1. Concept Overview
The angle of contact tells us how a liquid behaves when it touches a solid surface. Depending on the nature of the solid and the liquid, the surface of the liquid may curve upward or downward near the solid. This curvature is controlled by the balance between cohesive forces (within the liquid) and adhesive forces (between the liquid and solid).
2. Definition
3. Understanding How Angle of Contact Forms
3.1. Cohesive and Adhesive Forces
Two types of molecular forces decide how a liquid behaves near a solid:
- Cohesive forces: Attraction between molecules of the liquid.
- Adhesive forces: Attraction between liquid molecules and the solid surface.
The angle of contact is determined by the competition between these two forces.
3.2. Meaning of the Angle
The angle of contact is measured inside the liquid at the point where the liquid surface touches the solid. A smaller angle means the liquid wets the solid, while a larger angle means it does not.
4. Types of Angle of Contact
4.1. Acute Angle (\( \theta < 90^\circ \))
When adhesive forces are stronger than cohesive forces, the liquid climbs up the solid surface slightly. This produces a concave meniscus.
Examples:
- Water on glass
- Alcohol on glass
4.2. Obtuse Angle (\( \theta > 90^\circ \))
When cohesive forces are stronger, the liquid pulls itself downward, away from the solid surface. This produces a convex meniscus.
Examples:
- Mercury on glass
- Mercury in porcelain
5. Meniscus Formation
5.1. Concave Meniscus
If the liquid wets the surface, the edge of the liquid curves upward, making a concave shape. Water in a glass tube shows this behaviour.
5.2. Convex Meniscus
If the liquid does not wet the surface, the edge curves downward, making a convex shape. Mercury in a glass tube forms this type of surface.
6. Factors Affecting Angle of Contact
6.1. Nature of Liquid
Different liquids behave differently on the same solid surface. Water and mercury, for example, show opposite behaviours on glass.
6.2. Nature of Solid Surface
The same liquid may form different angles of contact on different surfaces. Water wets clean glass but does not wet waxed surfaces.
6.3. Impurities
Impurities dissolved in the liquid or present on the surface can change the adhesive and cohesive forces, thus changing the angle of contact.
6.4. Temperature
Increasing temperature generally decreases the angle of contact because surface tension decreases.
7. Significance of Angle of Contact
7.1. Wetting and Non-Wetting Behaviour
A small angle of contact (less than \(90^\circ\)) means the liquid spreads on the surface. A large angle (greater than \(90^\circ\)) means the liquid forms droplets instead of spreading.
7.2. Capillary Action
The rise or fall of liquid in a narrow tube depends directly on the angle of contact. A concave meniscus leads to capillary rise, while a convex meniscus leads to capillary fall.
8. Examples to Build Intuition
8.1. Water Spreading on Glass
Water wets the surface, forming a small angle of contact. This is why water spreads nicely on clean glass.
8.2. Mercury Forming a Bulging Surface
Mercury does not wet glass and forms an obtuse angle of contact. This creates a convex meniscus.
8.3. Droplets on Waxed Surfaces
On a waxy or oily surface, water tends to form beads because the angle of contact becomes large, showing non-wetting behaviour.