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Tracer diffusion in hard sphere fluids from molecular to hydrodynamic regimes

TitleTracer diffusion in hard sphere fluids from molecular to hydrodynamic regimes
Publication TypeJournal Article
Year of Publication2006
AuthorsSokolovskii, RO, Thachuk, M, Patey, GN
JournalJournal of Chemical Physics
Volume125
Pagination10
Date PublishedNov
Type of ArticleArticle
ISBN Number0021-9606
KeywordsBOUNDARY-CONDITIONS, BROWNIAN PARTICLE, COEFFICIENT, DYNAMICS SIMULATION, friction, KINETIC-THEORY, RIGID PARTICLES, SINGLE-PARTICLE MOTION, SIZE DEPENDENCE, TRANSPORT-COEFFICIENTS, VELOCITY-AUTOCORRELATION FUNCTION
Abstract

Molecular dynamics is employed to investigate tracer diffusion in hard sphere fluids. Reduced densities (rho(*)=rho sigma(3), sigma is the diameter of bath fluid particles) ranging from 0.02 to 0.52 and tracers ranging in diameter from 0.125 sigma to 16 sigma are considered. Finite-size effects are found to pose a significant problem and can lead to seriously underestimated tracer diffusion constants even in systems that are very large by simulation standards. It is shown that this can be overcome by applying a simple extrapolation formula that is linear in the reciprocal cell length L-1, allowing us to obtain infinite-volume estimates of the diffusion constant for all tracer sizes. For higher densities, the range of tracer diameters considered spans diffusion behavior from molecular to hydrodynamic regimes. In the hydrodynamic limit our extrapolated results are clearly consistent with the theoretically expected slip boundary conditions, whereas the underestimated values obtained without extrapolation could easily be interpreted as approaching the stick limit. It is shown that simply adding the Enskog and hydrodynamic contributions gives a reasonable qualitative description of the diffusion behavior but tends to overestimate the diffusion constant. We propose another expression that fits the simulation results for all densities and tracer diameters. (c) 2006 American Institute of Physics.

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