A new interpretation of contact angle variations in view of a recent analysis of immersion calorimetry

The values of the contact angles of different liquids on the same solid are classically interpreted by theories assuming that the vapor adsorption is negligible on solids presenting contact angles. This implies that the solid may be defined by a certain tension, called "critical tension", which is n ot clearly thermodynamically defined. These assumptions are in disagreement with modern theories of vapor adsorption science. In this paper, it is sho wn that it is possible to understand the contact angle values using a rigor ous derivation of the Young equation, which links the contact angle, the li quid-vapor surface tension of the drop, and the surface tension of the soli d in equilibrium with its own vapor. Numerical approximations allow the con tact angle variation to be predicted, without use of ad hoc definitions. It is then possible to link vapor adsorption results and contact angle experi ments. It is also possible to deduce the surface tension of solids simply a nd to compute contact angles on powders without using the Washburn method. (C) 2000 Academic Press.