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Surface Tension - Definition, Examples, Formula, Unit, Dimension, FAQs

Surface Tension - Definition, Examples, Formula, Unit, Dimension, FAQs

Edited By Vishal kumar | Updated on Jul 02, 2025 04:36 PM IST

Surface tension is the tendency of fluid surfaces to shrink into the minimum surface area possible. Have you noticed when you fill a glass up to the brim with water, you can still add a few more drops till it spills out? Or have you ever broken a thermometer and observed how the fallen mercury behaves? All these happen due to the surface tension of the surface. Let us understand the concept, surface tension definition along with its SI unit, formula and examples.

Surface Tension - Definition, Examples, Formula, Unit, Dimension, FAQs
Surface Tension - Definition, Examples, Formula, Unit, Dimension, FAQs

This concept is the part of properties of solids and liquids which is a crucial chapter in Class 11 physics. It is not only essential for board exams but also for competitive exams like the Joint Entrance Examination (JEE Main), National Eligibility Entrance Test (NEET), and other entrance exams such as SRMJEE, BITSAT, WBJEE, VITEEE and more. Over the last ten years of the JEE Main exam (from 2013 to 2023), a total of three questions have been asked on this concept. And for NEET one questions were asked from this concept.

Background wave

Surface Tension

Surface tension is the elastic tendency of a fluid surface which makes it acquire the least surface area.

If we draw an imaginary line on the free surface of the liquid as shown in the below figure.

Then Surface tension of a liquid is measured by the force acting per unit length on either side of an imaginary line.

So Surface tension of a liquid is given by

T=Fl

Where:
F→ force
l→ imaginary length

The direction of this force is perpendicular to the line and tangential to the free surface of the liquid.

It depends only on the nature of the liquid and is independent of the area of the surface or length of the imaginary line considered.

  • It is a scalar quantity.

Unit of Surface Tension

  • Force is measured in newtons ( N ).
  • Length is measured in meters (m).
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T=N m=N/m
SI Unit: N/m.

  • Dimensional Formula: [T]=MT2.

Example- Raindrops are spherical in shape because each drop tends to acquire minimum surface area due to surface tension, and for a given volume, the surface area of the sphere is minimum.

The surface tension of liquid decreases with the rise in temperature.

For More Information On Surface Tension, Watch The Below Video:

Solved Example Based On Surface Tension

Example 1: A drop of liquid of density ρ is floating half immersed in a liquid of density σ and surface tension 7.5×10−4Nem−1. The radius of drop-in will be :
(g=10 ms−2)

1) 15(2ρ−σ) 2) 15(ρ−σ) 3) 32(ρ−σ) 4) 320(2ρ−σ)

Solution:

B+T(2πR)=mgσ×2πR33 g+(2πR)=ϱ×2πR33 gσR2 g3+T=ϱR2 g37.5×10−2=R2 g3(ϱ−σ)R=32(ρ−σ)

Hence, the answer is option (3).

Example 2: A spherical soap bubble of radius 3 cm is formed inside another spherical soap bubble of radius 6 cm. If the internal pressure of the smaller bubble of radius 3 cm in the above system is equal to the internal pressure of another single soap bubble of radius r cm. The value of r is___________.

1) 2

2) 4

3) 3

4) 1

Solution:

Pi−P=4 Tr1P−Po=4 Tr2Pi−Po=4 Tr1+4 Tr2→(1)

Pi−Po=4 Tr→(2) From eq. (1) and (2) 4tr=4 Tr1+4 Tr2 (Given) 1r=1r1+1r2=13+16=918r=2 cm

Hence, the answer is option (1).

Example 3: A thin liquid film formed between a U- U-shaped wire and a light slider supports a weight of 1.5 ×10-2 N ( see figure) The length of the slider is 30 cm and its weight is negligible. The surface tension of the liquid film is:

1) 0.0125 Nm-1

2) 0.1 Nm-1

3) 0.05 Nm-1

4) 0.025 Nm-1

Solution:

The force due to the surface tension will balance the weight.

2TL=wT=w2L

Substituting the given values, we get
T=1.5×10−2 N2×30×10−2 m=0.025Nm−1

Hence, the answer is option (4).

