On the applicability of Fick's law to diffusion in inhomogeneous systems

Two alternative expressions exist for the diffusive flux in inhomogeneous systems: Fick's law and the Fokker–Planck law. Here we re-examine the origin of these expressions and perform numerical and physical experiments to shed light on this duality. We conclude that in general the Fokker–Planck expression should be conceded preference, in spite of the fact that Fick's law seems to be more popular.