page_title: Contact angle
section_title: Thermodynamics
hierarchical_section_title: Contact angle / Thermodynamics
context_page_description: The contact angle is the angle, conventionally measured through the liquid, where a liquid–vapor interface meets a solid surface. It quantifies the wettability of a solid surface by a liquid via the Young equation. A given system of solid, liquid, and vapor at a given temperature and pressure has a unique equilibrium contact angle. However, in practice a dynamic phenomenon of contact angle hysteresis is often observed, ranging from the advancing contact angle to the receding contact angle. The equilibrium contact is within those values, and can be calculated from them. The equilibrium contact angle reflects the relative strength of the liquid, solid, and vapour molecular interaction.
context_section_description: The shape of a liquid–vapor interface is determined by the Young–Dupré equation, with the contact angle playing the role of a boundary condition via the Young equation.
  The theoretical description of contact arises from the consideration of a thermodynamic equilibrium between the three phases: the liquid phase (L), the solid phase (S), and the gas or vapor phase (G) (which could be a mixture of ambient atmosphere and an equilibrium concentration of the liquid vapor). (The "gaseous" phase could be replaced by another immiscible liquid phase.) If the solid–vapor interfacial energy is denoted by , the solid–liquid interfacial energy by , and the liquid–vapor interfacial energy (i.e. the surface tension) by , then the equilibrium contact angle  is determined from these quantities by the Young equation:
  The contact angle can also be related to the work of adhesion via the Young–Dupré equation:
  where  is the solid – liquid adhesion energy per unit area when in the medium G.
caption: Schematic of a liquid drop showing the quantities in the Young equation.