How Surface Energy Affects Real-World Phenomena and Experiments
Surface energy is the work done on the outer area of a material where the atoms do not have a bond with another atom in their immediate neighborhood. It is imperative that the atoms in any material are bonded with other atoms. This is because the bulk of the unchanged physical aspect of the material is entirely surrounded by bonded atoms. However, when the material is nearing the end and reaching the surface, the bonds of the atom split open, and there are no bonds on the outer surface of the material. This is known as surface energy. Higher surface energy means that the atoms have a stronger drive for the reconnection of bonds.
Whenever a spring is stretched, we say that some work is done on it and it is stored in the form of Potential energy, known as Elastic potential energy. We can say if the body is in its undeformed state, there will be no potential energy.
Similarly, when we talk about free surfaces of the liquid, as we know the free surfaces of liquids are considered to be stretched membranes, then the surface will store some potential energy due to stretched surface and since it’s only at the surface of the liquid, it is termed as Surface energy or Surface free energy.
Surface Free Energy
Surface energy is the potential energy of the liquid molecules that will help the molecules to remain at the surface of the liquid.
All the molecules at the surface try to reach the bottom layer, which will decrease the surface energy. Surface energy is the amount of work done to increase the surface area of the liquid surface.
Mathematically, the surface energy is the work done per unit area of the liquid surface.
⇒ Surface energy = \[\frac{\text{Work done}}{\text{Area}}\] Joules/m\[^{2}\]
The SI unit of surface energy is Joules/m2 or Newton/metre(N/m).
If the surface is less, then the liquid surface will exert high surface energy (Ex: metals, oxides, ceramics). Similarly, if the surface area is more, then the surface will exert low surface energy (Ex: Plastics, Rubbers, etc.).
The materials having low surface energy are classified as low surface energy materials.
Relation Between Surface Tension and Surface Energy
Let us derive a relation between the surface tension and surface energy. Now, consider a rectangular wireframe with a slider attached to it as shown in the figure below. The rectangular wireframe is dipped in the soap water that will result in soap film due to the surface tension.
Due to surface tension, the soap film will exert force on the slider. Then, the net force on the slider due to surface tension is given by:
⇒ F = 2Tl ………..(1),
Where
T - Surface Tension of the soap film
l - Length of the slider
(The multiplicative factor 2 is used, as there are two surfaces of the slider, and the force exerted on both surfaces slide due to surface tension.)
For slowly moving the slider outwards, the external force acting on it will be
For slowly moving the slider outwards, the external force acting on it will be,
⇒ F\[_{ext}\] = 2Tl……….(2)
Where,
T - Surface Tension of the soap film
l - Length of the slider
Now, let us displace the slider by Δx distance outwards, then there will be some work done on it.
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Whatever work is done to move the slider will be stored in the form of its potential energy. Therefore, the work done in displacing the slider is given by:
⇒ dW = F\[_{ext}\] . Δx
⇒ ΔU = dW = 2Tl . Δx ………(3)
Where,
ΔU - Increase in the surface energy
On rearranging the above expression,
⇒ ΔU = T . ΔS ………….(4)
Where,
ΔS - Increase in surface area of the film
Therefore, the relation between surface tension and surface energy is given by the following expression:
⇒ T = \[\frac{\Delta U}{\Delta S}\] ……….(5)
Equation (5) gives the relation between surface tension and surface energy.
Example:
1) A Film of water is formed between two parallel wires each of length 20 cm and the separation between them is 1 cm. Calculate the work required to increase their surface area by 2 mm. (Surface tension of water is 72 х 10⁻³ N/m.)
Solution: Before starting the solution of the given example, let us first visualise the given data.
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It is given that
The length of the wire = 20 cm
The separation between the wires = 1 cm = 10⁻² m
The displacement due to the external force ∆x = 2 mm = 2 х 10⁻³ m
The surface tension of water = T = 72 х 10⁻³ N/m
Now, we know that the relation between surface tension and surface energy (work done),
⇒ ∆U = T.∆S …..(1),
Where
⇒ ∆S = 2l∆x (Increase in surface area of the film)
⇒ ∆S = 2 х 20 х 2 х 10⁻³ = 80 х 10⁻³m² …….(2)
Substituting equation (2) in (1),
⇒ ∆U = (72 х 10⁻³) . (80 х 10⁻³) = 5.76 х 10⁻³ J
⇒ ∆U = 5.76 mJ
Therefore, 5.76 mJ of work is done to increase the surface area by 2 mm.
Did You Know?
The surface tension of every material is different. Thus, the surface energy of the materials will also be varying from one material to another. The surface energy of liquid will be different from that of the surface energy of metals. The surface energy of water will be depending upon the angle of contact.
