Ficks laws of diffusion describe diffusion and were first posited by Adolf Fick in 1855 on the basis of largely experimental results. They can be used to solve for the diffusion coefficient, . Ficks first law can be used to derive his second law which in turn is identical to the diffusion equation.
Ficks first law: Movement of particles from high to low concentration (diffusive flux) is directly proportional to the particles concentration gradient.
Ficks second law': Prediction of change in concentration gradient with time due to diffusion.
A diffusion process that obeys Fick's laws is called normal or Fickian diffusion; otherwise, it is called anomalous diffusion or non-Fickian diffusion.