Diffusion as Hydrodynamic Motion

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Abstract
A phenomenological definition of diffusion in a binary mixture suggests a description of the motion by a hydrodynamic equation which contains a frictional force proportional to the relative diffusion velocity. Plausible assumptions give a set of hydrodynamic equations for any gaseous mixture, which may include charged molecules in a magnetic field. The equations determine the diffusion velocities in terms of the applied forces, the pressure gradients, and the binary diffusion coefficients. The same equations are also derived from kinetic theory in the first diffusion approximation. In higher approximations where thermal diffusion first appears, the equations are no longer valid.

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