ABSTRACT
Abstract-Notwithstanding the explicit limitations indicated in the original paper, the Washburn equation is often used to calculate the contact angles from wicking experiments on porous materials that do not fulfill the exact conditions of its validity. This procedure is implemented even in commercial software programs. It is based on the hypothesis that some aspects of the porosity can be expressed as "constant" characteristics of the material analyzed. However, doubts have been raised in the literature whether such a description is correct and acceptable in all cases. In the present paper some wicking measurements made on different natural and synthetic porous materials with water and different perfectly wetting (8=0) liquids are presented. It is shown that a weak correlation exists between the contact angles estimated through the Washburn equation and those directly measured. In particular we have emphasized that the value obtained depends on the initial choice of the reference liquid and on the sample size. It seems possible, however, to evaluate some aspects of the porosity from wicking experiments, although the correlation with the contact angle of the liquid on the porous material is certainly non-trivial.
