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