INTRODUCTION

Surface tension is a property of the surface of a liquid that allows it to resist an

external force. It causes liquid surfaces to behave as stretched elastic membranes. This

property is caused by cohesion of like molecules, and is responsible for many of the behaviors

of liquids. Its strength depends on the forces of attraction among the particles of the liquid

itself and with the particles of the gas, solid, or liquid with which it comes in contact. The

surface tension is very much visible to us in our everyday life, for instance in floating of some

objects on the surface of water, even though they are denser than water, and in the ability of

some insects (e.g. water striders) and even reptiles (basilisk) to run on the water surface. The

spherical shape of the liquid drops is also due to surface tension. Quantitatively, surface tension

is defined as the force acting normally per unit length of a line drawn on the surface of the

liquid. Surface tension has the dimension of force per unit length or of energy per unit area. The

two are equivalent—but when referring to energy per unit of area the term surface energy is

used which is a more general term in the sense that it applies also to solids and not just liquids.

In materials science, surface tension is used for either surface stress or surface free energy

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CAUSE OF SURFACE TENSION

The cohesive forces among the liquid molecules are responsible for this

phenomenon of surface tension. In the bulk of the liquid, each molecule is pulled equally in

every direction by neighbouring liquid molecules, resulting in a net force of zero. The molecules

at the surface do not have other molecules on all sides of them and therefore are pulled

inwards. This creates some internal pressure and forces liquid surfaces to contract to the

minimal area.

Surface tension is responsible for the shape of liquid droplets. Although easily

deformed, droplets of water tend to be pulled into a spherical shape by the cohesive forces of

the surface layer. In the absence of other forces, including gravity, drops of virtually all liquids

would be perfectly spherical. The spherical shape minimizes the necessary "wall tension" of the

surface layer according to Laplace's law.

Another way to view it is in terms of energy. A molecule in contact with a

neighbour is in a lower state of energy than if it were alone (not in contact with a neighbour).

The interior molecules have as many neighbours as they can possibly have, but the boundary

molecules are missing neighbours (compared to interior molecules) and therefore have a higher

energy. For the liquid to minimize its energy state, the number of higher energy boundary

molecules must be minimized. The minimized quantity of boundary molecules results in a

minimized surface area.

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The principle behind the phenomenon can be explained using the basic

molecular theory as follows.

Liquids, according to the Molecular theory, are made up of molecules. Let KLMN

represent a surface film of thickness LM, which is same as the molecular range. Consider three

molecules A, B, C at different positions. The molecule A experiences force of attraction equally

in all directions, due to its neighbouring molecules. The solid circle represents its sphere of

influence (whose radius is equal to the molecular range). Therefore, the net force acting on A is

zero. Consider the molecule at B (till below the surface). Like A, even B experiences a force of

attraction due to its neighbouring molecules. But unlike A, B is not pulled equally on all sides

and experiences a net pull downward. This is because it experiences more attraction due to

number of molecules inside the liquid. Coming to molecule 'C', we find that it experiences a

greater downward pull because it is attracted by even lesser numbers of molecules. The

downward force or pull experienced by molecules B and C is called the force of cohesion. In

other words, the force of cohesion represents the attractive force of two similar molecules.

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If one has to bring a molecule like A to the surface KL, then work has to be done

against this force of cohesion. Therefore, this work done is stored as potential energy of the

molecule. This means that the surface film has potential energy. Greater the number of molecules

on the surface, greater is the potential energy of the film. We know that every system in the

universe tends to acquire a minimum potential energy. In order to attain stable equilibrium, the

surface film also tends to have minimum P.E. and so, the number of molecules in the surface

film is minimum. Since the thickness of the film (LM or KN) is fixed, the surface area has to

minimum in order to acquire minimum volume. In an attempt to minimize the surface area, the

film contracts and acts like a stretched membrane.

