Editing Molecular diffusion (section)

== Non-equilibrium system ==
[[File:entropie.svg|thumb|280px|Illustration of low entropy (top) and high entropy (bottom)]]
Because chemical diffusion is a net transport process, the system in which it takes place is not an [[chemical equilibrium|equilibrium]] system (i.e. it is not at rest yet). Many results in classical thermodynamics are not easily applied to non-equilibrium systems. However, there sometimes occur so-called quasi-steady states, where the diffusion process does not change in time, where classical results may locally apply. As the name suggests, this process is a not a true equilibrium since the system is still evolving.

Non-equilibrium fluid systems can be successfully modeled with Landau-Lifshitz fluctuating hydrodynamics. In this theoretical framework, diffusion is due to fluctuations whose dimensions range from the molecular scale to the macroscopic scale.<ref>{{cite journal | last1=Brogioli | first1=Doriano | last2=Vailati | first2=Alberto | title=Diffusive mass transfer by nonequilibrium fluctuations: Fick's law revisited | journal=Physical Review E | publisher=American Physical Society (APS) | volume=63 | issue=1 | date=2000-12-22 | issn=1063-651X | doi=10.1103/physreve.63.012105 | page=012105| pmid=11304296 |arxiv=cond-mat/0006163| bibcode=2000PhRvE..63a2105B }}</ref>

Chemical diffusion increases the [[entropy]] of a system, i.e. diffusion is a spontaneous and irreversible process. Particles can spread out by diffusion, but will not spontaneously re-order themselves (absent changes to the system, assuming no creation of new chemical bonds, and absent external forces acting on the particle).