As mentioned above, chemical molar flux of a component A in an isothermal, isobaric system is defined in Fick's law of diffusion as: J A = − − D A B ∇ ∇ c A where the nabla symbol ∇ denotes the gradient operator, D AB is the diffusion coefficient (m ·s) of component A diffusing through component B, c A is the concentration (mol /m) of component A.