FICK'S LAWS OF DIFFUSION

• Describe how diffusion takes place
• First law - for steady-state diffusion (concentration does not change in time)
  • defines the flux of atoms as a function of atoms concentration: (Fick's first law of diffusion)

    • J = flux or net flow of atoms

    • D = diffusivity or diffusion coefficient

    • dC/dt = concentration gradient

  • The diffusion flux is proportional with the gradient concentration

  • the arrow in the picture below shows that the movement is in the sense of decreasing concentration

• Second law - for non steady-state diffusion (concentration varies in time)
  • shows how the concentration varies in time
  • It can be deducted from the continuity equation. If we consider a material of aria A, length dx (see picture below), we can write the continuity equation (mass conservation inside the material) for this case as:

• The variation of concentration in time is: (Fick's second law of diffusion)
• The lower picture shows how the concentration varies in space at different moments of time (different colours)