Example 4: On heating water, bubbles being formed at the bottom of the vessel detach and rise. Take the bubbles to be spheres of radius R and making a circular contact of radius r with the bottom of the vessel. If r << R, and the surface tension of water is T, value of r just before bubbles detach is :(density of water is ρw )

1) R22pwg3T
2) R2pwg6T
3) R2pwgT
4) R23pwgT

Solution:

Here T is the surface tension and we know that surface tension,

T=Fl=F=Tl=T(2πr),

where F is the force due to surface tension and its vertical component will be T(2πr)Sinθ. .

Now The bubble will detach, if Buoyant force ≥ vertical component of surface tension force, so for the bubble to just detach:-

vertical component of surface tension force=buoyant force(or upthrust)

T(2πr)Sinθ=Vρwg⇒T(2πr)rR=43πR3ρwg⇒r2=2R4ρwg3T⇒r=R22ρwg3T

Hence, the answer is option (1).

Example 5: A soap bubble, blown by a mechanical pump at the mouth of a tube, increases in volume, with time, at a constant rate. The graph that correctly depicts the time dependence of pressure inside the bubble is given by :

1)

2)

3)

4) none of these

Solution:

Pressure Difference in Water Droplet -

ΔP=2TR

wherein:

T- Surface tension

R- Radius

V=ct4/3=43πR3

R=Kt1/3
P=P0+4TKt1/3
P=P0+Ct1/3

Hence, the answer is option (4).

Frequently Asked Questions (FAQs)

1. Why are water drops spherical?

Raindrops are spherical in shape due to the presence of cohesive forces between liquid molecules and the surface tension of the liquid.

2. What is the formula of surface tension?

Formula of surface tension  is T = F/L

3. What is the unit of surface tension?

The unit of surface tension is Newton/meter(N/m).

4. What are the dimensions of surface tension?

The dimensions of surface tension are [MT^-2]

5. Write the surface tension of water at its boiling point?

It  is zero at boiling point.

