It is defined as the angle between the tangent to the liquid surface drawn at the point of contact and the solid surface inside the liquid.
Case I: When $\theta < 90^\circ $
The liquid surface curves up towards the solid. This happens when the force of cohesion between two liquid molecules is less than the force of adhesion between the liquid and solid.
If such a liquid is poured into a solid tube, it will have a concave meniscus.
For example: A glass rod dipped in water or water inside a glass tube.
Case II: When $\theta > 90^\circ $
The liquid surface get curves downward in contact with the solid. This happen when the force of cohesion is greater than the force of adhesion.
In such cases, solids do not get wet.
When such liquids are put into a solid tube, a convex meniscus is obtained.
For example, A glass rod dipped in mercury or mercury within a solid glass tube.
For example, A glass rod dipped in water or water inside a glass tube.
Case III: When $\theta = 90^\circ $
The liquid surface neither rise nor get depressed in a capillary tube. This happens when the force of cohesion is equal to the force of adhesion.
For example, Water inside a silver container.
Note:
- The value of angle of contact lies between $0^\circ $ and $180^\circ $
- The value of angle of contact for pure water and glass (without grease) is zero
- The angle of contact depends on the nature of solid and liquid both
- The angle of contact is different for different pairs of solids and liquids
- Angle of contact does not depend upon the inclination of the solid surface to the liquid surface
- Angle of contact increases with increase in temperature of liquid
- Angle of contact decreases on adding soluble impurity to a liquid
