The contact angle is an easy measurement method to determine the wettability of a solid substrate by a liquid. The liquid will form a drop shape when depositing a drop onto a solid substrate. The point where the solid, the liquid, and the vapor meet is called the three-face point, and it determines the contact angle. The contact angle is also an indicator of wettability, which depends on the combination of solid substrate and liquid and the environment.

The relationship between the contact angle, the solid surface free energy, the liquid surface tension, and the interfacial tension between solid and liquid is defined by Young’s equation.