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Essay:
Fick's lawDiffusion of water through materials is generally assumed to follow Fick's law. This states that the rate of diffusion is equal to the water potential gradient multiplied by a constant. If the process takes place at constant temperature, any unit of potential gradient will do: RH difference per metre or vapour pressure difference per metre will give the same result, as long as the constant matches the unit. However, at least one common absorbent material, wood, does not obey Fick's law (14). Furthermore, the "constant" depends on the water content of the material. Padfield, T., 1998
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