Surface Tension: Causes, Examples and Dimensions

Surface Tension is the tendency of a fluid to shrink into a minimum surface area. It is based on the principle that the molecules of the liquid at the surface area are in a different state than those in the center of the liquid. Surface tension allows objects with a higher density than water such as insects and razor blades to float on the water surface. It depends on the composition of the vapor phase. The surface tension of liquids has many important roles in daily life and also different industrial processes such as rounding off the sharp edges of glass using heat.

Key Terms: Surface Tension, Pressure, unit of surface tension, intermolecular forces, vapor phase, surface tension formula, fluid, fluid flow

---

What is Surface Tension?

[Click Here for Sample Questions]

Surface Tension is the phenomenon that acts on the surface of the liquid due to the attraction of the particles of the surface area by the underlying particles in the interior that tends to minimize the surface area.

• In a liquid, the interior molecules are surrounded by other molecules on all four sides.
• They face an equal attraction from all these sides, therefore the net attracting force acting on it becomes zero.
• However, the molecules on the surface are surrounded by the other molecules on three sides.
• Therefore it experiences more attraction from the molecules below than those lying in the vapor phase.
• As a result of this, it gets pulled inward and tension is created. This is known as surface tension.
• It depends upon the forces of attraction between particles within the given liquid, gas, or solid that is in contact with it. 
• Surface tension occurs when the surface of a liquid is in contact with a gas and acts as a thin elastic sheet

Surface tension of a Liquid

---

Read More: 

---

Causes of Surface Tension

[Click Here for Previous Year's Questions]

Surface Tension is caused by the intermolecular force of the liquid particles such as the Van der Waals force. The molecules at the surface are pulled towards the bottom of the liquid. 

• A molecule is tugged in all directions equally by neighboring liquid molecules due to cohesive forces, resulting in a net force of zero.
• As the molecules near the surface are not surrounded by identical molecules on all sides, they are pushed inward.
• This causes internal pressure which further causes the liquid surfaces to compress to the smallest possible area.
• Due to the cohesive structure of water molecules, there is also a tension parallel to the surface that will resist an external force.

• The forces of attraction that operate between molecules of the same kind are known as cohesive forces, while the forces that act between molecules of different types are known as adhesive forces. 
• The form of liquid droplets is determined by surface tension.
• The imbalance in cohesive forces of the surface layer tends to draw droplets of water into a spherical shape, despite their ease of deformation.
• Drops of almost all liquids would be roughly spherical in the absence of additional forces. 

---

Surface Tension Formula

Surface Tension is given by the ratio of the surface force (F) to the length of the force acting (L). The surface tension formula can be expressed as:

T = F/L

where,

• F - force per unit length
• L - length of the acting force
• T - surface tension

Surface Tension Forces

Read Also: Reynolds Number

---

Unit of Surface Tension

[Click Here for Sample Questions]

The SI surface tension unit is Newton per Meter, also expressed as N/m. The CGS unit is dyne/cm

---

Examples of Surface Tension

[Click Here for Previous Year's Questions]

Some of the examples of surface tension include –

Drops of Water

The shape of the drops is due to the surface tension of the water. Liquid drops are spherical because a sphere has the smallest surface area for a given volume. The force of gravity pulls down on the drop and therefore, the shape of the drop of water isn't completely spherical. In the absence of gravity, the drop would minimize the surface area in order to minimize tension, which would result in a perfectly spherical shape of the droplet.

Needle Floating on Water

Even though the density of the needle or a paper clip is greater than the water, the surface tension along the depression is enough to counteract the gravitational force that pulls down on the metal object.

 Surface Tension and Water

Insects Walk on Water

Several insects such as the water strider are able to walk on water. Their legs cause the surface of the liquid to become depressed, which minimizes the potential energy in order to create a balance of forces so the strider can move across the water surface without breaking it. 

Some of the other applications of surface tension include – 

• Jaundice - Clinical examinations and tests.
• When clothes are washed in soap or detergents, it lowers the surface tension of water.
• The Waterproof material for tents doesn’t allow water to seep into the tents, as the surface tension of the water bridges the pores in the tent material. 

Read More: Young's Modulus

---

Dimensional Formula of Surface Tension

[Click Here for Sample Questions]

The formula of Surface tension = F/L

Also, we know that Force = ma

Putting this value in the equation, we get Surface Tension = ma/L

Therefore, we get MLT^-2L^-1

On solving it further, we get MT^-2

So, the dimensional formula for surface tension is MT^-2.

Read More: Bulk Modulus

---

Methods of Measuring Surface Tension

[Click Here for Previous Year's Questions]

There are different methods of measuring Surface Tension. They are:

• Levitated drops’ vibrational frequency
• Wilhelmy plate method
• Spinning drop method
• Du Noüy–Paddy method
• Du Noüy ring method
• Capillary rise method
• Spherical resonant oscillations & hemispherical liquid drop
• Stalagmometric method
• Pendant drop method
• Sessile drop method
• Bubble pressure method

---

Read More:

---

Things to Remember

• Surface Tension is the tension that acts on the surface of the liquid to minimize the surface area due to the intermolecular force of the liquid particles such as the Van der Waals force.
• The surface tension formula can be expressed as T = F/L.
• The SI surface tension unit is Newton per Meter.
• Surface tension’s dimensional formula is MT^-2.
• An example of surface tension is a needle floating on the surface of the water
• The liquid rises (or sinks) in the capillary due to surface tension.
• The higher the surface tension, the higher will be the energy, therefore fluids take the shape with the least surface area to save energy.

---

Previous Year Questions

1. A ball whose density is 0.4×103kg/m³ falls into the water from a height of 9cm… [BITSAT 2007]
2. A boat with scrap iron in it is floating in a small tank of water… [JIPMER 1998]
3. The cylindrical tube of a spray pump has a radius of R… [NEET 2015]
4. A small spherical ball falling through a viscous medium of negligible density… [KEAM]
5. Fluid is in streamlined flow across a horizontal pipe of the variable area of cross… [NEET 2013]
6. A small hole of the area of cross-section 2mm² is present near the bottom of a fully… [NEET 2019]
7. A soap bubble, having a radius of 1mm, is blown from a detergent solution having… [NEET 2019]
8. A liquid is filled up to a height of 20cm in a cylindrical vessel… [KEAM]
9. A small sphere of radius ′r′ falls from rest in a viscous liquid… [NEET 2018]
10. A rectangular film of liquid is extended from… [NEET 2016]
11. Water rises to a height h in a capillary tube… [NEET 2015]
12. A certain number of spherical drops of a liquid of radius… [NEET 2014]
13. The force acting on a window of area 50cm×50cm of a submarine at a depth… [KEAM]
14. A spherical ball of diameter 1cm and density… [KEAM]
15. Ice pieces are floating in a beaker A containing water and also in a beaker… [KEAM]