Fig.3 Direction of Surface Tension

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FACTORS AFFECTING SURFACE TENSION

Surface tension varies from liquid to liquid and also with the change in the

conditions available. Primarily surface tension of a liquid is governed by the strength of

intermolecular attractive forces. Therefore, the magnitude of surface tension is a measure of

intermolecular attractive forces. The conditions affecting surface tension are as follows:

Temperature

Solute concentration

Presence of Contaminants

Effect of Temperature:

Surface tension is dependent on temperature. For that reason, when a value is

given for the surface tension of an interface, temperature must be explicitly stated. Surface

tension decreases with rise in temperature, almost linearly. The decrease of surface tension

with increase in temperature results because the kinetic energy (or speeds) of the molecules

increases. Thus, the strength of intermolecular forces decreases resulting in the decrease of

surface tension also. For example, clothes are washed more efficiently in hot water than in cold

water due to decreased surface tension in hot water.

The surface tension of all substances reduces to zero at a particular temperature known

as the critical temperature which is intrinsic to each of the substance.

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There are only empirical relations connecting surface tension and temperature.

The most accurate among them is the Eotvas equation. According to Eotvas the effect of

temperature on surface tension is given by the equation.

Where, γ = surface tension, k = constant, V=Molar volume of the substance, Tc =

critical temperature and T= temperature

As 'T' approaches critical temperature, the surface tension becomes zero. At this

stage the meniscus between the liquid and vapour disappears.

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Variation of surface tension with temperature

Effects of Solute Concentration:

Solutes can have different effects on surface tension depending on their

structure:

Little or no effect, for example sugar

Increase surface tension, inorganic salts

Decrease surface tension progressively. Alcohols, phenol etc.

Decrease surface tension and, once a minimum is reached, no more effect: surfactants

like detergents

What complicates the effect is that a solute can exist in a different concentration at the surface

of a solvent than in its bulk. This difference varies from one solute/solvent combination to another.

Effect of Contamination:

The presence of dust, oil or grease on the surface of water, reduces the surface

tension of water. Impurities affect surface tension appreciably. It is observed that impurities,

which tend to concentrate on the surface of liquids, compared to its bulk lower the surface

tension.

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EFFECTS OF SURFACE TENSION

Capillary Rise in a Vertical Tube

Capillary action is the result of adhesion and surface tension. Adhesion of water

to the walls of a vessel will cause an upward force on the liquid at the edges and result in a

meniscus which turns upward. The surface tension acts to hold the surface intact, so instead of

just the edges moving upward, the whole liquid surface is dragged upward. The height to which

water rises decreases with increase in the radius of the capillary tube.

Capillary action occurs when the adhesion to the walls is stronger than the

cohesive forces between the liquid molecules. The height to which capillary action will take

water in a uniform circular tube is limited by surface tension. Acting around the circumference,

the upward force is:

Fupward = T2πrWhere, T=Surface Tension;

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r = the radius of capillary tube. The height h to which capillary action will lift water depends upon the weight of

water which the surface tension will lift:

T2πr = ρg(hπr2)The height to which the liquid can be lifted is given by

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Pressure inside a Bubble

The surface tension of water provides the necessary wall tension for the

formation of bubbles with water and for the shape of liquid droplets. Although easily deformed,

droplets of water tend to be pulled into a spherical shape by the cohesive forces of the surface

layer. The spherical shape minimizes then necessary "wall tension" of the surface layer

according to Laplace’s law.

The pressure difference between the inside and outside of a bubble depends

upon the surface tension and the radius of the bubble. The relationship can be obtained by

visualizing the bubble as two hemispheres and noting that the internal pressure which tends to

push the hemispheres apart is counteracted by the surface tension acting around the

circumference of the circle. For a bubble with two surfaces providing tension, the pressure

relationship is:

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The net upward force on the top hemisphere of the bubble is just the pressure

difference times the area of the equatorial circle:

The surface tension force downward around circle is twice the surface tension

times the circumference, since two surfaces contribute to the force:

This gives

This latter case also applies to the case of a bubble surrounded by a liquid, such

as the case of the alveoli of the lungs.

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SURFACE TENSION OF WATER

Water is one of the liquids exhibiting great surface tension. The surface tension

of water is 71.97 dynes/cm at 25°C. It would take a force of 72 dynes to break a surface film of

water 1 cm long. The surface tension of water decreases significantly with temperature as

shown in the graph. The surface tension arises from the polar nature of the water molecule.

