When the free surface of a liquid comes in contact with a solid, it becomes curved at the point of contact. The angle between the tangent to the liquid surface at the point of contact of the liquid with the solid and the solid surface inside the liquid is called angle of contact.
Fig: Angle of contact
In Figure, QR is the tangent drawn at the point of contact Q. The angle PQR 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.
The angle of contact depends on the nature of liquid and solid in contact. For water and glass, B lies between 8° and 18°. For pure water and clean glass, it is very small and hence it is taken as zero. The angle of contact of mercury with glass is 138°.