6. What is the formula for surface tension, and what do its variables represent?
The formula for surface tension is γ = F/L, where γ (gamma) is the surface tension, F is the force acting on the surface, and L is the length of the surface. This formula represents the force per unit length required to create additional surface area.
7. What is the Young-Laplace equation, and how does it relate to surface tension?
The Young-Laplace equation describes the pressure difference across a curved interface between two fluids. It relates this pressure difference to the surface tension and the curvature of the interface. The equation is ΔP = γ(1/R₁ + 1/R₂), where ΔP is the pressure difference, γ is the surface tension, and R₁ and R₂ are the principal radii of curvature of the interface.
8. How does surface tension affect the boiling point of liquids?
Surface tension affects the boiling point of liquids by influencing the formation of vapor bubbles. A higher surface tension makes it more difficult for bubbles to form and expand, as they need to overcome the "skin" created by surface tension. This results in a slightly higher boiling point. Conversely, reducing surface tension (e.g., by adding surfactants) can lower the boiling point.
9. What is the concept of critical micelle concentration (CMC) in relation to surface tension?
The critical micelle concentration (CMC) is the concentration of surfactants above which micelles form and all additional surfactants added to the system go to micelles. At the CMC, the surface tension of the solution stops decreasing significantly with increasing surfactant concentration. This concept is important in understanding the behavior of surfactants and their effect on surface tension.
10. How does surface tension contribute to the phenomenon of supersaturation?
Surface tension contributes to supersaturation by creating an energy barrier for the formation of new phases. In a supersaturated solution, the formation of crystals or bubbles requires creating new surfaces, which has an energy cost due to surface tension. This energy barrier can prevent nucleation, allowing the solution to remain in a metastable supersaturated state until the barrier is overcome or nucleation sites are introduced.
11. What is surface tension, and why does it occur?
Surface tension is the tendency of liquid surfaces to shrink into the minimum surface area possible. It occurs due to the cohesive forces between liquid molecules being stronger than the forces between the liquid and air molecules. This creates a "skin-like" effect on the liquid surface, allowing small objects to float on water even if they are denser than the liquid.
12. How does surface tension explain why water droplets form spherical shapes?
Surface tension causes water droplets to form spherical shapes because a sphere has the smallest surface area for a given volume. The cohesive forces between water molecules pull the droplet inward from all directions, minimizing the surface area and creating a spherical shape to achieve the lowest energy state.
13. What are the SI units and dimensions of surface tension?
The SI unit of surface tension is Newton per meter (N/m) or Joule per square meter (J/m²). The dimension of surface tension is MT⁻² (Mass/Time²), which is the same as that of force per unit length.
14. How does temperature affect surface tension?
Generally, surface tension decreases as temperature increases. This occurs because higher temperatures increase the kinetic energy of molecules, weakening the cohesive forces between them. As a result, the liquid surface becomes less "tight" and more easily deformable at higher temperatures.
15. Why can some insects walk on water?
Some insects can walk on water due to surface tension. Their lightweight bodies and specially adapted hydrophobic (water-repelling) legs create small depressions in the water surface without breaking through it. The upward force from the surface tension balances the insect's weight, allowing it to stay afloat.
16. What is the capillary action, and how is it related to surface tension?
Capillary action is the ability of a liquid to flow against gravity in narrow spaces. It is closely related to surface tension because it results from the combined effects of cohesive forces (surface tension) and adhesive forces between the liquid and the surrounding material. This phenomenon explains how water can rise in plant stems or how a paper towel absorbs liquid.
17. How does adding soap to water affect its surface tension?
Adding soap to water reduces its surface tension. Soap molecules are surfactants, which means they have a hydrophilic (water-loving) end and a hydrophobic (water-repelling) end. The hydrophobic ends align at the water's surface, disrupting the cohesive forces between water molecules and lowering the surface tension.
18. What is the difference between cohesion and adhesion in relation to surface tension?
Cohesion refers to the attractive forces between molecules of the same substance (e.g., water molecules attracting other water molecules), which is responsible for surface tension. Adhesion, on the other hand, is the attraction between molecules of different substances (e.g., water molecules attracting glass molecules). Both forces play a role in phenomena like capillary action.
19. How does the shape of a meniscus relate to surface tension?
The shape of a meniscus (the curve formed by a liquid in a container) is determined by the balance between adhesive and cohesive forces, which are related to surface tension. In water, the meniscus is concave (curved upward at the edges) because water's adhesion to glass is stronger than its cohesion. In mercury, the meniscus is convex (curved downward at the edges) because its cohesion is stronger than its adhesion to glass.
20. How does surface tension contribute to the formation of soap bubbles?
Surface tension is crucial for the formation of soap bubbles. The soap solution forms a thin film with two surfaces (inner and outer) that have surface tension. This tension causes the bubble to minimize its surface area, resulting in a spherical shape. The soap molecules reduce the water's surface tension, making it easier to form and maintain bubbles.
