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What is Surface Energy?

Surface energy is a measure of the excess energy present at the surface of a material. Surface energy indicates the disruption of intermolecular bonds that occurs when a surface is created. This phenomenon is also called surface free energy or interfacial free energy. In simpler words, surface energy can be defined as the work per unit area done by the force that creates the new surface. It can be used to describe wetting and adhesion between materials, but is not often used quantitatively.

Surface energy is the energy required to create two new surfaces in a material

Surface Energy is the excess energy required to create a surface in a vacuum.

Surface energy’ is a relative measurement of the energy at the surface (which is a result of this incomplete bonding). It is positively correlated to the strength of bulk interactions and the level of surface exposure. Therefore, surface energy will be higher if the bulk interactions are stronger, or if the surface exposure is greater.

The Process

A surface inherently tries and reduces its energy. This is done by the process of adsorbing a material with a lower energy onto its surface. Through the adsorption process, the number of exposed surface atoms with high surface energy are minimized and replaced with lower energy atoms or molecules. Liquids will often have lower surface energies than solids (due to the weak forces interacting between molecules), which is why liquids usually spread out. Hence, Surface energy can be defined as the energy required per unit area to increase the size of the surface, and as such is often quoted in units of mN/m.

Surface energy is mostly used to describe a solid, although numerically equivalent to liquid surface tension. The tendency of a solution to spread out on a solid surface depends on several factors. It is assumed that the liquid and solid are capable of similar polarity interactions.

Usually a surface with a ‘low’ surface energy will cause poor wetting, and therefore result in a high contact angle. As we know, the surface is not capable of establishing strong adhesion and as there is inherently no energetic magnitude for the liquid to break bulk bonding in favor of interacting with the surface. Typical surfaces with low surface energy include hydrocarbons, as these are held together with weak forces.

The correlation between Surface Energy and Surface Tension (Formula) is given as follows:-

Surface Energy = Energy Area

\= Joulem2

\= Newton×mm2

\= Newtonm

\= ForceLength

\= Surface Tension

Excess pressure inside a liquid drop and a bubble:

Consider a droplet of water. The droplet doesn’t collapse because of the surface tension which is explained as follows:-

σ = F/L

Where,

  • σ is the surface tension of the liquid
  • F is the force per unit length
  • L is the line over which the force acts

If the drop doesn’t collapse, it implies that the pressure within the drop is greater than that outside. The extra pressure (p) is given by subtracting the outside pressure (PO) from the inside pressure (pi);

p = pi – po

Outside Force

\= p x surface area of the drop

\= (pi – po) x 4πr2

Where,

  • r is the radius of the drop

To satisfy the usual tendency for increasing surface area;

dW = Outside Force = (pi – po) x 4πr2.dr

Work done by the excess pressure is stored in the form of potential energy.

Increase in P.E.

\= surface tension × increase in surface area.

\= σ x [4π (r+dr2) – 4πr2

\= σ x 8πr.dr (after neglecting tiny terms)

dW = Increase in P.E.

This implies that,

 (pi – po) x 4πr2.dr = σ x 8πr.dr

 (pi – po) = 2σ/r

 p = 2σ/r

Since a bubble has two free surfaces:

p = 4σ/r