## Wikipedia - زاوية تماس contact angle

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@
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 [Terms] [Paper]