Surface Energy

Surface energy measures the disruption of intermolecular bonds that occur when a surface is created. It also goes by the name surface free energy or interfacial free energy.

It can basically be defined as the excess energies at the interface of a material compared to the bulk or the work required to build an area of a particular interface.

What is Surface Energy?

It is the relative measurement of energy at the interface/surface of the material. In the bulk area of a material, atoms are generally stable and have a balanced set of bonds/interactions. The interface area, in contrast to the bulk area, will have an incomplete, unbalanced set of bonds/interactions, hence they will have interfacial free energy.

The surface of a material will always try and minimize its energies. This is done by adsorbing material with lower energy onto its interface.

The number of exposed interface atoms with high interfacial free energy is minimized and replaced with lower energy atoms or molecules throughout the adsorption process. Liquids often have lower surface energies than solids due to the weak interaction forces between the molecules.

SI Unit: N/m (newton per meter)

Dimensions of Surface Energy: [MT-2]

\(\mbox{Surface Energy } = \frac{\mbox{Energy}}{\mbox{Area}}\)

Relation Between Surface Energy and Surface Tension:

\(\mbox{Surface Energy } = \frac{\mbox{Energy}}{\mbox{Area}}\)

= \(\frac{\mbox{Joule}}{m^2}\)

= \(\frac{\mbox{Newton}*m}{m^2}\)

= \(\frac{\mbox{Force}}{\mbox{Length}}\)

= \(\mbox{Surface Tension}\)

Solved Example

Question. A water drop of diameter 1 cm breaks into 1000 similar droplets of same diameter. What will be the gain or loss in the interfacial free energy? (Take surface tension as 0.05 N/m)

Solution. Let mass and density of big drop be \(M\), \(D\), and mass, the density of small drop be \(m\), \(d\).

\(D^3 = 1000d^3\)

⇒ \(D = 10d\)

Change in surface energies = Surface tension * change in area of material

⇒ \(ΔSE = 0.05 * (1000 * ⲡd^2 – ⲡD^2)\)

⇒ \(ΔSE = 0.05 * (10 * ⲡD^2 – ⲡD^2)\)

⇒ \(ΔSE = 0.05 * (9ⲡD^2)\)

⇒ \(ΔSE = 0.05 * 9ⲡ * 10^{-4}\)

∴ \(ΔSE = 0.1413mJ\)


What does surface tension mean?

Surface tension is the work required to increase the surface area of a liquid due to intermolecular forces.

What is the relation between surface energy and surface tension of a liquid?

The surface tension of a liquid is quantitatively(numerically) equal to its surface energy.

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