I was studying about drying phenomena in porous media and was left wondering whether capillary pressure is the extra pressure needed by the non wetting fluid to imbibe the capillary throat or not. Moreover, while drying at normal conditions, what are the expected values of the pressure exerted by wetting and non-wetting fluid?
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The capillary pressure is the pressure difference over the curved liquid-gas interface. Typical values can be computed using the Young-Laplace equation.
If the fluid is wetting (contact angle CA < 90), the pressure is positive, meaning that imbibition will occur spontaneously.
For non-wetting fluids (CA>90 degrees) the pressure is negative, meaning that the imbibition process in non-spontanious. In this case penetration of the porous medium will only occur if an external pressure exceeds the capillary pressure. An example of such a case is found in mercury-porosimetry.
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