What is Surface Tension: Definitions, SI Unit, Formula, Solved Examples

What is Surface Tension Formula?

The phenomenon of surface tension happens when the surface of a liquid comes into contact with another fluid (it can be a liquid as well). Surface tension is the propensity of fluid surfaces to contract into the smallest feasible surface area.Liquids have the lowest surface area imaginable.  The liquid's surface acts like an elastic sheet. Surface tension is determined not only by the forces of attraction between particles inside a specific liquid but also by the forces of attraction of solids, liquids, and gases in contact with it. Surface tension energy may be thought of as being roughly similar to the effort or energy necessary to remove the surface layer of molecules in a unit area.

Surface tension is commonly measured in dyn/cm, which is the force necessary to break a 1-centimeter film. Surface tension is the attractive force inherent in liquids that pulls surface molecules into the remainder of the liquid. It also reduces the surface area. The attracting forces are caused by electrostatic forces. This cohesiveness is usually referred to at the gas-liquid interface; remember, not liquid-solid or liquid-liquid. One can perceive electrostatic forces, which pull molecules together. The small dipole in water is capable of attracting molecules, particularly to each other. As a result, it is possible to form a tight-knit cohesive unit.

When in a liquid condition, the force has little effect since each molecule pushes in all directions towards the other molecules. However, because there are no molecules to draw the others up, the molecules do not pull up at all on the liquid's surface. A strong force between the molecules pulls them down. As a result, it forms a solid barrier at the liquid-gas boundary. As a result, breaking through the liquid's surface will take far more force than traveling through a liquid.

The surface tension of several liquids is shown in the table below-

The definition of surface tension states that the surface tension is primarily determined by the forces of attraction between the particles inside the provided liquid as well as the gas, solid, or liquid in contact with it.

What Causes Surface Tension Formula?

The liquid particles are drawn together by intermolecular interactions such as the Van der Waals force. The particles are drawn towards the remainder of the liquid along the surface. Surface tension is defined as follows-

1. Strong molecular interactions.
2. Cohesive interactions of liquid molecules. 
3. The molecules in the liquid's bulk have adjacent molecules on either side. 
4. Molecules are tugging on each other in all directions equally, resulting in a net force of zero. 
5. However, when compared to the rest of the liquid, the liquid molecules near the interface have just half the number of adjacent liquid molecules. 
6. This allows the molecule to form stronger bonds with the molecules on its sides, resulting in a net inward push towards the liquid. 
7. Surface tension is the force that prevents the surface from breaking.

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What are the Surface Tension Unit and Formula?

Surface Tension is measured in Newton per Meter (N/m) in the SI system. Check out the other units in the table below- 

Surface tension is defined as the ratio of the surface force F acting on a liquid to the length d along which the force operates. This is the same principle that allows insects like water striders to walk on water or a paperclip to float. 

As a result, the surface tension formula is-

γ = F /d

where,

• F stands for force per unit length.
• L denotes the length over which force acts.
• T denotes the liquid's surface tension.

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Surface Tension Dimension

The surface tension is provided by the formula,

Surface Tension = F/L

We know that F = ma, so we can solve the problem by inserting the value, we get

    = ma/L

By equating the essential quantities in the equation, we get 

MLT⁻² L⁻¹

Further, by solving,

= MT⁻²

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Surface Tension Formula Examples

The following are some surface tension examples-

1. Walking insects on water: Because their weight is inadequate to permeate the surface, water striders and other microscopic insects may walk on water. Water striders, which are little insects, can walk on water because their weight is far less than that required to breach the water's surface.
2. Floating a needle on the water's surface: Because of surface tension, a little needle may be made to float on the surface of the water despite being several times as thick as water.
3. Soaps and detergents: Soaps and detergents help to clean garments by reducing the surface tension of the water, enabling it to penetrate pores and contaminated areas more easily.
4. Bubbles have a spherical shape: Because the surface tension of water provides the essential wall tension for bubble formation. Because of the inclination to lower wall tension, the bubbles develop spherical forms. The surface tension of water supplies the wall tension for the creation of round bubbles. 
5. Jaundice clinical test: Normal urine has a surface tension of around 66 dynes/centimeter, but if bile is present (a test for jaundice), it reduces to about 55. During the Hay test, powdered sulphur is sprayed over the urine surface. When the surface tension of the urine is decreased by the bile, it floats but sinks.
6. Disinfectants for surface tension (disinfectants are solutions with low surface tension): Disinfectants are generally solutions with a low surface tension. This permits them to propagate and break down bacterial cell walls.
7. Easy washing of clothes in Hot Water: It is simple to wash clothing in hot water since the surface tension of the water is lower and it is a better wetting agent.

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How to Determine Surface Tension with Solved Examples

The following is an example of how to use the formula to calculate surface tension-

Example- Calculate the surface tension of a particular liquid with a dragging force of 7 N and a length of 2 m along which the force acts.

Ans. F = 7 N 

L = 2 m

According to the Formula, 

T = F/L

T = 7/2

T = 3.5 N/m

Example- You have a 1 cm long piece of metal that weighs 0.1 N. Determine the surface tension.

Answer- The F is 0.1 N and the d is 1 cm. 

Using the method, we can calculate surface tension-

γ= F /d

γ= 0.1N/1 cm

0.1 N/ 0.01

m = 10 N/m.

Therefore, the surface tension is 10 N/m.

Methods of Surface Tension 

The following are some surface tension measuring techniques:

1. Method of spinning drops
2. Method of bubble pressure
3. The frequency of vibration of levitated droplets
4. Sessile drop technique
5. Method of pendant dropping
6. Method of Du Noüy-Padday
7. The resonant oscillations of a spherical and hemispherical liquid drop.
8. Du Noüy ring technique
9. Wilhelmy plate technique
10. Method of pendant dropping
11. Stalagmometric technique
12. Method of capillary rise

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Conclusion

1. Surface tension is an important physical characteristic. 
2. Fluids always exhibit this characteristic.
3. Water and other liquids take on the form of the container in which they are stored due to surface tension. 
4. Students may readily comprehend why raindrops are spherical if they understand surface tension. 
5. Surface tension is vital in key engineering applications such as hydraulic pumps and hydraulic brakes. 
6. As a result, students preparing for the JEE Mains and JEE Advanced Exams 2022 must be well-versed in this subject. 
7. Surface tension questions can also be found in other olympiads, such as the national engineering olympiads.