diffusion

Diffusion is the net movement of particles such as atoms or molecules from areas of higher concentration to areas of lower concentration. If there is some random particle motion within a fluid producing random walks, diffusion occurs. A diffusion equation is an equation describing diffusion over time. Fick's laws include a diffusion equation describing what happens in many cases, a type of diffusion termed Fickian diffusion. The equation:

       dC
J = -D ——
       dx

• J - the diffusive flux, the flow of the diffusing substance as amount of substance/unit time through a unit area.
• D - the diffusion coefficient (aka diffusivity) of the substance.
• C - the concentration of the diffusing material, as in amount of substance per unit volume.
• x - measurement over a direction over which we are describing the diffusion.

Amount of substance is a measure of the number of particles (atoms or molecules) of the substance, quantified in moles, for example.