Fick's Second Law of Diffusion



From the law of conservation of matter, the change of solute concentration with time must be the same as the local decrease of the diffusion flux, in the absence of a source or a sink, that is




Substituting  from Fick's 1st law we get




When the concentration of the solute is low, the diffusivity at a given temperature can be considered as a constant and we get




Units of D (Diffusion Coefficient) are cm2/sec those of C (x, t) atoms/cm3