At very low pressures, Equation (8) approaches the expression for the diffusion coefficient in the Knudsen flow regime (Loeb, 1961):
In the Knudsen flow regime the diffusion coefficient is independent of pressure and the governing PDE simplifies to Fick's second law:
193 cm) the conditions for the Knudsen flow regime exist at p < 0.
13 Pa, the resulting D represents the limiting value for the Knudsen flow regime.
This can be: i) transport through a dense (nonporous) layer, ii) Knudsen flow in narrow pores, iii) viscous flow in wide pores or iv) surface diffusion along the pore wall (12).
05 [micro]m (13), the main transport mechanism is Knudsen flow.