1. Concept Overview
The surface of a liquid behaves as if it is covered with a thin, stretched membrane. This happens because liquid molecules at the surface experience unbalanced attractive forces. The tendency of this surface layer to contract and minimize its area is called surface tension.
This property explains why small insects can walk on water, why water drops are spherical, and why soap reduces the tightness of the surface layer.
2. Definition
3. Understanding Why Surface Tension Occurs
3.1. Cohesive Forces Between Molecules
Inside a liquid, each molecule is surrounded by other molecules and the cohesive forces cancel out. But at the surface, molecules have no liquid molecules above them, so they experience a net inward pull. This creates tension on the surface.
3.2. Surface Behaviour Like a Stretched Film
Because of this inward pull, the surface behaves as if it is stretched tightly. The liquid tries to keep its surface area as small as possible. This is why water forms droplets instead of spreading out flat.
4. Surface Energy and Minimization of Surface Area
4.1. Surface Energy
The surface of a liquid has extra energy because the surface molecules are in a higher energy state. Surface tension is related to this energy—reducing surface area reduces energy.
4.2. Tendency to Minimize Area
Liquids naturally try to minimize their surface area to reduce their surface energy. This is why droplets tend to become spherical, as a sphere has the minimum surface area for a given volume.
5. Formula and Meaning
5.1. Surface Tension Formula
The formula for surface tension is:
\( T = \dfrac{F}{L} \)
Where:
- \( T \): surface tension
- \( F \): force acting along the surface
- \( L \): length along which force acts
5.2. Pressure Difference in Curved Surfaces
Surface tension causes a pressure difference across curved surfaces such as droplets or bubbles. For a spherical drop:
\( \Delta P = \dfrac{2T}{r} \)
where \( r \) is the radius. Smaller droplets have higher internal pressure.
6. Factors Affecting Surface Tension
6.1. Temperature
Surface tension decreases with increase in temperature because molecular forces weaken as molecules move more freely.
6.2. Impurities
Impurities may increase or decrease surface tension depending on their nature. For example, detergent reduces the surface tension of water.
6.3. Nature of Liquid
Liquids with strong cohesive forces, like mercury, have high surface tension. Liquids with weaker forces, like water or alcohol, have lower surface tension.
7. Applications of Surface Tension
7.1. Formation of Droplets
Water dripping from a tap forms round droplets due to surface tension trying to minimize surface area.
7.2. Insect Walking on Water
Some insects can walk on water because their weight is not enough to break the surface layer created by surface tension.
7.3. Action of Soaps and Detergents
Soaps reduce the surface tension of water, making it spread more easily and helping in cleaning.
8. Examples to Build Intuition
8.1. Floating of a Needle
A steel needle can float on water if placed gently. It is supported by the tight surface layer created by surface tension, even though steel is denser than water.
8.2. Beading of Water on Leaves
Water forms round beads on waxy leaves because surface tension pulls the liquid surface inward.
8.3. Spherical Soap Bubbles
Soap bubbles become spherical as this shape minimizes the surface area for a given volume, reducing surface energy.