FIG. 22 Comparison of contact angles measured by the proposed method with those from a photograph: (a) advancing contact angle; (b) receding contact angle.

Fig. 23 schematically shows the principle of the method of using a circular cylinder as the test solid. S, L, and V in the figure indicate solid, liquid, and vapor phases, respectively. The cylinder held horizontally is first immersed and then slowly drawn from the liquid bath. We can see a pair of two-dimensional menisci formed under the cylinder, as shown in Fig. 23a. As the cylinder is raised to a certain critical height, the waists of the two meniscus curves contact each other and the liquid breaks off from the solid surface. The geometry of the two-dimensional meniscus can be determined from the Laplace equation (11) and the contact angle as a boundary condition, as mentioned in Section I. Hence we could calculate the contact angle using the