# What is Surface Tension?

Surface Tension (ST) is defined as the tensile force acting on the free surface of liquid which is in contact with a gas or on the surface in between two immiscible liquids such that the contact surface behaves like a thin membrane under tension. It is more interesting to know how this tensile force acts on the free surface of the liquid.

To understand that lets us take three molecules of liquid as A, B and C. Molecule A is completely lie inside the free surface of liquid, it experiences equal tensile force in all direction from its neighboring molecules. So the net resultant force on the molecule A is zero.

The molecule B is situated near the free surface of the liquid. It is experiencing upward and downward forces. But the magnitude of the downward force is greater than the upward force; hence it experiences a net downward force.

The molecule C which is situated on the free surface of the liquid experiences a net downward force. So all the molecules of liquid which is situated on the free surface of the liquid experiences a net downward force. Due to this, the free surface of the liquid acts likes a thin film under tension.

### Cause

Surface tension occurs due to cohesive forces existing in between the molecules of liquid. It is the cohesive forces that allows one molecule of liquid to attract its neighbouring molecules. Due to this cohesive forces the molecules of liquid on the free surface experiences a tensile force.

### Formula

When the tensile force acting on the free surface of the liquid is calculated on unit length than the quantity we get is called surface tension. It is also equal to the surface energy per unit area.
• It is denoted by the Greek letter σ (sigma)
• Its SI unit is N/m.

Mathematically,
Where

σ = Surface tension
F = Tensile force
l = length on which tensile force acts

• The ST of water in contact with air at 20 degree celsius is 0.0725 N/m. and
• The ST of mercury in contact with air at 20 degree celsius is 0.52 N/m.

### Examples

1. Drops of water: it is observed that the drops of water have spherical shape and this happens due to the ST of the water.
2. Insects walking on water: We have seen so many times that small insects are capable of walking on the surface of water. This is possible because of the ST of the water, it provides a thin film for the insects on which they can balance their body and prevent them from sinking.
3. Needle or paper clip floating on water: A needle or paperclip is a metallic part which has more density than water. But even after than they can float on the free surface of the water and this is due to the surface tension.