Fick's law in the context of Molecular diffusion

Diffusivity, mass diffusivity or diffusion coefficient is usually written as the proportionality constant between the molar flux due to molecular diffusion and the negative value of the gradient in the concentration of the species. More accurately, the diffusion coefficient times the local concentration is the proportionality constant between the negative value of the mole fraction gradient and the molar flux. This distinction is especially significant in gaseous systems with strong temperature gradients. Diffusivity derives its definition from Fick's law and plays a role in numerous other equations of physical chemistry.

The diffusivity is generally prescribed for a given pair of species and pairwise for a multi-species system. The higher the diffusivity (of one substance with respect to another), the faster they diffuse into each other. Typically, a compound's diffusion coefficient is ~10,000× as great in air as in water. Carbon dioxide in air has a diffusion coefficient of 16 mm/s, and in water its diffusion coefficient is 0.0016 mm/s.

In physics and engineering, mass flux is the rate of mass flow per unit of area. Its SI unit is kg⋅s⋅m. The common symbols are j, J, q, Q, φ, or Φ (Greek lowercase or capital phi), sometimes with subscript m to indicate mass is the flowing quantity.

This flux quantity is also known simply as "mass flow". "Mass flux" can also refer to an alternate form of flux in Fick's law that includes the molecular mass, or in Darcy's law that includes the mass density.Less commonly, the defining equation for mass flux in this article is used interchangeably with the defining equation in mass flow rate.