1. Concept Overview
Capillarity is the phenomenon where a liquid either rises or falls in a thin tube. This happens because of surface tension and the interaction between the liquid and the tube material. The effect becomes noticeable only in very narrow tubes, called capillary tubes.
Water rises in a thin glass tube, while mercury falls. The direction of movement depends on the angle of contact and the balance between cohesive and adhesive forces.
2. Definition
3. Why Capillarity Occurs
3.1. Role of Adhesive and Cohesive Forces
If adhesive forces between the liquid and the tube are stronger than cohesive forces within the liquid, the liquid rises. If cohesive forces dominate, the liquid falls.
3.2. Role of Surface Tension
Surface tension creates a curved surface (meniscus) inside the tube. This curvature generates an upward or downward force depending on the angle of contact, leading to rise or fall of the liquid column.
4. Types of Capillarity
4.1. Capillary Rise
Occurs when the liquid wets the surface, forming a concave meniscus. Adhesive forces are stronger than cohesive forces. Water in a glass tube is a classic example.
4.2. Capillary Fall
Occurs when the liquid does not wet the surface, forming a convex meniscus. Cohesive forces dominate. Mercury in a glass tube shows this behaviour.
5. Height of Capillary Rise or Fall
5.1. Formula for Capillary Height
The height \(h\) of capillary rise or fall is given by:
\( h = \dfrac{2T \cos \theta}{\rho g r} \)
Where:
- \(T\): surface tension
- \(\theta\): angle of contact
- \(\rho\): density of the liquid
- \(g\): acceleration due to gravity
- \(r\): radius of the capillary tube
5.2. Dependence on Tube Radius
The height of rise is inversely proportional to the tube radius. Thinner tubes show stronger capillary effects, which is why the phenomenon is visible only in narrow tubes.
6. Real-Life Examples
6.1. Rise of Water in Plants
Capillarity helps water move upward through tiny vessels (xylem) in plant stems. Narrow channels make capillary rise significant.
6.2. Wicking in Paper Towels
Paper towels absorb water because tiny pores act like capillaries, pulling water upward through capillary action.
6.3. Lamp Wicks Drawing Oil
In oil lamps, the wick draws oil upward from the container to the flame due to capillary rise.
6.4. Depression of Mercury in Glass
Mercury falls in a thin glass tube because it does not wet glass. Capillary fall occurs due to strong cohesive forces in mercury.