Applications of Surface Tension

Surface tension is a fascinating property of liquids that plays a vital role in both natural and technological processes. It arises due to cohesive forces between liquid molecules and manifests as the tendency of a liquid surface to minimize its area. While it may seem like a simple concept, surface tension is behind many everyday phenomena—from the way insects walk on water to the mechanism of water transport in plants. Understanding its applications is crucial, especially in biology and physics, where concepts like capillarity, droplet formation, and pulmonary function heavily rely on surface tension.

1.0What is Surface Tension?

Surface tension is basically a property of liquid. The liquid surface behaves like a stretched elastic membrane which has a natural tendency to contract and tends to have a minimum possible area. This property of liquid is called surface tension.

2.0Molecular Theory Behind Surface Tension

Intermolecular forces

• Forces of attraction or repulsion acting among the molecules are known as intermolecular forces.The nature of intermolecular force is electromagnetic.

There are two types of intermolecular attractive forces.

Laplace's Molecular Theory of Surface Tension 

• Surface tension arises due to intermolecular cohesive forces, which are electromagnetic in nature.
• Molecules attract each other only within a small distance (~10⁻⁹ m), called the molecular range.
• A sphere with this range as radius is called the sphere of molecular activity or sphere of influence.
• Only molecules within this sphere influence a given molecule.

Molecular Behavior at Different Positions

Molecule D (deep inside liquid): A molecule deep inside the liquid is symmetrically surrounded by other liquid molecules, experiencing equal cohesive forces in all directions. As a result, the net force acting on it is zero, keeping the molecule stable in its position.

Molecule C (just below the surface): A molecule just below the surface has nearly symmetric surroundings, with slightly fewer molecules above than below. This causes a minimal net downward force, though the overall force is approximately zero.

Molecule B (near the surface):A molecule near the surface has more neighboring molecules below than above, resulting in unbalanced cohesive forces that pull it slightly downward. This creates a net downward force, though it's not at its maximum.

Molecule A (at the surface):A molecule at the surface is exposed to air above and surrounded by liquid molecules below, causing a strong downward cohesive force due to the lack of attraction from above. This results in the maximum net downward force and is the cause of surface tension.

3.0Mathematical Formula of Surface Tension

• Surface tension is the force acting per unit length along an imaginary line on the liquid surface, perpendicular to the line and in the plane of the surface.
• Consider an imaginary line AB drawn on the liquid surface.The liquid on both sides of AB pulls inward, creating a force F perpendicular to the line.If the length of AB is L, then surface tension $T=\frac{F}{L}$
• SI units : N/m and J/m²
• CGS Units dyne/cm and erg/cm²
• Dimensions: [M¹ L⁻¹ T⁻²]

4.0Dependency of Surface Tension

1. Cohesive Force: Increasing cohesive forces between molecules increases surface tension; decreasing cohesive forces lowers surface tension.
2. Impurities:
   Completely soluble impurities (e.g., ionic salts) increase surface tension when dissolved in small amounts.
   Partially soluble impurities (e.g., detergents) decrease surface tension by weakening cohesive forces.
3. Temperature: Surface tension decreases as temperature rises and becomes zero at the critical temperature or boiling point.
   Note: Water’s surface tension is highest at 4°C.
4. Contamination: Dust or lubricants on the liquid surface reduce surface tension.

5.0Effects of Surface Tension

(1) Small liquid drops and soap bubbles are spherical.

(2) The hairs of the brush remain separated from each other inside water, but when the brush is taken out, the hairs stick together.

(3) Floatation of needle on water.

(4) Formation of lead shots.

(5) Dirty clothes become clean in hot detergent solution in comparison to pure water at room temperature.

6.0Applications of Surface Tension

1.Needle supported on water surface :A needle can be supported on the water surface because surface tension creates a “skin-like” layer that resists external force. This layer holds the needle up by balancing its weight, preventing it from sinking despite being denser than water.

2.Hair of Painting Brush Clinged Together:The hairs of a paintbrush stick together when removed from water because the thin water films on them contract to minimize their surface area.

3.Plateau’s Experiment:This experiment clearly shows that a liquid drop takes a spherical shape when gravitational forces are negligible. To perform the experiment, prepare a mixture of alcohol and water with a density equal to that of olive oil, and pour it into a glass beaker. Then, carefully introduce a large drop of olive oil into the mixture.

4.Shape of Mercury Droplets:Small mercury droplets are spherical, while larger droplets tend to become flattened. This is because surface tension forces cause small droplets to minimize their surface area, forming a sphere, which has the least surface area for a given volume.

5.Shape of Raindrops

Raindrops are generally spherical because surface tension causes them to minimize their surface area. Since a sphere has the smallest surface area for a given volume, raindrops naturally form this shape.