Contact angle measurements
The contact angle is the angle at which a liquid/vapor interface meets the solid surface. The contact angle is specific for any given system and is determined by the interactions across the three interfaces. Most often the concept is illustrated with a small liquid droplet resting on a flat horizontal solid surface. Ideally, the droplet should be as small as possible because the force of gravity, for example, can actually change the above-mentioned angle. The shape of the droplet is determined by the Laplace equation. The contact angle plays the role of a boundary condition.
