Surface Tension: Definition, Formula, Unit, Dimension and Real-Life Examples

Surface tension is the property of a liquid's surface that makes it behave like a stretched elastic membrane. It occurs due to the cohesive forces between the molecules of a liquid, which try to pull the surface molecules inward and reduce the surface area. Because of this property, water droplets become nearly spherical, and small objects like insects or needles can sometimes float on water.

This article gives a detailed explanation of surface tension, its causes, formula, SI unit, dimensional formula, and real-life examples.

Table of Contents

• What is Surface Tension
• What Causes Surface Tension
• Surface Tension Formula, Unit and Dimensional Formula
• Surface Tension of Water and Other Liquids
• Real-Life Examples of Surface Tension
• Different Methods of Measuring Surface Tension

What is Surface Tension

Surface tension is the property of a liquid that makes its surface behave like a stretched elastic membrane. It occurs because of the cohesive forces between the molecules of the liquid. Due to these forces, the liquid surface tries to occupy the smallest possible area.

In simple words, the molecules present at the surface are pulled inward by the molecules below them. This inward pull creates a kind of tension on the liquid's surface. 

Surface tension is defined as the force acting per unit length on the surface of a liquid that tends to minimize its surface area.

Because of this property:

• Water droplets become nearly spherical.
• A needle can float on the surface of water.
• Some insects can walk on water.
• Soap bubbles take a round shape.

Now, there's an interesting question that comes into the picture: why does this happen? Let’s find out.

What Causes Surface Tension

Surface tension is mainly caused by cohesive forces, which are the attractive forces between the molecules of the same liquid. These forces include intermolecular attractions such as hydrogen bonding and Van der Waals forces.

Balanced Forces Inside the Liquid

A molecule present deep inside the liquid is surrounded by other molecules on all sides. It is pulled equally in every direction, so the net force acting on it becomes zero.

Unbalanced Forces at the Surface

The molecules present on the surface do not have similar molecules above them. They are pulled only by the molecules beside and below them. As a result, they experience a net inward force.

This inward pull makes the surface molecules come closer to each other and reduces the surface area of the liquid.

The "Elastic Skin" Effect

Because of the inward pull on the surface molecules, the liquid surface behaves like a stretched elastic membrane or a thin skin. This is why:

• Water droplets become spherical.
• A needle can float on water.
• Small insects can walk on the surface of water.
• Soap bubbles take a round shape.

So, in simple words, surface tension occurs because the molecules at the surface experience an unbalanced inward pull due to cohesive forces between liquid molecules.

Surface Tension Formula, Unit and Dimensional Formula

After learning what surface tension is and why it occurs, the next step is to express it mathematically. Scientists use formulas and units to measure the amount of surface tension present in different liquids.

Let’s first look at the surface tension formula, followed by its SI unit of surface tension and surface tension dimensional formula.

Surface Tension Formula

Mathematically, surface tension is defined as the force acting per unit length on the surface of a liquid.

 T=Fl
Where T is the surface tension of the liquid, F is the force acting on the liquid surface, and l is the length along which the force acts

Interestingly, surface tension can also be expressed as the work done per unit increase in the surface area of the liquid.

 T=WA
Where T is Surface tension, W is Work done, and A is Increase in surface area

This relation shows that energy is required to increase the surface area of a liquid because the liquid naturally tries to keep its surface area as small as possible.

SI and CGS Units of Surface Tension

Since surface tension is the ratio of force to length, its SI unit is, Newton per metre (N/m)

In the CGS system, the unit of surface tension is dyne per centimetre (dyn/cm)

Surface tension is usually represented by the symbols T or σ (sigma).

Surface Tension Dimensional Formula

The surface tension dimensional formula can be obtained using its formula:

 T=ForceLength
Since,

 Force=MLT−2
Therefore,

 T=MLT−2L T=MT−2

Hence, the surface tension dimensional formula is:
 [M1L0T−2]

Or we can simply write

 [MT−2]
Where M represents mass and T represents time.

The dimensional formula helps in checking the correctness of equations involving surface tension.

Surface Tension of Water and Other Liquids

The value of surface tension is not the same for all liquids. It depends on the strength of the intermolecular forces between their molecules. Liquids with stronger attractive forces generally have higher surface tension.

The table below shows the surface tension of water and some other liquids:

Liquid	Surface Tension (N/m)
Hydrogen	0.0024
Helium	0.00016
Water	0.072
Ethanol	0.022
Molten Sodium Chloride	0.114

Interestingly! The surface tension of water is comparatively high because water molecules are strongly attracted to each other through hydrogen bonding. This is why water droplets form nearly spherical shapes and can support small insects on their surface.

Now you may ask, why do different liquids have different surface tensions? The answer lies in the strength of the attractive forces between their molecules. The stronger the intermolecular forces, the greater the surface tension of the liquid.

Real-Life Examples of Surface Tension

Do you know? Surface tension can be seen almost everywhere around us.

• Insects Walking on Water: Water striders can stand and move on water because the surface acts like a stretched sheet.
• Floating Needle: A carefully placed needle can float due to the surface tension of water.
• Formation of Water Droplets: Raindrops and dewdrops become round because liquids try to reduce their surface area.
• Soap Cleaning Clothes: Soap lowers the surface tension of water, allowing it to spread easily and remove dirt.
• Soap Bubbles: Round soap bubbles are formed because surface tension pulls the liquid evenly in all directions.
• Waterproof Tents: Rainwater does not easily pass through some tent materials because of the surface tension of water.
• Mercury Droplets: Broken mercury forms shiny spherical drops because it has very high surface tension.

Many natural processes depend on this simple property of liquids.

Different Methods of Measuring Surface Tension

Over the years, several methods have been developed to determine the surface tension of liquids. The choice of method depends on the type of liquid, the amount of sample available, and the level of accuracy required.

The most commonly used methods of measuring surface tension are given below:

• Wilhelmy Plate Method: A thin platinum plate is partially dipped into the liquid. The force due to surface tension acting on the plate is measured using a sensitive balance. This method is highly accurate and widely used in laboratories.
• Du Noüy Ring Method: A platinum-iridium ring is placed on the liquid surface and slowly pulled upward. The maximum force required to detach the ring from the liquid surface is used to calculate the surface tension.
• Pendant Drop Method: In this method, a drop of liquid is suspended from a needle. The shape of the hanging drop is analyzed, and the surface tension is calculated from its profile.
• Spinning Drop Method: A liquid drop is placed inside another liquid in a rotating tube. The shape of the rotating drop helps determine the surface tension and interfacial tension.
• Drop Weight or Drop Count Method: Liquid drops are allowed to fall slowly from a capillary tube. The number or weight of the drops is measured and compared with a reference liquid to calculate surface tension.
• Capillary Rise Method: A thin glass tube is placed in a liquid, causing the liquid to rise inside the tube due to surface tension. By measuring the height of the liquid column, the surface tension can be determined.
• Maximum Bubble Pressure Method: Gas is passed through a capillary tube immersed in the liquid to form bubbles. The maximum pressure needed to form a bubble is used to calculate the surface tension.

Interestingly! The capillary rise method and the Du Noüy ring method are among the most commonly used techniques for measuring the surface tension of liquids in laboratories and industries.

In this article, we have seen that surface tension is a special property of liquids that makes their surface behave like a stretched film. We also studied its causes, formula, SI unit, dimensional formula, examples, and different methods used to measure it. Surface tension plays an important role in many natural phenomena and everyday activities around us.