Wetting Phenomena: Line Tension and Gravitational Effect.

An apparent contact angle is formed when a droplet is deposited on a solid substrate. Young's law has been employed to describe the equilibrium contact angle. Often in experiments, the equilibrium contact angle deviates from Young's law and depends on the volume of the droplet, known as the line tension effect. However, the physical origin of the line tension is quite controversial. Especially, the sign and the quantity of the line tension spanning 6 orders of magnitude are unsolved problems. Here, we quantify the line energy in terms of physical parameters and demonstrate that both positive and negative line tensions exist. The results are quantitatively compared with existing experiments as well as with previous theories.