Diffusivity defines the diffusion, or movement, of one material through another.
You can specify isotropic, orthotropic, or anisotropic diffusivity. If you specify
orthotropic diffusivity, the material exhibits directional dependency in the principal
directions only. If you specify anisotropic diffusivity, the material exhibits full
directional dependency, and you can define a full symmetric tensor.
The governing equations for mass diffusion are an extension of Fick's equations: they allow
for nonuniform solubility of the diffusing substance in the base material and for mass
diffusion driven by gradients of temperature and pressure.