Short Answer:
Capillarity is defined as the phenomenon of rise or fall of a liquid in a narrow tube (capillary tube) due to surface tension. It occurs because of the combined effect of cohesive forces (between liquid molecules) and adhesive forces (between liquid and tube surface).
In simple terms, capillarity is the ability of a liquid to move upward or downward in a small tube without any external force. For example, water rises in a glass tube while mercury falls. This happens due to the difference in adhesive and cohesive forces acting on the liquid.
Detailed Explanation :
Capillarity
Capillarity, also known as capillary action, is the tendency of a liquid to rise or fall in a narrow tube or in small pores of a material. It occurs due to the surface tension of the liquid and the interaction between the liquid molecules and the solid surface.
The word “capillary” comes from the Latin word capillus, meaning “hair,” which indicates that this phenomenon is observed in very thin tubes, much like the diameter of a hair. Capillarity is an important property in engineering, biology, and daily life because it helps explain how liquids move through narrow spaces such as soil, paper, and plant stems.
Causes of Capillarity
Capillarity results from two opposing molecular forces:
- Cohesive Forces:
These are the forces of attraction between similar molecules of the liquid. For example, water molecules attract each other through hydrogen bonding. - Adhesive Forces:
These are the forces of attraction between the liquid molecules and the surface of the solid (such as the tube).
The interaction of these two forces determines whether the liquid will rise or fall in the capillary tube:
- If adhesive forces > cohesive forces, the liquid rises (e.g., water in a glass tube).
- If cohesive forces > adhesive forces, the liquid falls (e.g., mercury in a glass tube).
Capillary Rise and Capillary Fall
When a thin glass tube is dipped into a liquid, the liquid either rises or falls in the tube. This is due to surface tension acting at the liquid-air interface.
- Capillary Rise:
- Occurs when the liquid wets the tube (adhesive force is stronger).
- Example: Water rises in a glass capillary because it wets the surface.
- Capillary Fall:
- Occurs when the liquid does not wet the surface (cohesive force is stronger).
- Example: Mercury falls in a glass tube because it does not wet the surface.
The shape of the liquid surface in the tube is called the meniscus.
- For water, the meniscus is concave (curves upward).
- For mercury, the meniscus is convex (curves downward).
Mathematical Expression for Capillarity
The height (or depth) to which a liquid rises or falls in a capillary tube is given by the formula:
where,
- h = height of rise or fall (m)
- σ = surface tension of the liquid (N/m)
- θ = contact angle between the liquid and the tube surface
- r = radius of the capillary tube (m)
- ρ = density of the liquid (kg/m³)
- g = acceleration due to gravity (9.81 m/s²)
From this relation, we can see that the height of capillary rise or fall depends on:
- Surface tension of the liquid
- Radius of the tube
- Density of the liquid
- Contact angle between liquid and solid surface
Thus, smaller the tube radius, higher will be the rise or fall of the liquid.
Example:
For water in a glass tube (σ = 0.0728 N/m, θ = 0°, ρ = 1000 kg/m³, r = 0.5 mm):
This means water will rise about 3 cm in a fine glass tube of 1 mm diameter.
Factors Affecting Capillarity
- Surface Tension:
The higher the surface tension of a liquid, the greater will be the capillary rise or fall. - Radius of the Tube:
Capillary rise is inversely proportional to the radius of the tube. Narrower tubes show a higher rise or fall. - Contact Angle (θ):
- When θ < 90°, the liquid rises (wetting surface).
- When θ > 90°, the liquid falls (non-wetting surface).
- Density of Liquid:
A denser liquid shows a smaller capillary rise because its weight resists the upward pull of surface tension. - Temperature:
Increase in temperature decreases surface tension, hence reducing capillary rise.
Applications of Capillarity in Engineering and Daily Life
- Soil Moisture Movement:
In agriculture, capillarity helps water rise through soil pores to nourish plant roots even when the surface appears dry. - Ink Flow in Pens:
In fountain pens and markers, ink flows through small capillary channels from the reservoir to the nib due to capillarity. - Action in Paper and Cloth:
Liquids like water or ink spread through paper or fabric fibers because of capillary action. - Lubrication:
In machines, capillary action helps lubricating oil spread evenly between moving parts, reducing friction and wear. - Wicking in Medical Devices:
Capillarity is used in medical test strips and absorbent pads to move small volumes of liquid for analysis. - Refrigeration Systems:
Capillary tubes in refrigeration units use this property to control refrigerant flow at low pressure.
Examples of Capillarity in Nature
- Water rises in the narrow vessels (xylem) of plants, helping them transport nutrients from roots to leaves.
- Oil spreads easily on metal surfaces due to capillary action.
- Candle wicks draw melted wax upward through fine capillary pores to sustain the flame.
These natural examples show how capillarity enables movement of fluids in narrow spaces without external force.
Conclusion
In conclusion, capillarity is the phenomenon of rise or fall of a liquid in a narrow tube due to surface tension and the interaction between cohesive and adhesive forces. It plays an important role in various natural and engineering processes such as ink flow, soil moisture movement, lubrication, and plant water transport. Capillarity depends on factors like tube radius, surface tension, density, and contact angle. Understanding capillarity helps engineers design systems where liquid movement in narrow spaces is essential.