Fick's law of diffusion formula JA = - DAB. dCA/dz
Where, JA is the molar flux of A in the z-direction relative to the molar average velocity [moles per unit area per unit time, kmol/m2.s]
CA is the concentration of A [moles of A per unit volume]
dCA/dz is the concentration gradient in the z-directions.
DAB is the proportionally constant, known as the molecular diffusivity or diffusion coefficient for component A diffusing through B, m2/s, and distance in the direction of diffusion m. The negative sign indicates/implies that diffusion occurs in the direction of a drop in concentration.