Explain the angle of contact in the context of intermolecular forces.

(N/A) The surface of a liquid near the plane of contact with another medium is curved.
Angle of contact: The angle between the tangent to the liquid surface at the point of contact and the solid surface inside the liquid is known as the angle of contact. It is denoted by $\theta$.
It varies for different pairs of liquids and solids.
In the figure,$(a)$ shows a water droplet on a lotus leaf,and $(b)$ shows water spreading over a clean plastic plate.
Let the surface tensions be:
$S_{la} = \text{surface tension of liquid-air interface}$
$S_{sa} = \text{surface tension of solid-air interface}$
$S_{sl} = \text{surface tension of solid-liquid interface}$
At the line of contact,the surface forces between the three media must be in equilibrium.
For figure $(a)$:
$S_{sa} = S_{sl} + S_{la} \cos \theta$
$\cos \theta = \frac{S_{sa} - S_{sl}}{S_{la}}$
If the cohesive forces between liquid molecules are stronger than the adhesive forces between liquid and solid molecules,then $S_{sl} > S_{sa}$,resulting in $\cos \theta < 0$,which means $\theta$ is obtuse. In this case,the liquid does not wet the solid surface,and the meniscus is convex.
If the adhesive forces are stronger than the cohesive forces,then $S_{sa} > S_{sl}$,resulting in $\cos \theta > 0$,which means $\theta$ is acute. In this case,the liquid wets the solid surface,and the meniscus is concave.