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Solids and Surface Tension for JEE

Last updated date: 04th Dec 2023
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What is Surface Tension?

Surface tension, which tends to minimise surface area, is the tension of a liquid's surface film brought on by the attraction of the surface layer's particles to the liquid's bulk. The forces of attraction between the particles in the provided liquid as well as the forces of attraction of the solid, liquid, or gas in contact with it affect surface tension. The effort or energy necessary to remove the surface layer of molecules in a unit area can be conceived of as being roughly equivalent to the energy responsible for the phenomenon of surface tension. 

The forces of attraction between the particles in the given liquid, as well as the gas, solid, or liquid in contact with it, are the fundamental determinants of surface tension. Water has a surface tension of 0.07275 joule per square metre at 20 °C (68 °F).

Surface Tension Formula

Surface tension can be defined mathematically as follows:


where F is the force applied per unit length, L is the length over which the force is applied, and T is the liquid's surface tension.

Unit of Surface Tension

Newton per Metre, or N/m, is the metric measurement for surface tension (SI unit). Dyn/cm is the CGS unit for surface tension.

Dimensional Formula of Surface Tension

The formula, as we all know, provides the surface tension.

Surface tension \[=\frac{F}{L}\]

Knowing that F = ma, we may substitute the value for it in the equation to obtain

Surface tension \[=\frac{ma}{L}\]

When we put the basic quantities into an equation, we get

Surface tension = \[\left[ ML{{T}^{-2}}{{L}^{-1}} \right]\]

Continuing to solve, we obtain

The dimensional formula of surface tension is \[\left[ M{{T}^{-2}} \right]\].

Surface Energy

The breakdown of intermolecular connections that takes place when a surface is formed is quantified as surface energy. Surface free energy and interfacial free energy are other names for it. The work performed by the force that forms the new surface per unit area can be used to define surface energy.

Surface Tension in Solids

The tangential stress (force per unit length) in the surface layer is known as surface tension. This stress needs to be countered by either external forces or volume tensions within the body.

The relationship \[y=F+A\left(\dfrac {dF}{dA} \right)\], where A is the surface area, links the surface free energy to the surface tension of a crystal face. Surface free energy and tension are equivalent for a liquid with a single component. Surface tension and surface energy are not identical for crystals.

Contact Angle

When a gas-liquid interface comes into contact with a solid surface, such as the container's walls, surface tension arises; however, the interface typically curves up or down near the solid surface. A meniscus is a concave or convex surface with such a form.

It is possible to establish a connection between the liquid-solid surface tension and the liquid-gas surface tension using the contact angle, theta, as follows:

$\gamma{{a}_{ls}}=-\gamma{{a}_{\lg }}\cos \theta $

where $\gamma{{a}_{ls}}$ is the liquid-solid surface tension, $\gamma{{a}_{\lg }}$ is the liquid-gas surface tension, and $\theta $ is the contact angle.

Solid-Liquid-Gas Interface

Solid-Liquid-Gas Interface

One thing to remember about this equation is that the cosine component will be negative, which indicates that the liquid-solid surface tension will be positive, in situations where the meniscus is convex (that is, the contact angle is greater than 90 degrees).

The connection would produce a negative liquid-solid surface tension if, on the other hand, the meniscus is concave (i.e., dips down, causing the contact angle to be less than 90 degrees). In general, this means that the liquid is sticking to the container's walls and trying to increase the area in contact with the solid surface in order to reduce the total amount of potential energy.

Methods of Measurement of Surface Tension

Using the following techniques, we may measure the surface tension:

  1. Spinning Drop Method: Measurements are performed in a spinning horizontal tube that is filled with a dense fluid.

  2. Stalagmometric Method: The system is built around a capillary glass tube with a wider centre.

  3. Pendant Drop Method: The balance of forces, which includes the liquid's surface tension, determines the shape of the drop hanging from a needle.

  4. Capillary Rise Method: The increase of a liquid above zero pressure level known as capillary rise is brought on by the net upward force generated by the attraction of water molecules to a solid surface.

  5. Bubble Pressure Method: It enables monitoring the growth of surface tension at an emerging interface.

  6. Sessile Drop Method: Using a superoleophobic surface created by chemical acid etching, anodizing, and fluorination treatment, the sessile drop method measures surface tension.

Applications of Surface Tension

  • Small insects, like the water strider, can only float on the water's surface due to their extremely low weight, which prevents them from penetrating it.

  • On the water's surface, a tiny needle can be floated.

  • Surface tension is crucial for the characterization of products used in food, medicine, and packaging.

  • Different disorders in the human body can be identified by changes in surface tension in biological processes.

  • The reason why water bubbles are round is because surface tension reduces the bubble into spherical shapes while still providing the tension necessary to form the bubble with the water.

  • Surface tension is also the basis for how soaps and detergents function. They reduce the water's surface tension so that soaps and detergents can penetrate pores and holes with ease.

  • When we use disinfectants in the field, they can float on the water and spread out on the cells to kill them because they are mostly low surface tension solutions.

  • Surface tension phenomenon is used by R&D departments in all industrial plants to raise product quality.


Fluid surfaces have a tendency to contract into the smallest surface area feasible due to surface tension. Surface tension is described as a phenomenon that happens when the surface of a liquid comes into contact with another phase (it can be a liquid as well). The least amount of surface area is often acquired by liquids. The liquid's surface behaves like an elastic sheet. The relationship between surface force F and the length L along which the force acts is known as surface tension.

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FAQs on Solids and Surface Tension for JEE

1. What Causes Surface Tension?

The liquid particles are drawn together by intermolecular forces like the Van der Waals force. The particles are drawn toward the liquid's interior along the surface. The relationship between surface force F and the length L along which the force acts is known as surface tension.

Surface tension can be defined mathematically as follows:


2. What Distinguishes Surface Energy from Surface Tension?

Surface tension and surface energy differ primarily in that surface tension measures the force per unit length of the surface whereas surface energy calculates the amount of effort required to stretch it per unit area. In contrast to surface energy, which is often used to describe the same forces between solid-state molecules, surface tension is typically used to describe the attractive interactions between liquid molecules.