In Figure 1, the droplet on ideal and real surfaces is presented.
On an ideal surface the Young equation applies and the measured
contact angle is equal to the Young contact angle (Figure
1(a)). On a real surface the actual contact angle is the angle
between the tangent to the liquid-vapor interface and the actual,
local surface of the solid (Figure 1(b)). However, the measured (apparent) contact angle is the angle between the tangent to the
liquid-vapor interface and the line that represents the apparent
solid surface, as seen macroscopically. Actual and apparent
contact angle values can deviate substantially from each other. To
calculate real surface free energies of the solid the actual contact
angles should be used.