Alternative Forms of Ficks Law

In place of the molar diffusion flux with respect to the molar average velocity we may use the diffusion fluxes in the volume average reference velocity frame uY, in which case Fick s law takes the form 

Molar concentrations c, are not suggested by solution theories as convenient concentration variables (even in ideal solutions) to represent the thermodynamically based activity a.  

The third commonly encountered form of Fick s first law is in the mass average reference velocity frame v. 

This form of Fick s law is most convenient when we must solve the mass continuity equations simultaneously with the equations of motion (see, e.g.. Chapter 10). 

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