Surface Energy

What is Surface Energy? 

The surplus energy that is exhibited at a liquid surface when we compare their energy content with the molecules away from the surface is known as surface-free energy. In other words, surface energy is the work performed per unit area to build the surface of any liquid. Smaller insects and light objects like leaves, paper clips, etc. can easily move on this surface provided that their weight is light enough to be supported by the surface film.

How to evaluate surface energy?

The most popular technique involves the implementation of the contact angle method of measurement. Here common liquids are taken whose surface tension is already known. A Force tensiometer is another alternative equipment to fulfill the purpose.

In addition to one polar liquid, we require a dispersive liquid-like α – iodomethane to execute the sessile drop contact angle method. The α – iodomethane has an intrinsic high surface tension despite being a dispersive fluid, therefore it is taken. We should only consider liquids with high surface tension because it will be feasible to evaluate the contact angles. 

For solids, we need to apply the method of force tensiometers. Measurements are taken in immersed liquids for both advancing and diminishing contact angles. The value obtained in this way is not recorded as it cannot be used in the future as many people prefer considering the diminishing values while others take the advancing ones. Thus, the purpose of using a tensiometer is to determine the average surface energy of the whole area. 

Surface Energy Formula

The surface energy gets reduced as the molecules on the surface constantly try to travel downwards. Stable potential energy restricts this phenomenon, intending to expand the liquid’s surface area. This is known as interfacial free energy or simply surface energy. 

In mathematical terms, surface energy is the total work performed per unit area. 

Therefore, surface free energy = work performed / area. 

From the surface energy formula, we may easily infer the S. I. unit of surface energy: Joules/m2. 

When we break a solid object into pieces, the bonds are disturbed. There is an increase in surface area. Thus, the liquids can employ greater surface energy on the individual pieces if they are immersed in them. 

Relation between surface energy and surface tension 

Through a brief discussion let us determine the relationship between surface energy and surface tension. 

A rectangular wire frame having a mobile arm is dipped within a soapy solution horizontally. A film is created in the middle. As the solution’s surface tension will be similarly exerted on all sides of the wire, its moveable arm will move inwards.

 The magnitude of this resultant tension is F = 2TI

Externally we decide to apply another force named F’ which will displace the mobile arm by a tiny distance. 

Work performed by F’ (dW): F’dx = 2. TIdx

We can express 2. TIdx alternatively as dA. This dA signifies the increase in the soap film’s surface area. 

This concludes that, dW = T. dA

We notice that the work performed in expanding the soap film is restored in the surplus area ‘dA’ in the form of potential energy. This potential energy is nothing but interfacial free energy or surface energy. 

Surface Energy (E) = T. dA ⇒ T = E / dA

 Therefore, in Physics, surface tension is the measurement of surface energy per unit region.

 Surface tension is the work performed within a liquid’s unit area to keep it intact. This is the same answer to the question – what is surface energy? 

Conclusion

The creation of a liquid surface is a result of the intrinsic thermodynamics of a system. The surface area of any liquid tends to get reduced because of surface tension. This has to be checked. To expand the area, work needs to be performed. This work is stored in the constituent molecules of the liquid as surface energy. It helps the liquid surface to support solid objects that are light enough but have densities greater than the liquid itself. An alternative way of defining surface energy is the amount of work that is needed to separate a mass material into two different surfaces.