Diffusion in Binary Mixtures Ficks First Law

Let us consider in more detail diffusion in a simple system made up of components 1 and 2. Let and represent the velocities of transfer of components 1 and 2 and u = + 

X2U2 represents the molar average velocity of the mixture. If Cj and C2 are the molar concentrations of 1 and 2 and is the total mixture molar concentration, then the diffusion flux is usually related to the mole fraction gradient by the constitutive relation 

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