Capillary Converging diverging

It has been widely demonstrated that injection of water into both PTFE-treated and untreated GDMs requires positive capillary pressure while water withdrawal requires negative capillary pressures, where capillary pressure is defined as Pc = Pl — Pg- Various explanations for this behavior have been put forward, including contact angle hysteresis,59 the converging-diverging nature of void space... 

We have dealt with Knudsen flow in a straight capillary and in a converging/diverging pore, and we see that the shape of a pore can contribute to the flux calculation. Now we turn to dealing with practical solids where pore orientation is rather random. 

Because imbibition and drainage behaviour are to be modelled, we require the capillary properties of the individual tetrahedral pores. This Involves calculating the curvature of menisci in non-axisymaetric, converging-diverging pores -something that cannot yet be done - and consequently we have fallen back on the... 

The capillary assumed so far is cylindrical in shape and its size is uniform along the tube. If the capillary size is not uniform, but either converging or diverging, the Knudsen diffusivity and flux given in eqs. (7.4-10) and (7.4-11) are still valid and they hold for every point along the capillary, provided that the diffusion process is still dominated by Knudsen mechanism. Evaluation of the Knudsen flux for such a capillary is dealt with in the next section. 

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