contact angle

When a liquid does not spread on a substrate (usually a solid), a contact angle( $θ$ ) is formed which is defined as the angle between two of the interfaces at the three-phase line of contact. It must always be stated which interfaces are used to define $θ$ . It is often necessary to distinguish between the 'advancing contact angle' ( $θ a$ ), the 'receding contact angle' ( $θ r$ ) and the 'equilibrium contact angle' ( $θ e$ ). When $θ r ≠ θ a$ the system is said to exhibit contact angle hysteresis

Source:

PAC, 1972, 31, 577 (Manual of Symbols and Terminology for Physicochemical Quantities and Units, Appendix II: Definitions, Terminology and Symbols in Colloid and Surface Chemistry) on page 598