Surface tension of water

Surface tension is the property of liquid by virtue of which the free surface of liquid tends to shrink into the minimum surface area possible.  It is denoted by S. Surface tension is only concerned with liquids not gases as gases do not have free surface. Examples of surface tension are:  oil and water do not mix, hair of a paint brush do not cling together when dry and even when dipped in water, can water rise up to the top of the leaves of the tree all these are the examples of surface tension. Liquid do not have a definite shape but they do have a definite volume it means the acquired of free space when poured in a container. These surfaces have some additional energy. This phenomenon is known as surface tension. Formula of surface tension – γ=1/2*F/L

γ – Surface tension, F- Force, L- length

So, by the formula we can say that it is defined as the force acting per unit length on the surface of the liquid.

Surface energy 

In any material atoms reside in two categories one is at the bulk of that material where atoms are  stable and have balanced interaction, the other one is at the surface of the material these are not so stable. Surface energy may be defined as the excess energy at the surface of a material compared to the bulk. The potential energy at the bulk is higher.

Surface energy is work done per unit area to increase the size of the surface in the material.

Surface energy = to work done / area

Relationship between surface tension and surface energy

F = 2Tl 

Due to the displacement dx surface area of the film increases, the increase in its surface area is

∆A = 2l*dx

Thus, work done per unit surface area is

W/A = 2Tldx / 2ldx =T

This work done is stored in the unit surface area the in the form of potential energy and this potential energy is known as surface energy.

Cause of surface tension

Surface tension arises due to the cohesive interactions between the molecules in the liquid. At the bulk of liquid molecules are surrounded by neighbouring molecules which are pulling each other equally in all directions causing the net force zero.

Cohesive and Adhesive forces.

Cohesive force is the intermolecular force of attraction among molecules of the same substance. This is the reason that water droplets attain a spherical shape.

Adhesive forces are the forces of attraction between unlike molecules. They are caused due to the forces acting between two substances as the mechanical forces and electrostatic forces. Ink sticks to paper while writing is an example of adhesive force.

Excess pressure on curved surface area of a liquid

1. When the surface area of a liquid is curved, there is a pressure difference between the liquid side and the vapour side of the surface.
2. Each molecule of liquid on the surface is attracted by cohesive force in all direction and the force of surface tension acts tangentially to the liquid surface.
3. If the surface is plane then the molecule on the surface will be equally attracted from all the sides, making the net force on the molecule equal to zero. Hence the pressure is the same on both sides.
4. If the liquid surface is convex, the resultant force R acts downward. To maintain equilibrium there will be pressure on the paper side.
5. If the liquid surface is concave, then the resultant force R will act upward on the molecule. To maintain equilibrium the pressure on the upper side must be greater than the pressure on the liquid side.

Therefore, whenever a liquid surface is curved the pressure on its concave side is greater than the pressure on the convex side.

Examples of surface tension are- 

1. When we mix soap and water, the surface tension of liquid reduces along the liquid to mix with the dirt on our hands and clean it.
2. This spherical shape of Mercury drops is due to the surface tension.
3. Small insects can walk on water because of their light weight.
4. If we place a small needle carefully in water it will float on water because of the force of surface tension.
5. Soap bubbles .

Conclusion 

The tendency of a liquid by virtue of which it occupies minimum surface area within a particular volume is called surface tension. Surface tension (γ) =1/2*F/L

when we apply some force on a liquid to create a space in between then the work done by force will be stored in the form of energy. This is called elastic potential energy or surface energy. Formula of surface energy is, Surface energy = to work done / area.

There are two types of forces acting on a body, cohesive force and Adhesive force.

Intermolecular force of attraction between  same molecules is known as Cohesive force whereas intermolecular force of attraction between unlike molecules is known as adhesive force.