Some Examples Explaining Surface Tension and Surface Energy

Examples of Surface Tension

Thread on a soap film

Take a frame of wire, dip it in soap solution and then take it out. A soap film will be formed in the frame. Place a loop of wet thread gently on the film. It will remain in the form as we place it. Now pierce the film will a pin at any point inside the loop, the lop will immediately take circular form as shown in figure.

Floating needle

Take a greased needle on a piece of blotting paper and place it gently over water surface. Blotting paper soaks water and soon sinks down but the needle keeps floating. The neeedle creates depression in the liquid surface so that surface tension forces have an upward component which is capable to support the weight of needle. If we add oil in water the needle sinks. It is because oil lowers the surface tension of water and weight of needle exceeds the upward directed component of surface tension force.

When a dry brush is diped into water, its hair spread out

When a brush is dipped into water, the hair do not cling together but when the brush is removed from water, its hair cling together as shown in figure. When the brush is in water, there is water all around its hair. Consequently, there is no free surface and the effect of surface tension is absent. However when the brush is taken out from water, the force of surface tension contract the hair.

Formation of lead shots

Inorder to manufacture lead shots, melted lead is allowed to fall in water by spraying it from a sufficient height. During its fall, the melted lead forms small spherical drops due to surface tension forces and on entering water, they become solid.

Oil has less surface tension than water

When a drop of oil id dropped on the surface of water, the oil is streched in all directions as a thin film. Mosquitoes breed on the free surface of stagnant water. Due to surface tension, the liquid layer supports the eggs laid by the mosquitoes. Whenil is spread in water, there is little surface tension and the mosquitoes cannot bread.

Surface Energy

The free surface of a liquid always has a tendency to contract and occcupies minimum surface area. If the surface area of a liquid has to be inceased, work has to be done. The work done is stored in the liquid. The work done is stored in the liquid surface film as its potential energy.The potential energy per unit area of the surface film is surface energy. It is also defined as the amount of workdone in increasing the area of a surface film through unity.

$$\text {Surface energy,} \sigma = \frac {\text {work done in increasing surface area} }{\text {increase in surface area}}$$

It is also called free energy because the mechanical work done can be released when the surface contracts.

Relation between Surface Tension and Surface Energy

Let us consider a rectangular frame of wire ABCD made up of a metal wire in which position EF is movable. It is dipped in a soap solution and is taken out.The force due to surface tension on \(EF = T \times 2l\) in ward direction on there exists two free surfaces.

The part EF is moved to a distance x to the position E'F' by external agent \(=F \times x = T \times 2l \times x \)

Equation (i) becomes

$$\sigma = \frac {T\times 2l \times x}{2lx}$$

$$\therefore \sigma = T$$

Thus surface tension T is numerically equal to surface energy.