When a liquid does not spread on a substrate (usually a solid), a contact angle \(\left(\theta\right) \) 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 \(\theta\). It is often necessary to distinguish between the 'advancing contact angle' \((\theta _{\rm{a}}) \), the 'receding contact angle' \((\theta _{\rm{r}}) \) and the 'equilibrium contact angle' \((\theta _{\rm{e}}) \). When \((\theta _{\rm{r}}) \neq (\theta _{\rm{a}}) \) the system is said to exhibit contact angle hysteresis