Stokes-Einstein relations for a square-well fluid.
A Stokes-Einstein relation, relating the shear viscosity eta to the self-diffusion coefficient D, is constructed for a classical fluid subject to an effective two-body intermolecular force, derived from a square-well potential, undergoing dynamics as described by a Smoluchowski equation for pair diffusion. The time correlation functions for eta and 1D are separated into contributions from delta function, hard-sphere forces, and from delta function, square-well soft forces. Furthermore, D is separated into its two- and three-body time correlation functions, and eta into its two- to four-body terms. D shows activated diffusion, as in Arrhenius behavior, and on the level of two-body dynamics, the Deta product adheres to the Stokes-Einstein relation, subject to a small correction for potential softness. Three-body time correlation functions increase D, whereas three- and four-body correlation functions in eta are partially offsetting. The deviation of Deta product from the Stokes-Einstein law arises from the three-body time correlations functions in D.
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Department of Chemistry, Oregon State University, Corvallis, 97331, USA.
Evans
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