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Diffusion
Fick's Laws

A Phenomenological Approach
· The redistribution of material that occurs through diffusion can be described by Fick's laws which describe the material flux, and the rate of change of material concentration at a fixed position.
· Many diffusion systems can be considered in a one-dimensional approximation. In this limit, Fick's first law has the form:
(Diffusion Flux) = J = - D(T) (dc/dx)
where (dc/dx) is the concentration gradient, and D(T) is the temperature dependent diffusion coefficient. 
· The diagram illustrates the steady state situation in the presence of a constant concentration gradient.
From: Newey & Weaver, 
"Materials Principles and Practice," 
Butterworths (1990)