Macroscopic Wetting Behavior and a Method for Measuring Contact Angles

Abstract

Macroscopic wetting behavior is investigated theoretically from a thermodynamic viewpoint. The axisymmetric liquid meniscus formed under a conical solid surface is chosen as the subject of the theoretical analysis. Using the meniscus configuration obtained by the Laplace equation, the total free energy of the system is calculated. In the case of the half vertical angle of the cone φ = 90 deg (horizontal plate), the system shows thermodynamic instability when the meniscus attaches to the solid surface at the contact angle. This result, unlike the conventional view, agrees well with the practical wetting behavior observed in this study. On the other hand, when 0 deg < φ < 90 deg, the system shows thermodynamic stability at the contact angle. However, when the solid cone is held at a position higher than the critical height from a stationary liquid surface, the system becomes unstable. It is possible to measure the contact angle easily using this unstable phenomenon.