21. What is the Marangoni effect, and how is it related to surface tension?
The Marangoni effect is the mass transfer along an interface between two fluids due to a gradient of surface tension. This gradient can be caused by variations in temperature or chemical composition. The effect causes liquids to flow from areas of low surface tension to areas of high surface tension, leading to phenomena like "tears of wine" in a glass.
22. What is the role of surface tension in the process of wetting?
Surface tension plays a crucial role in wetting, which is the ability of a liquid to maintain contact with a solid surface. The balance between the cohesive forces within the liquid (surface tension) and the adhesive forces between the liquid and the solid determines whether a liquid will wet a surface. If adhesive forces are stronger, the liquid will spread out, while if cohesive forces dominate, the liquid will bead up.
23. How does the curvature of a liquid surface affect the pressure beneath it?
The curvature of a liquid surface creates a pressure difference across the interface, as described by the Young-Laplace equation. A convex surface (curved outward) creates higher pressure beneath it, while a concave surface (curved inward) creates lower pressure. This principle explains phenomena like why small water droplets have higher internal pressure than larger ones.
24. How does surface tension contribute to the lotus effect?
The lotus effect refers to the high water repellency (superhydrophobicity) exhibited by lotus leaves. Surface tension plays a role in this effect by interacting with the micro- and nano-scale structures on the leaf surface. These structures minimize the contact area between water droplets and the surface, causing the droplets to bead up due to surface tension and easily roll off the leaf.
25. What is the relationship between surface tension and contact angle?
The contact angle is the angle formed where a liquid-vapor interface meets a solid surface. It is directly related to surface tension through Young's equation: γsv = γsl + γlv cos(θ), where γsv, γsl, and γlv are the solid-vapor, solid-liquid, and liquid-vapor surface tensions, respectively, and θ is the contact angle. This relationship shows how surface tensions determine the wetting behavior of a liquid on a solid surface.
26. How does surface tension affect the formation of raindrops?
Surface tension plays a crucial role in the formation and size of raindrops. As water vapor condenses in clouds, surface tension causes small water droplets to form spherical shapes. As these droplets collide and coalesce, surface tension continues to maintain their rounded shape. However, as drops grow larger, air resistance becomes more significant, causing larger drops to deform or break apart, limiting their maximum size.
27. What is the Du Noüy ring method, and how does it measure surface tension?
The Du Noüy ring method is a technique used to measure surface tension. It involves slowly lifting a platinum ring from the surface of a liquid. The force required to pull the ring free from the liquid surface is measured and related to the liquid's surface tension. This method takes into account both the weight of the liquid lifted and the force due to surface tension, providing an accurate measurement of surface tension.
28. How does surface tension affect the process of emulsification?
Surface tension plays a crucial role in emulsification, which is the process of mixing two immiscible liquids. Emulsifiers work by lowering the surface tension between the two liquids, making it easier to form and stabilize small droplets of one liquid within the other. By reducing the energy required to create new interfaces, lower surface tension allows for the formation of smaller, more stable emulsion droplets.
29. What is the relationship between surface tension and surfactant concentration?
As surfactant concentration increases, surface tension generally decreases. This relationship is often represented by the Gibbs adsorption isotherm. Initially, adding surfactants causes a rapid decrease in surface tension. However, as the concentration approaches the critical micelle concentration (CMC), the rate of decrease slows down. Beyond the CMC, further additions of surfactant have little effect on surface tension as excess molecules form micelles in the bulk solution.
30. How does surface tension affect the shape of liquid bridges between solid surfaces?
Surface tension significantly influences the shape of liquid bridges between solid surfaces. These bridges, also known as capillary bridges, form due to the balance between surface tension, which tends to minimize the liquid's surface area, and the adhesive forces between the liquid and solid surfaces. The resulting shape is typically a catenoid, which minimizes surface area for a given volume and separation distance between the solids.
31. What is the role of surface tension in foam stability?
Surface tension plays a crucial role in foam stability. Foams are dispersions of gas bubbles in a liquid, and their stability depends on the strength of the liquid films between bubbles. Surface tension contributes to film stability by resisting film thinning and rupture. Surfactants in foams lower surface tension, making it easier to create bubbles, but they also create surface tension gradients that oppose film drainage, enhancing stability.
32. How does surface tension contribute to the coffee ring effect?
The coffee ring effect, where particles in an evaporating droplet accumulate at the edges, is influenced by surface tension. As the droplet evaporates, its contact line with the surface is often pinned due to surface roughness. Surface tension maintains the droplet's shape, causing fluid flow from the center to the edges to replace evaporated liquid. This flow carries suspended particles to the edge, creating the characteristic ring pattern.
