Implementation of a perturbation model for a dilute binary liquid and comparison of model mass diffusion coefficients with microgravity experiment results for liquid Pb 1 wt % Au

Abstract

The estimation of mass diffusion coefficients, through Earth-bound experiments, remains difficult, due to the frequent occurrence of dominating convective flows resulting from gravity-driven density gradients caused by temperature and concentration gradients. To partly remedy this, a series of capillary mass diffusion experiments has been performed in microgravity on a number of different space platforms, sometimes performed on a microgravity isolation mount, to further reduce the platform operational noise referred to as “g-jitter”. Theoretical comparisons are sought for the experimental observations. Two numerical models have been developed based on perturbation theory. We have used the Lado criteria for minimizing the difference in free energy between the multispecies liquid of interest and a reference hard-sphere liquid. The hard-sphere liquid is characterized by the rational function approximation of the partial radial distribution functions. The effective embedded atom like glue potential has been used to model the liquid of interest. This has necessitated introducing the mean coordination number as an additional parameter. Isothermal compressibility has been used to determine the mean coordination number. The initialization of the numerical solutions, and the extension of solutions over the experimental range of temperatures have been demonstrated for Pb 1 wt % Au. The model results have been used to estimate the mass diffusion coefficients by applying the Enskog equation to the reference hard-sphere liquid. For consideration of capillary experiments, a definition of total mass diffusion coefficient, Dtot, has been introduced to characterize reverse Kirkaldy simultaneous diffusion. The mixed diffusion coefficient estimate is in good agreement with the mixed diffusion coefficient estimated from the velocity correlation of molecular dynamic simulations. Dtot and D11 are in good agreement with the experimental results indicating that reverse Kirkaldy simultaneous diffusion has had an influence on the experiment. Good agreement between the mixed mass diffusion coefficient and the result from molecular dynamic simulation indicates the perturbation models can predict the mixed coefficient. These models may assist in the analysis of data from both Earth-bound and microgravity, mass diffusion experiments when the required embedded atom type potentials are available.