this question requires some philosophical thinking and depends on how you define wetness. The debate over whether water is wet is likely to continue for as long as the planet is awash with the stuff.
Most scientists define wetness as a liquid’s ability to maintain contact with a solid surface, meaning that water itself is not wet, but can make other sensation.
But if you define wet as ‘made of liquid or moisture’, as some do, then water and all other liquids can be considered wet. Some people describe wetness as a physical, cooling sensation experienced when water takes in energy to evaporate into surrounding air.
The balance between adhesive and cohesive forces determines the degree of wetting. Cohesive forces, such as hydrogen bonds, hold water molecules to each other and create a surface tension. With strong cohesive forces, water tends to stay as spherical droplets to minimise contact with a surface.
Adhesive forces attract the water to the surface of another material and encourage it to spread. If the adhesive forces are stronger than the cohesive ones, then a surface will become wet. Other liquids – such as alcohol – are better than water at wetting. Adding detergents can improve water’s wetting ability by lowering the cohesive forces.
The nature of the surface exposed to water affects how wet it will become. Water-hating (hydrophobic) surfaces, such as waterproof fabrics, push liquid drops to have as little contact as possible.
You can define a material’s hydrophobicity in terms of the internal contact angle that a water droplet makes with the surface. A perfectly hydrophobic surface is totally water repellent with a contact angle of 180°, while a perfectly wettable surface has a contact angle of zero.
Read more:
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- How does spider silk collect water?
- How old is water?
Asked by: Everett Manning, via email
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