When a system contains two or more components whose concentration vary from point to point, there is a natural tendency for mass to be transferred, minimizing any concentration difference within the system. Mass transfer in a system is governed by Ficks first law: Diffusion flux from higher concentration to lower concentration is proportional to the gradient of the concentration of the substance and the diffusivity of the substance in the medium.' Mass transfer can take place due to different driving forces. Some of them are:
* Mass can be transferred by the action of a pressure gradient (pressure diffusion)
* Forced diffusion occurs because of the action of some external force
* Diffusion can be caused by temperature gradients (thermal diffusion)
* Diffusion can be caused by differences in chemical potential
This can be compared to Fick's law of diffusion, for a species A in a binary mixture consisting of A and B:
where D is the diffusivity constant.