It was recalled that 2 alternative expressions existed for the diffusive flux in inhomogeneous systems: Fick's law and the Fokker–Planck law. Here, the origins of these expressions were re-examined and numerical and physical experiments were performed in order to shed light on this duality. It was concluded that, in general, the Fokker–Planck expression should be given preference; in spite of the fact that Fick's law seemed to be more popular.

On the Applicability of Fick’s Law to Diffusion in Inhomogeneous Systems. B.P.van Milligen, P.D.Bons, B.A.Carreras, R.Sánchez: European Journal of Physics, 2005, 26[5], 913-25