Characterization of Porous Media by the Kinetics of Liquid Penetration: The Vertical Capillaries Model☆

The characterization of porous media by measuring and analyzing the kinetics of vertical capillary penetration is demonstrated. The advantage of vertical over horizontal penetration experiments lies in the possibility of independently characterizing the equivalent pore radius and contact angle. Methods for analyzing the experimental data are discussed in terms of two models: a modified presentation of the well-known equation for penetration into a single vertical cylindrical capillary, and a simple new model for penetration into an assembly of vertical cylindrical capillaries with a radius distribution. It is shown that presenting the data in the form of the cube of the height of rise over time vs time is useful for deciding which model is more suitable and for elucidating the equivalent radii and contact angle. The present approach is demonstrated by discussing two experimental examples: capillary rise in filter paper and in sand. The former is well described by the single capillary model, while the latter requires the capillary assembly model. A striking common result of the analysis of the two cases is that the equivalent contact angle comes out to be very far from zero, although the intrinsic contact angle may be expected to be close to zero.