All questions of Surface Tension for UPSC CSE Exam

Understanding Capillarity
Capillarity, or capillary action, is the ability of a liquid to flow in narrow spaces without the assistance of external forces. This phenomenon is critical in various biological and mechanical systems.
Capillarity in Plants
- Water Transport: In plants, capillarity is essential for the movement of water from the roots to the leaves. Tiny tubes called xylem vessels utilize capillary action to draw water upward against gravity.
- Nutrient Absorption: The capillary spaces in soil allow plants to absorb water and minerals effectively, ensuring proper growth and survival.
Capillarity in Animals
- Blood Circulation: In animals, capillarity is vital for the circulation of blood. Capillaries, the smallest blood vessels, utilize capillary action to facilitate the exchange of gases, nutrients, and waste between blood and tissues.
- Fluid Regulation: Capillary action helps in maintaining fluid balance in tissues, ensuring that cells receive the necessary hydration.
Capillarity in Machines
- Ink in Pens: Many writing instruments, like fountain pens, rely on capillarity to draw ink to the nib for smooth writing.
- Absorption in Sponges: Machines and devices that utilize sponges or absorbent materials benefit from capillary action to manage liquids efficiently.
Conclusion
Capillarity is a fundamental phenomenon that plays a crucial role in the functioning of plants, animals, and machines. It is evident in various processes, highlighting its importance across different domains. Understanding capillary action helps in appreciating its significance in both natural and artificial systems.

Understanding Surface Tension
Surface tension is a fundamental property of liquids that arises due to the cohesive forces between liquid molecules. It can be effectively understood through the following points:
Definition
- Surface tension is defined as the elastic tendency of a fluid surface that makes it acquire the least surface area possible. This phenomenon occurs because molecules at the surface of a liquid experience a net inward force due to cohesive interactions with neighboring molecules.
Why Option 'A' is Correct
- Option 'A' states that surface tension is "the force that acts perpendicular to the surface of a liquid." This is accurate because the molecules at the surface are subjected to forces from the molecules below them, creating a tension that acts inward, perpendicular to the surface.
Characteristics of Surface Tension
- It causes the surface of a liquid to behave like a stretched elastic membrane.
- Surface tension is responsible for the formation of droplets, allowing them to maintain a spherical shape.
- It enables small objects, like a needle, to float on water despite being denser than the liquid.
Applications of Surface Tension
- Surface tension plays a crucial role in various fields, from biology (e.g., the functioning of lungs) to engineering (e.g., in the design of liquid containers).
- It is a key factor in processes like capillary action, which allows liquids to flow in narrow spaces against gravity.
Understanding surface tension is essential, especially in contexts like the Delhi Police Constable examination, where scientific principles can be applied in real-world scenarios.

Understanding Contact Angle
The contact angle is a crucial parameter in understanding the interaction between a liquid and a solid surface. It defines the degree to which a liquid droplet spreads or contracts on a surface.
Contact Angle of 180 Degrees
When the contact angle is 180 degrees, it indicates that the liquid does not wet the solid surface at all. Here’s why:

• No Interaction: A contact angle of 180 degrees suggests that the liquid droplet is perfectly spherical and does not spread on the solid surface. This means there is no attractive force between the liquid molecules and the solid surface molecules.
• Complete Non-Wetting: In this scenario, the cohesive forces within the liquid are much stronger than the adhesive forces between the liquid and the solid. As a result, the liquid retains its shape and does not adhere to the surface, leading to a state of complete non-wetting.
• Hydrophobic Surfaces: Surfaces that exhibit such behavior are typically hydrophobic in nature. These surfaces repel water, which is why the droplet remains intact and does not spread out.
• Implications: This behavior is significant in various applications, including coatings, paints, and surface treatments, where non-wetting properties are desired.

Conclusion
In summary, a contact angle of 180 degrees clearly indicates that there is no wetting of the solid by the liquid, confirming that option 'B' is correct. Understanding this concept is essential for applications in material science, engineering, and various industrial sectors.