Simulation of self-diffusion of point-like and finite-size tracers in stochastically reconstructed Vycor porous glasses

Aim of the present study is to simulate self-diffusion in three-dimensional images of reconstructed Vycor porous glass, which have the same statistica l content as the actual material in terms of porosity and autocorrelation f unction. Effective diffusivities are determined from a step-by-step random walk process at different porosities and diffusion regimes. In all cases, t he effective diffusivity curves show a sharp decrease below 20% porosity an d drop to zero below a porosity of about 15%, a value suggested independent ly from the theory of spinodal decomposition. Comparison between the comput ed and experimental diffusivity values obtained in the Knudsen regime, show s a relative difference of less than 6%. Additional simulations in the mole cular diffusion regime are performed using inert tracers of finite size. In these simulations, a transition is found in the value of diffusivity from a high value at small time scales, to a lower constant value achieved at la rge times. The time at which this crossover takes place corresponds to a me an-square displacement value whose square root is roughly equal to the aver age pore radius of the material. Comparison between computed and experiment al results taken from the literature, shows very good agreement, as in the case of point-like tracer diffusion. Thus, the reconstructed Vycor is shown to represent properly not only structural properties such as porosity and correlation function, but also dynamic properties such as effective diffusi vity of inert gas molecules of various sizes. (C) 1999 American Institute o f Physics. [S0021-9606(99)51630-9].