|MadSci Network: Chemistry|
Spitting or adding water may seem counter intuitive for defogging goggles.
Basically the ability of air to contain water vapor is related to its temperature -- warmer air can hold more water. When you wear air tight goggles the air on the eyeball and face side of the goggles gets warmed by the nearby skin and may also have water added to it in the form of evaporating sweat. The air on the outside of the goggles will tend to be much cooler than the air on the eyeball side. This makes the goggle plastic lense cooler than the moist air on the eyeball side and when the air touches the plastic lense the water vapor condenses back to liquid and leaves a very thin layer of water that fogs up.
A slightly thicker layer of water will not cause the lense to fog up. The thicker layer of water creates a barrier that acts to insulate the goggle lense from the warm eyeball-side air. The proper balance of water is needed. Too much water, and it will splash into the eyes. Too little water and the fog effect can be amplified. With leaky goggles this can be a real annoying problem of adding/removing water. Some people like to keep a small pool of water/sweat in the goggles to slosh around in the goggle to defog along the way by shaking their head -- this works too, but could be a problem in a competitive environment of swimming or bike racing.
Many companies have worked on developing a coating that will prevent fogging. These coating are usually promoted as "secret formulaes" but the idea is to develop a material (usually a polymer) that is less favorable for water to stick to, so that the water vapor on the eyeball-side of the lense condenses on the side of the goggles and not in the lense.
A good quick description of goggles with an experimental fog proof coating can be found at:
Tom "Sweat Hog" Cull
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