Soaps and detergents further lower the surface tension. Critical temperature of water is 374 °C

or 647K.

Temperature Surface Tension

0 75.64

25 71.97

50 67.91

100 58.85

COMMON EXAMPLES OF SURFACE TENSIONPage | 12

Graph Showing variation of Surface Tension with Temperature

Cleansing Action of Detergents

Detergents and soaps are used for cleaning because pure water can't remove

oily, organic soiling. Soap cleans by acting as an emulsifier. Basically, soap allows oil and water

to mix so that oily grime can be removed during rinsing. Detergents are primarily surfactants,

which could be produced easily from petrochemicals. Surfactants lower the surface tension of

water, essentially making it 'wetter' so that it is less likely to stick to itself and more likely to

interact with oil and grease.

Washing with cold water

The major reason for using hot water for washing is that its surface tension is

lower and it is a better wetting agent. But if the detergent lowers the surface tension, the

heating may be unnecessary.

Surface tension disinfectants

Disinfectants are usually solutions of low surface tension. This allows them to

spread out on the cell walls of bacteria and disrupt them. One such disinfectant, S.T.37, has a

name which points to its low surface tension compared to the 72.8 dynes/cm for water.

Clinical test for jaundice

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Normal urine has a surface tension of about 66 dynes/cm but if bile is present (a

test for jaundice), it drops to about 55. In the Hay test, powdered sulfur is sprinkled on the

urine surface. It will float on normal urine, but sink if the S.T. is lowered by the bile.

Shape of Liquid Droplets

Surface tension is responsible for the shape of liquid droplets. Although easily

deformed, droplets of water tend to be pulled into a spherical shape by the cohesive forces of

the surface layer. A water droplet can act as lens and form an image as a simple magnifier. The

relatively high surface tension of water accounts for the ease with which it can be nebulized, or

placed into aerosol form.

Floating of needle on water

If carefully placed on the surface, a small needle can be made to float on the

surface of water even though it is several times as dense as water. If the surface is agitated to

break up the surface tension, then needle will quickly sink.

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METHODS OF MEASURING SURFACE TENSION

Because surface tension manifests itself in various effects, it offers a number of

paths to its measurement. Which method is optimal depends upon the nature of the liquid

being measured, the conditions under which its tension is to be measured, and the stability of

its surface when it is deformed

Capillary rise method : The end of a capillary is immersed into the solution. The height at

which the solution reaches inside the capillary is related to the surface tension by the

equation discussed below.

Stalagmometric method : A method of weighting and reading a drop of liquid.

Wilhelmy plate method : A universal method especially suited to check surface tension

over long time intervals. A vertical plate of known perimeter is attached to a balance,

and the force due to wetting is measured.

Spinning drop method : This technique is ideal for measuring low interfacial tensions.

The diameter of a drop within a heavy phase is measured while both are rotated.

Pendant drop method : Surface and interfacial tension can be measured by this

technique, even at elevated temperatures and pressures. Geometry of a drop is

analyzed optically. For details, see Drop.

Sessile drop method : A method for determining surface tension and density by placing a

drop on a substrate and measuring the contact angle (see Sessile drop technique).

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EXPERIMENT TO DETERMINE THE SURFACE TENSION OF WATER BY CAPILLARY RISE METHOD

Aim: To determine the surface tension of water by capillary rise method

Apparatus: Capillary tube, needle, a beaker of clean water, travelling microscope.

Theory:

A capillary tube, open at both ends when dipped vertically in a liquid the liquid

level rises in the tube due to surface tension. Let h be the capillary ascent of liquid in the tube

and ρ the density of the liquid. The surface tension is given by the formula:

T= r (h+r ¿¿3) ρ g2cosθ

¿

Where, r is the radius of capillary tube,

h the capillary ascent

ρ the density of water,

g the acceleration due to gravity and

θ is the angle of contact

Procedure:

Place the adjustable height stand on the table and make its base horizontal by

leveling screws. Place a beaker containing clean water on the stand. Find the least count of the

travelling microscope for the horizontal and vertical scale. Raise the microscope to a suitable

height, keeping its axis horizontal and pointed towards the capillary tube. Make the horizontal

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cross wire just touch the central part of the central part of the concave meniscus (seen convex

through the microscope as in Fig. 1). Note the reading of the microscope on the vertical scale.

