Surface Tension

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, science, surface tension is used for either e ither 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 molecules are missing m issing neighbours neighbours (compared to interior molecules) 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 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 t he 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 sur face 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 intermolecular attractive forces. The conditions c onditions affecting surface tension are as follows:

y

Temperature

y

Solute concentration

y

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,  = surface surface tension, k = constant, V=Molar 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.

Variation of surface tension with temperature

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Effects of Solute Concentration: Concentration:

Solutes can have different effects on surface tension depending on their structure:

y

Little or no effect, for example sugar

y

Increase surface tension, inorganic salts

y

Decrease surface tension progressively. Alcohols, phenol etc.

y

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 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:

      











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       The height h to which capillary action will lift water depends upon the weight of  water which the surface tension will lift:



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 f orce:

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

Graph Showing variation of Surface Tension with Temperature

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COMMON EXAMPLES OF SURFACE TENSION 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 dynes/cm for water.

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

y

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.

y

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

y

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.

y

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.

y

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.

y

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



     

Where, r is the radius of capillary tube, h the capillary ascent  the density of water, g the acceleration due to gravity and

 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 Page | 16

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 8. Therefore =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 =

  

=





= 0.01mm

Table for Capillary Rise

Sl

Position of the cross

No. wire

Microscope Reading MSR

VSR

(mm)

Total=MSR+VSRXLC (mm)

Capillary Rise (Difference between the readings) (mm)

1

At the Meniscus Meniscus

43.5

14

43.64

2

At the tip of the

10.0

10

10.1

33.54

Pointer

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Table for the inner diameter of the capillary tube

Sl No. Measurement

1

2

Vertical Inner Diameter

Horizontal inner Diameter

Microscope Microscope Reading Position of  the cross wire

(i)Upper edge of the tube (ii)At the tip of the Pointer (i)Left Edge of  the tube (ii)Right Edge of the tube

MSR (mm)

VSR

119

39

Total=MSR+VSRXLC (mm)

Capillary Rise (Difference between the readings) (mm)

119.39 1.01

120

40

120.40

1

15

1.15 0.85

0

30

0.30

-3

Mean Diameter = 0.93mm = 0.93x10 m

Result:

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

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

   

= 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 bubbles and the rise of water in a capillary tube.

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BIBLIOGRAPHY Books

y

Concepts of Physics 1 by H.C.Verma

y

Mechanics 2 by D.C.Pandey

y

NOOTAN ISC Physics for class XI

y

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

Websites

y

en.wikipedia.org

y

www.britannica.com

y

www.tutornext.com

y

hyperphysics.gsu.edu

y

www.tutorvista.com

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