When the free surface of a liquid comes in contact with a solid, it becomes curved near the place of contact. The free surface of water curves upward when it touches a vertical glass surface. On the other hand, the free surface of mercury curves downwards when it touches the vertical glass surface.
The angle between the tangent at the liquid surface at the point of contact and the solid surface inside the liquid is called the angle of contact for a given pair of solid and liquid. It is represented by θ
The angle of contact remains the same whether the liquid is contained in a glass vessel or a glass plate is inserted in the liquid or a drop of given liquid rests on the glass.
In other words, the angle of contact does not depend on the manner of contact.
In Figs. , QR is the tangent drawn at the point of contact. The angle RQP which the tangent makes with the portion QP of the plate inside water is called the angle of contact.
When a liquid has concave meniscus, the angle of contact is acute. When it has a convex meniscus, the angle of contact is obtuse.
For pure water and perfectly clean glass, the angle of contact is 0°. For ordinary water and glass, it lies between 8° and 18°. For pure water and pure silver, the angle of contact is 90°. When pure water is put in pure silver vessel, the surface of water is flat.
The angle of contact of water with greased glass surface may be obtuse. The angle of contact of water with chromium may be as high as 160°. The angle of contact of mercury (exposed to air) with lass glass is nearly 138°.