WHAT IS SURFACE TENSION?

Surface Tension is the force that occurs on the surface of the liquid. The surface of the liquid due to inward pull of the molecules acts like a streched elastic membrane. This membrane can support light objects.

It is a property by virtue of which, the free surface of a liquid possesses a tendency to contract so as to acquire a minimum surface area.

When the things don’t sink beneath the fluid. i.e. They “stand “on the top.

Surface tension is the property of the surface of the liquid. it is what it caused the surface portion of the liquid to be attracted to another surface such as that of another portion of the liquid.

water

air

No net force

Net forceinward

Interface

DEFINITION OF SURFACE TENSION

The surface tension γ is the magnitude F of the force per unit length on either side of the imaginary line drawn on free surface of liquid. The surface tension is denoted by gamma. It is given as followings:

Formula to calculate Surface Tension is:

γ = F\L The direction of the force F being perpendicular to the line and tangential to the free surface of the liquid.

Units Of Surface Tension

SI Unit of Surface Tension : N/m CGS unit of Surface Tension : dyne/cm

Various intermolecular forces, such as Van der Waals forces, draw the liquid particles together. Along the surface, the particles are pulled toward the rest of liquid, as shown in the picture to the right.

The forces acting on the liquid causes surface tension.

Cause Of Surface Tension

Surface Tension Phenomenon

What’s going onat the surfaceof a liquid?

Let’s takea look!

Particles that make up a liquid are in constant random motion they are randomly arranged.

You might expect the particles at the surface,at the micro level, to form a random surface,as shown below.

You might expect the particles at the surface,at the micro level, to form a random surface,as shown below.

But how do intermolecular forces influence the surface?

= intermolecu

larattractions

Under the surface, intermolecular attractions pull on individual molecules in all directions.

= intermolecularattractions

= intermolecularattractions

= intermolecular

attractions

At the surface, pull on the molecules is laterally and downward;there is negligible intermolecular attractionsabove the molecules (from the medium above, such as air).SO, the net force on surface molecules is downward

The result of this downward force is thatsurface particles are pulled down untilcounter-balanced by the compressionresistance of the liquid:

Surface molecules are compressed more tightly together, forming a sort of skin on the surface, with less distance between them compared to the molecules below.

Surface molecules also form a much smoother surface than one would expect from randomly moving molecules.

This explains the characteristic rounded shape that liquids form when dropping through the air: The molecules are all being pulled toward the center.

This explains the characteristic rounded shape that liquids form when dropping through the air: The molecules are all being pulled toward the center.

This explains the characteristic rounded shape that liquids form when dropping through the air: The molecules are all being pulled toward the center.

Water in particular has a very high surface tension.

What property doesWater have that wouldgive it such a strongsurface tension?

Example Of Surface Tension

The surface tension of the water allows the insect to walk on thewater without sinking.

This is water strider insect

A iron needle floats on the surface of water .

Iron screen resting on the surface of water

CAPPILARY ACTION

The tendency of liquids to rise up in narrow tubes is capillary action

Surface Tensions of Common Liquids

Liquid SurfaceTension γ (N/m)Benzene (20 °C) 0.029Blood (37 °C) 0.058Glycerin (20 °C) 0.063Mercury (20 °C) 0.47Water (20 °C) 0.073Water (100 °C) 0.059

Applications Of Surface Tension

It helps in making a spinning Snowman.

The use of sprayers to paint a surface.

In cleaning action of clothes.

Factors Affecting Surface Tension

The presence of impurities either on the surface or dissolved in it, affect surface tension of the liquid.

Highly soluble substances increase the surface tension of water, whereas sparingly soluble substances reduce the surfaceTension.

Surface Tension of a liquid decreases with rise in temperature and becomes zero at critical temperature ( where interface between liquid and vapour’s disappear).

It depends upon the nature of the liquid and is independent of the area of the surface or length of line considered.