Young's equation

When a liquid comes into contact with a solid in a bulk, gaseous phase, according to Young’s equation, there is a relationship between the contact angle θ, the surface tension of the liquid σlg, the interfacial tension σsl between liquid and solid and the surface free energy σsg of the solid:

σsg = σsl + σlg ⋅ cosθ

Schematic diagram of a drop on a solid

Schematic diagram of the components of a three-phase system which are relevant for wetting

The equation is valid for three-phase systems in thermodynamic equilibrium for ideal (smooth and chemically homogenous) solids and pure liquids. It is the basis of all models for determining the surface free energy of solids by means of contact angle measurements.