Short Answer:

Surface tension is defined as the property of a liquid by which its free surface behaves like a stretched elastic sheet. It occurs because the molecules at the surface of a liquid experience an unbalanced molecular attraction. The molecules on the surface are pulled inward, creating tension along the surface.

In simple terms, surface tension is the force acting along the surface of a liquid, which tries to minimize its surface area. It is denoted by the symbol σ (sigma) and its SI unit is N/m. Examples include the spherical shape of water droplets and the floating of small insects on water.

Detailed Explanation :

Surface Tension

Surface tension is a molecular phenomenon that occurs at the interface between a liquid and a gas (or between two immiscible liquids). It is the property that allows the surface of a liquid to resist an external force. This happens because the molecules at the liquid surface experience cohesive forces (attraction between similar molecules) that are not balanced by forces above the surface.

Inside the liquid, each molecule is equally attracted in all directions by neighboring molecules. However, at the surface, the molecules are only pulled inward and sideways, resulting in a net inward force. This inward pull makes the surface behave as if it were covered by a stretched elastic membrane.

The magnitude of this force per unit length acting along the surface and perpendicular to the line is called surface tension.

Mathematically,

where,
σ = surface tension (N/m),
F = surface force (N),
L = length of the line on the surface (m).

Molecular Theory of Surface Tension

To understand surface tension, it is important to study the molecular behavior in different regions of a liquid.

• Molecules deep inside the liquid experience equal forces of attraction from all sides. Hence, the net force on them is zero.
• Molecules near the surface experience unbalanced forces. They are pulled inward because there are no molecules above them to balance the downward attraction.
  This inward force creates potential energy on the surface and makes the liquid tend to minimize its surface area.

This is why droplets of liquids tend to form spherical shapes — a sphere has the smallest possible surface area for a given volume.

Units and Dimensions

• SI Unit: Newton per meter (N/m)
• CGS Unit: Dyne per centimeter (1 N/m = 1000 dyne/cm)
• Dimensional Formula: M¹T⁻²

The surface tension of water at 20°C is approximately 0.0728 N/m, while that of mercury is 0.485 N/m (very high due to strong cohesive forces).

Factors Affecting Surface Tension

1. Temperature:
   Surface tension decreases with an increase in temperature. As temperature rises, molecular motion increases, which weakens the cohesive forces between molecules. At the boiling point, surface tension becomes zero.
2. Impurities:
   Impurities can either increase or decrease surface tension depending on their nature.
   • Soluble impurities like soap or detergent reduce surface tension.
   • Insoluble impurities like dust can increase it.
3. Nature of the Liquid:
   Liquids with strong cohesive forces, like mercury, have higher surface tension compared to liquids with weak cohesive forces, like alcohol.
4. Presence of Surfactants:
   Substances such as soaps and detergents are called surface-active agents (surfactants). They reduce surface tension, allowing liquids to spread more easily over surfaces.

Measurement of Surface Tension

Several methods are used to determine surface tension experimentally. The most common methods are:

1. Capillary Rise Method:
   When a thin tube (capillary) is dipped into a liquid, the liquid either rises or falls in the tube due to surface tension.
   The height (h) of the liquid column is given by:

where,
σ = surface tension (N/m),
h = height of rise or fall (m),
r = radius of capillary (m),
ρ = density of liquid (kg/m³),
g = acceleration due to gravity (9.81 m/s²),
θ = contact angle.

Example: Water rises in a glass tube (θ = 0°), while mercury falls (θ = 128°) due to the difference in adhesive and cohesive forces.

1. Drop Weight Method:
   The weight of a liquid drop detaching from a capillary is proportional to the surface tension of the liquid.
2. Ring Method (Du Noüy Method):
   A metal ring is dipped into a liquid and then pulled upward. The force required to detach the ring from the surface gives the value of surface tension.

Practical Examples of Surface Tension

• Formation of Water Droplets: Water droplets form spherical shapes to minimize surface area.
• Floating of Small Objects: Light objects like needles or insects (water striders) can rest on water because surface tension supports them.
• Capillary Action: The rise or fall of liquids in narrow tubes is due to surface tension combined with adhesive and cohesive forces.
• Soap Bubbles: Soap decreases surface tension, allowing bubbles to form easily.
• Ink Flow in Pens: Capillary action and surface tension together help ink flow in fountain pens.

Importance of Surface Tension in Engineering

1. Cooling Systems:
   In boilers and condensers, surface tension affects the formation of droplets during condensation and evaporation. Lower surface tension promotes better heat transfer.
2. Lubrication:
   Surface tension helps lubricants spread uniformly between moving parts, reducing friction and wear.
3. Detergents and Cleaning:
   Soaps and detergents reduce surface tension, allowing water to wet and clean surfaces more effectively.
4. Aerospace and Biomedical Fields:
   Surface tension plays a role in the behavior of fuel droplets in combustion and in the movement of liquids through capillary tubes in medical equipment.

Typical Values of Surface Tension

• Water (20°C): 0.0728 N/m
• Alcohol: 0.022 N/m
• Mercury: 0.485 N/m
• Benzene: 0.0289 N/m
  These values show that mercury has the highest surface tension, while alcohol has one of the lowest among common liquids.

Conclusion

In conclusion, surface tension is the property of a liquid that causes its surface to act like a stretched elastic sheet. It arises due to cohesive forces among liquid molecules. Surface tension depends on temperature, impurities, and the nature of the liquid. This property is very important in many engineering and natural processes such as droplet formation, capillary rise, and lubrication. Understanding surface tension helps engineers design systems where liquid behavior at surfaces and interfaces is critical.