Now lower the travelling microscope so that the horizontal cross-wire coincides with the tip of

the pointer. Note down the reading. The difference in their readings gives the capillary rise in

the tube.

Fig.1 Water Meniscus through microscope Fig.2 Measurement of inner diameter

To measure the inner diameter of the capillary tube, place the tube horizontally

on the stand. Focus the microscope on the end of the tube which was earlier dipped in water. A

white circle (the inner bore) surrounded by a green strip (glass cross-section) will be seen as

shown in Fig.2. Make the horizontal cross wire touch the inner circle at A. Note the microscopic

reading on the vertical scale. Now lower the microscope so that the horizontal crosswire

touches the inner circle at B. Again note down the reading. The difference of these values gives

the vertical inner diameter of the capillary tube. Now move the microscope on the horizontal

scale and make the vertical cross wire touch the inner circle at C. Move it to the right to make

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the vertical crosswire touch the circle at D. Note the reading. The difference of the readings

gives the horizontal inner diameter of the capillary tube.

The angle of contact of water in glass is 8⁰. Therefore cosθ=0.99027 ≈ 1

The following Observations are recorded:

Preliminary Observations for the Travelling Microscope

Value of one division on the main scale = 0.5 mm

No: of divisions on the Vernier Scale = 50

Least Count = Valueof 1division onthemain scaleNo .of divisions on themain scale =0.5

50 = 0.01mm

Table for Capillary Rise

Sl

No.

Position of the cross

wire

Microscope Reading Capillary Rise

(Difference between

the readings)

(mm)

MSR

(mm)

VSR Total=MSR+VSRXLC

(mm)

1 At the Meniscus 43.5 14 43.64

33.54 2 At the tip of the

Pointer

10.0 10 10.1

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Table for the inner diameter of the capillary tube

Sl No.

Measurement Position of the cross wire

Microscope Reading Capillary Rise(Difference

between the readings)

(mm)

MSR (mm)

VSR Total=MSR+VSRXLC (mm)

1

Vertical Inner Diameter

(i)Upper edge of the tube

119 39 119.391.01

(ii)At the tip of the Pointer

120 40 120.40

2 Horizontal inner Diameter

(i)Left Edge of the tube

1 15 1.15 0.85

(ii)Right Edge of the tube

0 30 0.30

Mean Diameter = 0.93mm = 0.93x10-3m

Result:

1. Capillary Rise (h) = 30.4 mm = 30.4x10-3m

2. Radius of Capillary Tube (r)=d/2= 0.47mm=0.47x10-3m

3. Surface Tension of Water = r (h+r¿¿3) ρ g

2 cosθ¿ = 7.18x10-2 N/m

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CONCLUSION

Physics, the science of matter and its motion, space-time and energy is relevant

in each of our activities. Everything surrounding to us is made of matter and Physics explains

matter as combinations of fundamental particles which are interacting through fundamental

forces. It will not be an exaggeration if it is said that Nature is almost Physics (in fact the word

Physics itself is derived from the Greek word physis meaning nature). Physics is all around us.

The importance of physics to society today is most easily represented by our reliance on

technology. Surface tension is just one of the innumerous physical phenomena that influence

our day to day life. Even a concept wise small effect like it influences our life to an unimaginable

extent. The application of it ranges widely from the washing action of detergents to surface

travel of water striders. The phenomenon is also evident in the spherical shape of liquid

droplets, bubbles and the rise of water in a capillary tube.

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BIBLIOGRAPHY

Books

Concepts of Physics 1 by H.C.Verma

Mechanics 2 by D.C.Pandey

NOOTAN ISC Physics for class XI

ISC Physics Practical for class XI by K.K.Mohindro Pitambar

Websites

en.wikipedia.org

www.britannica.com

www.tutornext.com

hyperphysics.gsu.edu

www.tutorvista.com

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