33. How does surface tension affect the process of nucleation in phase transitions?
Surface tension plays a critical role in nucleation during phase transitions, such as in the formation of liquid droplets from vapor or solid crystals from a solution. Creating a new phase interface requires energy, which is provided by surface tension. This energy requirement creates a nucleation barrier, making spontaneous nucleation difficult. The size of this barrier depends on the surface tension, explaining why supercooled liquids or supersaturated solutions can exist without immediate phase change.
34. What is the relationship between surface tension and the shape of sessile drops?
The shape of a sessile drop (a drop resting on a solid surface) is determined by the balance between surface tension and gravity. Surface tension tends to minimize the drop's surface area, pulling it into a spherical shape, while gravity flattens the drop. For small drops, surface tension dominates, resulting in nearly spherical shapes. For larger drops, gravity becomes more significant, causing flattening. The contact angle between the drop and the surface is also influenced by the balance of surface tensions at the three-phase contact line.
35. How does surface tension contribute to the phenomenon of liquid jet breakup?
Surface tension plays a crucial role in liquid jet breakup. As a liquid jet moves through air, small perturbations on its surface grow due to the Rayleigh-Plateau instability, which is driven by surface tension. Surface tension acts to minimize the jet's surface area, causing these perturbations to grow and eventually pinch off into droplets. The wavelength of the fastest-growing perturbation, and thus the size of the resulting droplets, is related to the jet's diameter and the liquid's surface tension.
36. How does surface tension affect the process of spreading of liquids on solid surfaces?
Surface tension significantly influences the spreading of liquids on solid surfaces. The process is governed by the balance between the liquid's cohesive forces (surface tension) and its adhesive forces with the solid. Spreading occurs when the adhesive forces overcome the cohesive forces. The spreading coefficient, S = γSV - (γSL + γLV), where γ represents the respective interfacial tensions, determines whether complete wetting (S > 0) or partial wetting (S < 0) occurs.
37. How does surface tension contribute to the phenomenon of fingering instability?
Fingering instability, such as in the Saffman-Taylor instability, occurs when a less viscous fluid displaces a more viscous one in a porous medium or between closely spaced plates. Surface tension plays a stabilizing
38. What is the Kelvin equation, and how does it relate surface tension to vapor pressure?
The Kelvin equation relates the vapor pressure of a curved liquid surface to the vapor pressure of the same liquid with a flat surface. It states that the vapor pressure over a curved surface is different from that over a flat surface due to surface tension. The equation is: ln(P/P₀) = (2γVm)/(rRT), where P is the vapor pressure over the curved surface, P₀ is the vapor pressure over a flat surface, γ is surface tension, Vm is molar volume, r is the radius of curvature, R is the gas constant, and T is temperature.
39. How does surface tension contribute to the phenomenon of coalescence?
Coalescence is the process by which two or more droplets or bubbles merge to form a single larger one. Surface tension plays a key role in this process. When two droplets come into contact, surface tension causes the combined droplet to minimize its surface area, driving the merging process. The reduction in total surface area leads to a decrease in surface energy, making coalescence energetically favorable.
40. What is the Worthington jet, and how is it related to surface tension?
The Worthington jet is a thin column of liquid that shoots upward when a drop impacts a liquid surface. Surface tension plays a crucial role in this phenomenon. As the drop creates a crater in the liquid surface, surface tension along with pressure differences cause the crater to collapse rapidly. This collapse ejects a narrow jet of liquid upward. The interplay between inertial forces, gravity, and surface tension determines the characteristics of the jet.
41. What is the Rayleigh-Plateau instability, and how is it related to surface tension?
The Rayleigh-Plateau instability is a physical phenomenon where a falling stream of fluid breaks up into droplets. It's driven by surface tension, which acts to minimize the surface area of the fluid. Small perturbations in the stream's radius grow exponentially when their wavelength exceeds the stream's circumference. Surface tension then causes these perturbations to pinch off, forming droplets. This instability explains phenomena like why a thin stream of water from a faucet breaks into droplets.
42. What is the Marangoni bursting effect, and how is it related to surface tension?
The Marangoni bursting effect is a phenomenon where bubbles at a liquid surface suddenly burst and eject small droplets. It's caused by surface tension gradients. When a bubble reaches the surface, its thin film drains and eventually ruptures at a point. This rupture creates a hole with a high-curvature rim. The difference in surface tension between the bubble film (which may contain surfactants) and the surrounding liquid drives rapid film retraction, sometimes fast enough to eject droplets.
43. What is the role of surface tension in the formation of Plateau borders in foams?
Plateau borders are the channels formed where soap films meet in a foam structure. Surface tension plays a crucial role in their formation and shape. The films in a foam tend to minimize their surface area due to surface tension. When three films meet, they form a Plateau border to balance the surface tensions. The cross-section of a Plateau border is a concave triangle, with each side having a radius of curvature that balances the pressure difference across the films.

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