The contact angle measurement is a way of examining the wettability of a solid by a liquid in a quantitative way.  The contact angle (Θ) itself is defined as the angle between the tangentials of the phase boundaries of gas to liquid and liquid to solid respectively at the point where the three medium meet.  

The contact angle is determined by the equilibrium between adhesive and cohesive forces.  Adhesive forces on a liquid-solid boundary cause a liquid drop to spread across the surface. Cohesive forces among the liquid particles however cause the drop to minimize its surface to the solid thus by forming small drops on the surface. The tendency of a drop to spread out over a flat, solid surface increases as the contact angle decreases. Thus, the contact angle provides an inverse measure of wettability. The used measurement technique is the sessile drop method.