Hauptseminar Porous Media SS 2021/Random walk models diffusion in porous media
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- Random walks in porous media: Diffusion at the pore and at the pore-network scale
- Alexander Schlaich
Diffusion in confined geometries - in contrast to transport - refers to the case where no driving force (chemical potential gradient, e.g. density/pressure/electric field) is applied, or where the transport due to the driving force can be neglected due to the vanishing gradients (which is typically the case in smallest pores). In this seminar, in a first step, the different diffusion mechanisms that pertain to fluid transport in nanoporous media are presented with special attention to their dependence on pore size vs. fluid molecule size and thermodynamic variables such as fluid density, pressure or temperature T. Then, in a second step, combination rules which allow one to describe the transport resulting from different diffusion mechanisms are introduced.
In detail, different diffusion mechanisms often coexist in nanopores due to the inhomogeneous density profile that results from adsorption. Of special importance are thus so-called correlation-function approaches which can be probed directly in experiments pulsed-field-gradient nuclear magnetic resonance (PFG-NMR), which allow to establish an effective diffusion constant in analogy with electrical transport in resistance networks. In a further refinement one has to take into account that the contribution of each equivalent resistor must be weighted by the local density of the fluid and that molecules can only diffuse if a diffusion path is available.
Points to be discussed
- Knudsen diffusion with slip corrections
- Molecular diffusion (Random walk and transition state theory)
- Reed–Ehrlich model
- Combination rules from correlation function approaches and intermittent Brownian motion
- Density-dependent diffusion and free volume theory
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Diffusion in porous networks