Surface Tension- Its Measurement, Force of Cohesion and Adhesion

Consider there are two glass slabs with you. Both are wet and stuck to each other. Now if you try to separate them it is very hard. You will have to apply a large amount of force to separate them. It feels like these slabs are glued together because of surface tension. So in this article we will be studying about surface tension in detail. 

Table of Contents

• Surface Tension
• Causes of Surface Tension
• Force of Cohesion 
• Force of Adhesion
• Measurement of Surface Tension
• Effect of Impurity and Temperature on Surface Tension 
• Practice Problems
• FAQs

Surface Tension

The property of a liquid at rest by virtue of which its free surface behaves like a stretched membrane under tension and tries to occupy as small an area as possible is known as surface tension.It is present only on the free surface of the liquid because of the unbalanced or asymmetric force on the liquid molecules residing at the surface as shown in the figure below.

Consider an imaginary line AB of length L on the free liquid surface. The liquid at the right part of that imaginary line pulls the liquid on the left part by a tensile force F and vice-versa as shown in the figure below. 

Mathematically, surface tension (S) is defined as tensile force (F) per unit length (L),

Note: 

• Surface tension is a scalar quantity.
• SI unit of surface tension is and dimension is .

Causes of Surface Tension

The free surface of the liquid acts like a stretched membrane, i.e. the surface of a liquid is in a state of tension as the surface of an inflated balloon. 

(Recreated)

Surface tension is due to intermolecular attraction. In the volume of liquid, every molecule such as A, as shown in figure, is surrounded by an equal number of molecules on all sides. Therefore, no net force acts on this molecule. On the other hand, a molecule on the surface of liquid such as B is surrounded with very molecules on the vapour side as compared with the liquid below. Thus, it experiences a net force F in downward direction as shown in the figure. If one pulls the molecule B upwards, then one has to work against the intermolecular attraction. Therefore, molecules on the free surface of a liquid have potential energy. If molecule A is shifted to the position of the molecule B, then external work has to be done in breaking the bonds because at position A molecule forms more bonds with the neighbours as compared to the position of molecule B. Thus, molecules on the surface of liquid have more potential energy as compared to the molecules in the body of liquid. Under the action of surface tension forces, the free surface of a liquid tends to have the least area for a given volume of liquid in order to minimise its potential energy. Since the sphere has a minimum area for a given volume, therefore raindrops and small droplets of mercury are approximately spherical. 

Note:

• The magnitude of surface tension depends on the temperature of the liquid and on the medium on the other side of the free surface. The surface tension decreases by increasing the temperature. 
• The surface tension of liquid decreases by adding impurities.
• At , the surface tension of water in contact with air is . At , the surface of mercury in contact with air is .

Force of Cohesion 

Cohesion is the force of attraction between molecules of the same material. In the case of the solids, the force of cohesion is very large and due to this solids have definite shape and size. On the other hand, the force of cohesion is weaker in liquids as compared to solids. Hence liquid does not have definite shape but has definite volume. In the case of gases, the cohesive force is insignificant. Because of this, gases have neither definite shape nor definite volume. 

Example:

Because of the cohesive force, two droplets of liquid coalesce into one when they come into contact.

Force of Adhesion

The force of attraction between the molecules of different substances is called adhesion. Electrostatic forces are exerted on various substances, and adhesion forces might be one of the outcomes.

Example:

Adhesion force helps us to write on paper with ink. 

Measurement of Surface Tension

Suspend a rectangular glass plate from one arm of a sensitive balance. Put some weight on the other pan so as to balance the glass plate.

(Recreated)

A beaker containing some liquid is placed below the glass plate. The beaker is raised gradually till the liquid just touches the glass plate and pulls it down a little because of surface tension. Weights are added on the other side till the glass plate just clears the liquid surface. If the weight required to do so is W, the surface tension of liquid-air interface is given by,

: length of the plate edge

, here m is the mass added to the pan.

Effect of Impurity and Temperature on Surface Tension 

Highly soluble impurities like sodium chloride and sugar in water increases the surface tension. But sparingly soluble substances like phenol, detergent in water reduce the surface tension. 

When the temperature of a liquid rises, the surface tension of the liquid drops, and vice versa. According to formula,

Where, is surface tension at , is the absolute temperature of the liquid, is the critical temperature and is the constant varies slightly from liquid to liquid, has mean value of . This formula shows that surface tension becomes zero at the critical temperature, this is why machinery parts get jammed in winter. 

Practice Problems

Q. A cube of mass and edge length floats on the surface of water such that it wets completely. By what additional distance is it buoyed up or down by surface tension? Surface tension of water is .

A. If surface tension is neglected the condition of floating gives,

Here x is the distance by which a block is submerged in the water.

Now,

Therefore, additional distance is .

Q. A circular plate with a radius of 14 cm and a mass of 10 g is placed on the water's surface. Find a minimum pull required to clear the plate off the water surface?

A. Downward force on plate because of surface tension is,

Downward gravitational force on plate is,

Minimum pull force required is

Q. A ring is cut from a platinum of internal and external diameter. It is suspended horizontally from a balancing pan, allowing it to come into touch with the water in a glass vessel. What is the surface tension of water if an extra 3.87 g weight is required to pull it away from water?

(Recreated)

A. The ring is in contact with water along its inner and outer circumference; so when pulled out the total force on it due to surface tension will be,

Q. A small insect of about long is posed upon the liquid. The surface tension of the liquid is . Calculate the estimated mass of the insect?

A.

Weight of the insect is balanced using surface tension force, so

FAQs

Q. Can fluid have zero surface tension?
A. In the case of an ideal fluid, the intermolecular forces are assumed to be very less, that's why only ideal fluids have zero value surface tension.

Q. How can we reduce the surface tension of water?
A. We can reduce the surface tension of water by adding impurities such as detergent or by increasing the temperature of water. 

Q. Does surface tension of liquid depend on the area of the container?
A. No, surface tension does not depend on the area of the container as it is a property of liquid. 

Q. What is the cause of surface tension?
A. Surface tension is caused by the cohesive force which is the force of attraction between the molecules of the same substance.