Tortuosity model analysis

Nielsen s tortuosity model [4] is perhaps the most cited model for the estimation of barrier property of flake-filled membranes. It predicts that Fq/F/ increases linearly with (a. However, as indicated by our nmnerical analysis as well as the experimental data on some systems, this simple model is likely to work well only at low (ct ) levels. The other model proposed by Cussler et al. [5] predicts that when (a ) is large, Fo/Ff increases more rapidly, at a leading rate of The experimental data on a number of systems show that the latter model predicts the permeability behavior reasonably well at high (ct )... 

A microscopic description characterizes the structure of the pores. The objective of a pore-structure analysis is to provide a description that relates to the macroscopic or bulk flow properties. The major bulk properties that need to be correlated with pore description or characterization are the four basic parameters porosity, permeability, tortuosity and connectivity. In studying different samples of the same medium, it becomes apparent that the number of pore sizes, shapes, orientations and interconnections are enormous. Due to this complexity, pore-structure description is most often a statistical distribution of apparent pore sizes. This distribution is apparent because to convert measurements to pore sizes one must resort to models that provide average or model pore sizes. A common approach to defining a characteristic pore size distribution is to model the porous medium as a bundle of straight cylindrical or rectangular capillaries (refer to Figure 2). The diameters of the model capillaries are defined on the basis of a convenient distribution function. 

The results from this analysis can now be used to construct geometrically accurate models of the diffusive transport in porous polymers. Previous models of diffusion in these polymers have used an empirically determined tortuosity factor as a lumped parameter to account for the retardation of release by all mechanisms (7-8). 

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