Diffusion PoiseuiUe

Knudsen diffusion [20,30,32,37 ] depending on gas pressure and mean free path in the gas phase applies to pores between 10 A and 500 A in size however, there are examples in the literature where it was observed for much larger pores [41 ]. In this region, the mean free path of molecules in gas phase A is much larger than the pore diameter d. It is common to use the so-called Knudsen number K = X d o characterize the regime of permeation through pores. When 1, viscous (PoiseuiUe) flow is realized. The condition for Knudsen diffusion is 1. An intermediate regime is realized when A), 1. 

Figure 4.5 Example of Taylor dispersion in a microchannel with a steady PoiseuiUe flow (a) initial flat concentration of the solute, (b) stretching of the solute to a paraboloid-shaped plug neglecting diffusion, and (c) solute plug with diffusion indicated by vertical arrows...

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