A 2005 paper by van Milligen asserted that the Fokker-Planck law should be used over this law in inhomogeneous systems.  Its 
A 2005 paper by van Milligen asserted that the Fokker-Planck law should be used over this law in inhomogeneous systems.  Its coefficient, dependent on temperature, fluid viscosity, and particle size, can be determined via the Stokes-Einstein relation.  Generalized by the Onsager reciprocal relations, the second one describes changes in volume over time, while the first one describes steady-state volumes.  FTP, identify this set of laws named after a German scientist which describe the process of diffusion.
A 2005 paper by van Milligen asserted that the Fokker-Planck law should be used over this law in inhomogeneous systems.  
Fick's law(s)