Surface Tension for Engineering Physics.pptx

Surface Tension
for
Basic Physics
1st
Semester Diploma Course
Streams: DETCE, DEE
(Lecture 2)
Course Coordinator
Mrs. Soumi Patra
Assistant Professor of Physics
Guru Nanak Institute of Technology(GNIT),Kolkata

Table of Contents
 Introduction
 Concept of Cohesive and Adhesive force
 Effect of Surface Tension
 Surface Tension
 Molecular theory of Surface tension

Introduction
 Surface tension is the elastic tendency of a fluid surface
which makes it acquire the least surface area possible.
 Surface tension allows insects (e.g. water striders), usually
denser than water, to float and stride on a water surface.
 At liquid–air interfaces, surface tension results from the
greater attraction of liquid molecules to each other (due to
cohesion) than to the molecules in the air (due to adhesion).
 The net effect is an inward force at its surface that causes
the liquid to behave as if its surface were covered with a
stretched elastic membrane. Thus, the surface becomes
under tension from the imbalanced forces, which is
probably where the term "surface tension" came from

Concept of Cohesive and Adhesive force
• Cohesion is the property of like molecules
(of the same) to stick to each other due to
substance mutual attraction.
• Adhesion is the property of different
molecules or surfaces to cling to each other.
• When the cohesive force of the liquid is
stronger than the adhesive force of the
liquid to the wall, the liquid concaves down
in order to reduce contact with the surface
of the all.
• When the adhesive force of the liquid to
the wall is stronger than the cohesive force
of the liquid, the liquid is more attracted to
the wall than its neighbors, causing the
upward concavity.
Concave up Meniscus
Concave down Meniscus

Effect of Surface Tension
Water strider walking
on the water surface
Water beading on a leaf
Paper clip floats on water

Surface Tension
• The property of a liquid due to which its free surface tries to have minimum
surface area and behaves as if it were under tension some what like a
stretched elastic membrane is called surface tension
• Surface tension of a liquid is measured by the
force acting per unit length, the direction of this
force being perpendicular to the line and tangential
to the free surface of liquid. So if F is the force
acting on one side of imaginary line of length L,
then T = (F/L)
• It depends only on the nature of liquid and is independent of the area of
surface.
• It is a scalar as it has a unique direction which is not to be specified.
• Dimension : [MT – 2
]. (Similar to force constant)
• Units : N/m (S.I.) and Dyne/cm [C.G.S.]
• It is a molecular phenomenon.

Molecular Theory of Surface Tension
 The maximum distance up to which the force of attraction between two
molecules is appreciable is called molecular range . A sphere with a molecule
as centre and radius equal to molecular range is called the sphere of
influence. The liquid enclosed between free surface (PQ) of the liquid and an
imaginary plane (RS) at a distance r (equal to molecular range) from the free
surface of the liquid form a liquid film. To understand the tension acting on
the free surface of a liquid, let us consider four liquid molecules like A, B, C
and D. Their sphere of influence are shown in the figure.

Examples of Surface Tension
(1) When mercury is split on a clean glass plate, it forms globules. Tiny
globules are spherical on the account of surface tension because force of
gravity is negligible. The bigger globules get flattened from the middle but
have round shape near the edges, figure
(2) When a greased iron needle is placed gently on the surface of water at rest,
so that it does not prick the water surface, the needle floats on the surface of
water despite it being heavier because the weight of needle is balanced by the
vertical components of the forces of surface tension. If the water surface is
pricked by one end of the needle, the needle sinks down
.

Table of Contents
 Factors affecting Surface Tension
 Applications of Surface Tension
 Surface Energy
 Angle of Contact
 Capillarity and Capillary Action
 Shape of Liquid Meniscus
 Question and Answer

Factors affecting Surface Tension
 Impurities: The presence of impurities in a liquid appreciably
affect surface tension.
 Temperature: The surface tension decreases with rise in
temperature. The temperature at which the surface tension of a
liquid becomes zero is called critical temperature of the liquid. The
variation in surface tension with temperature is linear and is given
by the relation
Tt = T0(1-αt)
where Tt , T0 are the surface tensions at t 0
C and 0 0
C respectively
and α is the temperature coefficient of surface tension.
Examples:
(i) Hot soup tastes better than the cold soup.
(ii) Machinery parts get jammed in winter.

Applications of Surface Tension
 The oil, grease and dirt particles get mixed with soap solution
easily. Hence clothes are washed easily.
 Due to low surface tension, the antiseptics spreads properly over
wound.
 Surface tension of all lubricating oils and paints is kept low so that
they spread over a large area.
 Oil spreads over the surface of water because the surface tension of
oil is less than the surface tension of cold water.
 A rough sea can be calmed by pouring oil on its surface.
 In soldering, addition of ‘flux’ reduces the surface tension of molten
tin, hence, it spreads.

