2008 Volume 16 Issue 1 Pages 49-60
The Young’s equation describes the interfacial equilibrium condition of a liquid droplet on a smooth solid surface. This relation is derived by Thomas Young in 1805. It has been discussed until today after his work. In general, the Young’s equation is discussed from the viewpoint of thermodynamics and derived by minimizing the total free energy of the system with intensive parameters in the total free energy kept constant, i.e., the variation of the total free energy is zero. In the derivation, the virtual work variations in the horizontal and vertical directions of the droplet on the smooth solid are considered independently. However, the virtual work variation at the droplet surface depends on the variation of the horizontal and vertical directions. This point has been overlooked in the past studies. In this study, by considering this directional dependency, we derived the modified Young’s equation based on the thermodynamics. Finally, we evaluated the modified Young’s equation by comparing the analytical solution of the relationship between a contact angle and the contact line radii of the droplet with some experimental data. Moreover, we investigated the line tension itself.