Q.Define angle of contact for a given pair of solid and liquid.

When the free surface of a liquid meets a solid surface, the liquid surface curves right at the point of contact, and the ANGLE OF CONTACT $\theta$ is defined as the angle, measured INSIDE the liquid, between the tangent to the liquid's curved surface at the point of contact and the solid surface. Its value depends on the specific solid-liquid pair, and it is exactly this angle that decides whether a given liquid SPREADS over a chosen solid or instead beads up into separate droplets on it -- this is the primary factor deciding the WETTABILITY of a surface by a liquid. Consider three interfacial surface tensions meeting at the point of contact O: solid-air ($T_{sa}$), solid-liquid ($T_{sl}$), and liquid-air ($T_{la}$). Since the liquid is in stable equilibrium at O, these three forces balance: $T_{sa}=T_{sl}+T_{la}\cos\theta$, i.e. $\cos\theta=\dfrac{T_{sa}-T_{sl}}{T_{la}}$. Three cases follow. If $T_{sa}>T_{sl}$, $\cos\theta$ is positive and $\theta$ is ACUTE (less than 90 degrees) -- the liquid wets and tends to spread over the solid (e.g. water on plastic). If $T_{sa}<T_{sl}$, $\cos\theta$ is negative and $\theta$ is OBTUSE (greater than 90 degrees) -- the liquid tends to bead up rather than spread (e.g. water on a waxy leaf). If $T_{sa}>T_{sl}+T_{la}$, no equilibrium angle exists at all, and the liquid spreads over the solid completely and indefinitely -- exactly the case of a drop of oil placed on water, which spreads out into a thin film rather than staying a bead, because oil's surface tension against air together with the oil- …