The surface energy of few materials is as listed below:
FAQs on Surface Energy: Definition, Examples & Key Concepts
1. What is meant by surface energy in Physics?
Surface energy is the excess potential energy that molecules at the surface of a liquid possess compared to the molecules in the bulk (interior) of the liquid. It can also be defined as the work done per unit area to increase the surface area of a liquid under isothermal conditions. Because of this excess energy, liquids tend to minimise their surface area to achieve a more stable, lower-energy state.
2. How is surface energy different from surface tension?
While numerically equal for a given liquid, surface energy and surface tension are conceptually different.
- Surface Tension is a force phenomenon, defined as the force acting per unit length on an imaginary line drawn on the liquid's surface. Its unit is N/m.
- Surface Energy is an energy phenomenon, defined as the potential energy per unit area of the surface film. Its unit is J/m².
In essence, surface tension is the cause, and surface energy is the effect or the stored energy resulting from that tension.
3. How is the surface energy of a liquid calculated?
Surface energy (S) is calculated using the formula: S = W / ΔA, where:
- W is the work done to change the surface area.
- ΔA is the change (increase) in the surface area.
This formula shows that surface energy is the amount of work required to create a new unit of surface area.
4. What are the SI unit and dimensional formula for surface energy?
The SI unit for surface energy is Joules per square metre (J/m²). Since Joule is kg⋅m²/s², the unit can be simplified to kg/s². Therefore, the dimensional formula for surface energy is [M¹L⁰T⁻²] or simply [MT⁻²], which is the same as that for surface tension.
5. Can you provide some real-world examples of surface energy in action?
Surface energy is responsible for many observable phenomena in our daily lives. Examples include:
- Spherical Droplets: Raindrops and morning dew form nearly perfect spheres because a sphere has the minimum surface area for a given volume, thus minimising surface energy.
- Soap Bubbles: A soap bubble is a thin film of soapy water enclosing air. The surface energy of the soap solution holds the bubble together in a spherical shape.
- Wetting and Coating: The ability of a liquid (like paint) to spread over a solid surface depends on the relative surface energies of the liquid and the solid.
- Insects on Water: Insects like water striders can walk on water because their weight is not enough to overcome the energy required to break the water's surface.
6. Why do liquids inherently try to minimise their surface area?
A liquid tries to minimise its surface area to reach the lowest possible energy state, which is the most stable configuration. Molecules inside the liquid are attracted equally in all directions by neighbouring molecules (cohesive forces). However, molecules at the surface experience a net inward pull because there are no molecules above them. This inward pull gives them higher potential energy. To reduce this overall energy, the liquid system naturally adjusts to have the fewest possible molecules at the surface, which corresponds to the minimum possible surface area.
7. How does temperature affect the surface energy of a liquid?
Surface energy decreases as the temperature of the liquid increases. When a liquid is heated, its molecules gain kinetic energy and move more vigorously. This increased motion weakens the cohesive forces between the molecules. With weaker intermolecular attraction, less work is required to bring molecules from the bulk to the surface. Consequently, the excess energy at the surface (surface energy) reduces.
8. What are the main applications and importance of understanding surface energy?
Understanding surface energy is crucial in many scientific and industrial fields. Its main applications include:
- Detergents and Soaps: Detergents work by lowering the surface energy of water, allowing it to wet clothes and remove dirt effectively.
- Paints and Adhesives: For a paint to coat a surface or an adhesive to stick, its surface energy must be lower than the surface energy of the solid it is applied to, ensuring proper wetting and spreading.
- Capillary Action: The rise or fall of liquids in narrow tubes, vital for plants absorbing water and for the functioning of ink pens, is governed by surface energy and adhesion.
- Metallurgy and Welding: In processes like soldering and welding, molten metal must have the correct surface energy to flow properly and form a strong bond between parts.
9. Is surface energy a form of potential or kinetic energy?
Surface energy is a form of potential energy. It is the extra energy stored by the molecules at the surface of a substance. This energy arises because work has been done against the net inward cohesive forces to bring these molecules from the interior (a lower energy state) to the surface (a higher energy state). It is not related to the motion of the molecules, so it is not kinetic energy.
10. Why is the surface energy of mercury much higher than that of water?
The surface energy of a liquid is directly related to the strength of its intermolecular cohesive forces.
- Mercury is a metal with very strong metallic bonds between its atoms. These powerful cohesive forces mean a large amount of energy is required to break these bonds and create a new surface.
- Water molecules are held together by hydrogen bonds, which are significantly weaker than mercury's metallic bonds. Therefore, less work is needed to increase the surface area of water.
This difference in cohesive forces is why mercury has exceptionally high surface energy and forms distinct, rounded droplets that do not easily wet surfaces.
