https://doi.org/10.1351/goldbook.C01290
When a liquid does not spread on a @S06084@ (usually a solid), a contact @A00346@ \(\left(\theta\right) \) is formed which is defined as the @A00346@ 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 @A00346@' \((\theta _{\text{a}}) \), the 'receding contact @A00346@' \((\theta _{\text{r}}) \) and the 'equilibrium contact @A00346@' \((\theta _{\text{e}}) \). When \((\theta _{\text{r}}) \neq (\theta _{\text{a}}) \) the system is said to exhibit contact @A00346@ @H02930@