People experience the laws of Physics in daily life. Surface tension is one of the properties of physics we see every day. Even after filling the entire cup with water, one can still add a few drops before the overflow begins. This is due to the phenomenon known as surface tension. The characteristic of fluid surfaces shrinking to the smallest possible surface area is surface tension. Due to surface tension, the liquid can have a concave, convex, or plain shape on the meniscus. It is also responsible for making the objects wet. In this article, a student will learn about surface tension, its definition, SI unit, related formulas, and practical applications.
Surface tension for CBSE class 11th students is an important Physics concept. Many questions in the competitive exams like JEE 2022, NEET 2022, and other olympiads arise from this topic. The notes and formulas given in this article are made by experts in this field. A student will find this chapter mentioned in JEE Main Physics syllabus and JEE Advanced Physics syllabus. Without further ado, let us begin.
Surface Tension Definition
Surface tension is a feature of any liquid that seeks to keep its free surface area as small as possible. The force operating per length on an imaginary line drawn tangentially on the free surface of a liquid is known as surface tension.
Surface Tension Formula and SI Unit
Surface tension S = Force/Length = F/l = (Work done)/(Change in area). The SI unit of surface tension is Nm-1 or Jm-2. Its dimensional formula is [MT-2]. Surface tension is a scalar quantity.
Surface tension is a molecular phenomenon caused by a cohesive force, and the force’s fundamental source is electrical. The nature of a liquid’s surface tension is unaffected by the surface area of the film or the length of the line. Due to the feature of surface tension, little liquid drops are spherical.
Forces Related To Surface Tension
Adhesive Force
Adhesive force is the force of attraction between the molecules of various substances, such as between the molecules of paper and ink, water and glass, and so on.
Cohesive Force
Cohesive force is the attraction force between molecules of the same material. For example, the attraction between molecules of water, glass molecules, and so on.
Cohesive forces and adhesive forces are Van Der Waals’ forces. These forces vary inversely as the seventh power of the distance between the molecules.
Molecular Range
The molecular range is the greatest distance at which a molecule may exert a force of attraction on neighbouring molecules.
- Distinct compounds have different molecular ranges. In solids and liquids, it is of the order of 10-9 m.
- The force of attraction between molecules is minimal if the distance between them is larger than 10-9 m.
Surface Energy
When we expand a liquid’s free surface area, we must fight against the force of surface tension.
This work is stored as potential energy in the liquid surface, and this increased potential energy per unit area of the free surface of the fluid is referred to as surface energy.
Surface energy (E) = T x & ΔAr
where. T = surface tension and ΔAr = increase in surface area.
(i) Work Done in Blowing a Liquid Drop: If a liquid drop is blown up from a radius of r1 to r2 then work done for that is
W = T . 4π (r22 – r12)
(ii) Work Done in Blowing a Soap Bubble: Because a soap bubble has two open surfaces, the effort done to expand the radius of a soap bubble from r1 to r2 is given by
W = T.8π(r22 – r12)
(iii) Work Done in Splitting a Bigger Drop into n Smaller Droplets: If a liquid drop of radius R is divided into ‘x’ identically sized droplets. Then each droplet’s radius
r = R. (x)-1/3
Work done, W = 4π(xr2 – R2)
= 4πSR2 (x1/3 – 1)
(iv) Coalescence of Drops: When n tiny liquid drops of radius R=n1/3.r merge to produce a single larger drop of radius R= x(⅓).r, energy is released in the process. The energy release is provided by
ΔU = S.4π(xr2 – R2)
= 4πSπx(1 – n1/3)
The formulas mentioned above are important from a competitive examination point of view. The questions from these formulas appear in exams like JEE Main 2022, JEE Advanced 2022, NEET exam, and other olympiads in India.
Angle of Contact
The angle formed by the tangents drawn at the liquid surface and the solid surface within the liquid at the point of contact is the contact angle. The nature of the liquid and solid contact and the medium above the liquid’s free surface measure the angle of contact.
A capillary glass tube becomes water-proof when the wax is applied to it. The contact angle widens and thus becomes obtuse. It does not contain any water. Rather, it falls into the tube because of the obtuse angle of contact.
If is an acute angle, i.e; <90°, then the liquid meniscus will be concave upwards.
- If is 90°, then liquid meniscus will be plain.
- If is obtuse, i.e; >90°, then the liquid meniscus will be convex upwards.
- If the angle of contact is acute, i.e; <90°, then the liquid will wet the surface.
- If the angle of contact is obtuse, ie; > 90°, then the liquid will not wet the surface.
Note: With an increase in temperature, the angle of contact rises.
When soluble impurities are added to a liquid, the angle of contact drops. For pure water and glass, the angle of contact is zero. It is 8° for ordinary water and glass. It’s 140° for mercury and glass. Silver is 90 degrees for distilled water. 0° for alcohol and a clean glass.
The angle of contact, meniscus, shape of the liquid surface
| Property | Angle of Contact < 90° | Angle of Contact = 90° | Angle of Contact > 90° |
| Materials | Glass and Water | Silver and Water | Glass and Mercury |
| Contact Angle | Less than 90° (Acute Angle) | Equal to 90° (Right Angle) | Greater than 90° (Obtuse Angle) |
| Shape of the Meniscus | Concave | Plane | Convex |
| Capillary Action | Liquid rises | No effect | Liquid Falls |
Factors Affecting Surface Tension
- The surface tension reduces as the temperature rises, eventually reaching zero at the critical point.
- The surface tension is zero at the boiling point and highest at the freezing point.
- The surface tension lowers when partly soluble contaminants such as soap, detergent, Dettol, phenol, and other substances are added to water.
- When highly soluble contaminants like salt are added to water, the surface tension rises.
- The surface tension of water diminishes as dust particles or oil spreads over it.
When a soap bubble is charged, the surface tension of the liquid drops as a result of the electrification. The liquid does not rise in a capillary tube when it is weightless.
Wetting Agents
If the angle of contact 0 is obtuse, the liquid will not be able to moisten the solid. You may have noticed that it is difficult to moisten some garments in our homes. This is due to the obtuse angle at which such clothing comes into contact with water.
In these circumstances, we add detergents and dyes to the water to minimise the angle of contact and aid water penetration into the cloth.
As a result, the clothing was soaked. Wetting Agents are compounds that, when combined with a fluid, lessen the angle of contact and improve the hydrophilicity of the fluid.
Waterproof Agents
Wetting agents function in the opposite direction as these chemicals. To generate a large angle of contact, waterproofing chemicals are applied. For instance, the waterproofing paints applied on the roof of a house contains chemicals that prevent water from penetrating the walls.
Detergents and Surface Tension
The contact angle between water and oil is obtuse. Water could not moisten the oil in this scenario. As a result, washing clothes with water does not remove the oil filth from the garments.
Detergents are compounds having hairpin-shaped molecules with the head bonding with water molecules and the tail bonding with oil molecules. As a result of the force of attraction from the top side, the surface tension of the oil layer is lowered.
After a while, it weakens to the point where the detergent molecules create a globe around the dirt molecules. Water is an easy way to remove this globe.
Conclusion
Surface tension is an important property of physics. Fluids depict this property every time. Surface tension is why water and other liquids take the shape of the container in which they are stored. If students understand surface tension, they can easily identify why the raindrops are spherical. For major engineering purposes like hydraulic pumps and hydraulic brakes, the concept of surface tension is important. Thus having sound knowledge of this topic is critical for students aiming for JEE Mains 2022 and JEE Advanced Exam 2022. Questions from surface tension also appear in other olympiads like the national engineering olympiads, etc.
