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The disruption and quantification of the intermolecular bonds that is observed when a surface is created is defined as surface energy. In simpler terms it is defined as the work done per unit area by the force during the creation of a new surface.
Mathematically, the surface energy is the work done per unit area of the liquid surface.
⇒ Surface energy = workdone/area
The SI unit of Surface energy is Joules/m2 or newton per meter (N/m)
Surface energy is a form of the potential energy of the liquid molecules that helps them to remain on the surface of the liquid. By liquid action all the molecules on the surface try to reach the bottom layer of the liquid which reduces the surface energy. Thus, the liquid surface tries to expand the surface area and the work done to increase the surface area is termed as Surface Energy. If the surface area is lower the liquid will exert higher surface energy and vice–versa.
What is Surface Energy
Have you ever tried to drown an ant? But somehow the ant does not drown but rather the body seems to be sliding across the water’s surface till they reach solid ground. Also, this phenomenon seems illogical since their breathing mechanism is located under the belly.
In reality, the body is not sliding. They are simply walking across like they would on land, just with a different maneuvering technique. The cohesive forces across liquid molecules the surface energy and tension create a thin film layer across the surface. The ants are extremely light to cause any damage to this layer or cause it to be brittle and fail, making it the most ideal support to walk across the water surface.
Surface energy and hence surface tension of different materials is different and will be varying from one material to another. The surface energy will be dependent on the angle of contact. Generally, a surface with a low value of surface energy will cause extremely poor wetting of the surface and thereby have a high contact angle. This is because the surface is not capable of forming high-strength bonds and hence lower energy to break bonds to have any interaction with the surface molecules.
Relation Between Surface Energy and Surface Tension (Formula):
Surface Energy = Energy / Area
= Joules / m2
= Newton × m / m2
= Newton / m
= Force / Length
= Surface Tension
Excess pressure inside a liquid drop and a bubble:
Consider a small drop of water that does not collapse because of the surface tension whose formula is given by:
σ (Surface Tension) = 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
Since the pressure inside the drop is greater than the external pressure being applied on the drop, the drop does not collapse. The difference or the extra pressure is given by subtracting the outside pressure (po) from the inside pressure (pi).
P = pi-po
The external force applied is given by:
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+dr)2 – 4πr2)
= σ x 8πr.dr (after neglecting tiny terms)
dW = Increase in P.E.
This implies that,
⇒ (pi – po) x 4πrr2.dr = σ x 8πr.dr
⇒ (pi –po) = 2σ/r
⇒ p = 2σ/r
The adhesive and cohesive forces of the molecules of similar and dissimilar materials depends on the forces of attraction and repulsion. If the molecules attract each other, the surface tension is reduced, while if the molecules repel, the surface energy increases. For two materials, the surface energy is the energy of intermolecular interaction between them and depends on each of their characteristics. A surface will always try to minimize the energy it possesses to reach a state of no energy. It can do this easily by adsorbing a material lower in energy onto its surface. Through this process of adsorption, the number of exposed high surface energy atoms are minimised and thereby replaced with lower energy atoms or molecules. Liquids tend to have lower surface energies than solids due to weak molecular forces which causes liquid to spread out easily.
