Mechanical Basis of Diffusion

Four types of theory of diffusion are distinguished, presented, and compared: kinematical, hydrodynamical, kinetic, thermodynamic. A simple mechanical theory, based upon recognizing the diffusive drags as forces that produce motions, is shown to include and unify all earlier attempts. In this theory a hypothesis of binary drags is formalized and shown to lead to the symmetry relations proposed originally by Stefan. The Onsager relations for pure diffusion are proved to be equivalent to Stefan's relations. Known results show that these relations hold as a first approximation in the kinetic theory of monatomic gas mixtures. Whether or not they hold in higher approximation is unknown.