If the change of concentration of a component A of the diffusive constituents in a mixture occurs over a time at a point, the Fick’s law of diffusion at unsteady state condition can be expressed for Z-direction as
Both the diffusive and non-diffusive constituents affect the rate of unsteady state diffusion. The diffusivity at unsteady state condition can be expressed in terms of activation energy and the temperature as
The activation energy (ED) for the diffusion decreases the rate of diffusion whereas temperature increases the diffusion rate.