Surface Energy
 The molecules on the liquid surface experience net downward force. So
to bring a molecule from the interior of the liquid to the free surface,
some work is required to be done against the intermolecular force of
attraction, which will be stored as potential energy of the molecule on
the surface. The potential energy of surface molecules per unit area of
the surface is called surface energy.
 Unit : Joule/m2 (S.I.) erg/cm2 (C.G.S.) Dimension : [MT–2
]
Work Done in Blowing a Liquid Drop or Soap Bubble.
(1) If the initial radius of liquid drop is r1 and final radius of liquid drop is
r2 then W = T x Increment in surface area
W = T x 4 π[r2
2
– r1
2
] [drop has only one free surface]
(2) In case of soap bubble
W = T x 8 π[r2
2
– r1
2
] [Bubble has two free surfaces]

Angle of Contact
 When a liquid is in contact with a solid, the angle between tangent drawn
to the free surface of the liquid and the surface of solid at the point of
contact measured inside the liquid is called angle of contact.
Characteristics:
i. The angle of contact is constant for a given liquid-solid pair.
ii. The value of angle of contact depends upon nature of liquid and solid in
contact.
iii. It depends upon the medium which exists above the free liquid surface.
iv. The angle of contact changes due to impurity or temperature.

Capillarity and capillary action
 The phenomenon of rise or fall of liquid level inside a capillary tube when it is
dipped in the liquid is called capillary action or capillarity.
 Example: Blood capillaries, vascular bundles in plants tissue etc.
 If a capillary tube is dipped in a liquid which partially or completely wets the
solid then the liquid level rises inside the tube.
 If a capillary tube is dipped in a liquid which does not wet the solid, then the
liquid level falls inside the capillary tube.
Applications of capillarity:
i) Blotting paper contains small pores. These pores act as capillaries and hence
quickly absorb ink.
ii) Wick in oil lamp contains threads, which act as capillaries and oil rises up in
the wick of the lamp.
iii) Sap and water rise upto the topmost leaves in the tree by capillary action.
iv) Towel used in every day life is made up of cotton. It absorbs water or moisture
from the body due to capillarity of towel.
v) Piece of sponge retains water, due to capillarity.

Shape of Liquid Meniscus
 When a capillary tube is dipped in a liquid, the liquid surface becomes
curved near the point of contact. This curved surface is due to the resultant of
two forces i.e. the force of cohesion and the force of adhesion. The curved
surface of the liquid is called meniscus of the liquid. If liquid molecule A is
in contact with solid (i.e. wall of capillary tube) then forces acting on
molecule A are
 (i) Force of adhesion Fa (acts outwards at right angle to the wall of the tube).
 (ii) Force of cohesion Fc (acts at an angle 45degree to the vertical).
 Resultant force FN depends upon the value of Fa and Fc.
Water in silver
coated capillary tube
Water in glass
capillary tube
Mercury in glass
capillary tube

Question and Answer
1. Explain why oil spreads over the surface of water while water does not
do so?
i. The surface tension of oil is less than the surface tension of water
ii. Thus, when oil is dropped on the surface of water, it spreads in all
directions due to the higher force of surface tension of water.
2. How are small insects able to walk on the water surface?
i. The free surface of water behaves as stretched membrane due to
surface tension.
ii. Due to weight of insect, the membrane is stretched. Thus surface
tension acts in an inclined manner
iii. The vertical component of surface tension balances the weight of the
insect.

Question and Answer
3. What is the effect of temperature on angle of contact?
As temperature increases, surface tension decreases thus angle of
contact decreases.
4. The surface tension of water at 0 0
C is 70 dyne/cm. Find surface
tension of water at 25 0
C. [α for water = 0.0027/ 0
C]
Given: T0 = 70 dyne/cm, t1 = 0 0
C, t2 = 25 0
C, α= 0.0027 /0
C
To find: Surface tension (T)
Formula: T = T0[1 – α(t2 -t1)] =70[1-0.0027(25-0)] = 65.275 dyne/cm
5. Why Rain drops are spherical?
Due to surface tension, the liquid tries to acquire minimum surface
area. For a given volume, sphere has minimum surface area,
therefore the liquid assumes the shape of a drop.

Question and Answer
6. Oil spreads on cold water but remains as a drop on hot water?
It is due to the fact that surface tension of oil is less than that of the
cold water but is greater than that of the hot water.
7. Calculate the force required to take away a flat circular plate of
radius 0.01 m from the surface of water. The surface tension of water
is 0.075 N/m.
Solution: Given: r = 0.01 m, T = 0.075 N/m
Formula: F = T x l The plate is flat
Force due to surface tension acts on only one face
l = 2r , F = 0.075 (2 x 3.142 x 0.01)
F = 0.004717 N

Question and Answer
8.A horizontal circular loop of wire of radius 0.02 m is lowered into
crude oil and forms a film. The force due to surface tension of the
liquid is 0.0113 N. Calculate the surface tension of crude oil.
Solution: Given: r = 0.02 m, F = 0.0113 N
Formula: F = Tl
As there are two sides of a circular loop acted upon by the force,
l = 2 x 2πr.
From formula, F = 4πrT
=> T = F / 4πr
=> T = 0.0113 / 4 x 3.142 x 0.02
= 0.04495 N/m

Question and Answer
9. A soap bubble of radius 12 cm is blown. Surface tension of soap
solution is 30 dyne/cm. Calculate the work done in blowing the soap
bubble.
Solution: Given: r = 12 cm, T = 30 dyne/cm
Formula: W = T x ∆A
Initial surface area of soap bubble = 0
Final surface area A = 2 x 4πr2
Increase in surface area ∆A = 2 x 4 πr2
From formula, W = T x 8 π r2
= 30 x 8 x 3.14 x (12)2
= 108518.4 erg
= 108518.4 x 10-7
J

Surface Tension for Engineering Physics.pptx

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