2. CONTENTS
Surface Tension
Angle of contact
Determination theory of surface tension
Problems
SURFACE TENSION
2
3. DEFINITION:
The tension of the surface film of liquid
caused by the attraction of the particles in
the surface layer by the bulk of the liquid,
which tends to minimize surface area.
SURFACE TENSION
3
5. Surface tension is the
property of the free
Surface of a liquid at rest
behave like a stretched
membrane in order to
acquire minimum surface
area i.e contractive
tendency.
SURFACE TENSION
6. DEFINITION
Surface tension can be defined as the force
acting per unit length perpendicular on an
imaginary line drawn on the liquid surface,
tending to pull the surface apart along the
line.
If F is the force acting on the length l of the line
AB, then surface tension is given by,
Its unit is N m-1
Dimension is [MT-2 ]
SURFACE TENSION
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7. IMPORTANCE
1. Separation of oil and water is caused by a
tension in the surface between dissimilar liquids
(‘’Interface Tension’’).
2. A spider can walk on a surface of water but on
the ethanol it will drown. Why?
Because the surface
Tension of water is high
enough to ethanol.
SURFACE TENSION
8. 3. High surface tension of water
is also the reason for
spherical shape of rain drop.
Actually the surface tension
of water would be much
lower, nothing would really
float on top. Even the
smallest particles would sink
to the bottom which causes
the failure of the ecosystem.
SURFACE TENSION
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8
9. COHESIVE FORCE
Cohesive force is the force of attraction
between the molecules of the same
substance.
This cohesive force is very strong in solids,
Weak in liquids and extremely weak in
gases.
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9
SURFACE TENSION
10. ADHESIVE FORCE
Adhesive force is the force of attraction between
the molecules of two different substances.
For example: Fevicol, gum etc exhibit strong
adhesive property.
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10
SURFACE TENSION
11. Angle of Contact (AoC)
When the liquid is in contact
with solid, the angle
between the solid surface
and the tangent to the free
surface of the liquid at the
point of contact, measured
from inside the liquid is
called the angle of contact.
SURFACE TENSION
12. CHARACTERISTICS
1. The AoC is constant for a given liquid-
solid pair.
2. The AoC between the liquid and solid
surface is small (acute), the liquid is said
to wet the surface (Water-Glass)
3. if the AoC is larger, the surface is not
wetted. (Mercury-Glass)
SURFACE TENSION
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12
13. 4. If there are impurities in the liquid, then
they alter the values of the AoC
5. The AoC decreases with an increase in
temperature.
6. A liquid will completely wet the solid if the
AoC is zero.
SURFACE TENSION
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13
14. Let us consider a capillary tube. Due to surface
tension, water rises to a height ‘h’ in the
capillary tube .
The surface tension T of the water acts inwards
and the reaction of the tube R outwards where
R = T but opposite in direction.
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14
SURFACE TENSION
Surface tension of a liquid by using a capillary tube
16. The reaction R can be resolved into two rectangular
components.
(i) Horizontal component R sin θ acting radially
outwards
(ii) Vertical component R cos θ acting upwards
The horizontal components are canceled each other
whereas the vertical component balances the
weight of water column in the tube.
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SURFACE TENSION
17. Total upward force = R cos θ × circumference of
the tube
F = 2πr R cos θ
F = 2πr T cos θ ...(1) [∵ R = T ]
This upward force is responsible for the capillary
Rise which is equal to weight of the water
column acting downwards.
F = W ……………...(2)
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SURFACE TENSION
18. volume of water in the tube:
a. Volume of cylindrical water
column = πr2h
b. Volume of water in the meniscus =
(Volume of cylinder of height r and
radius r) – (Volume of hemisphere)
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SURFACE TENSION
20. Substituting (1) and (3) in (2)
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20
SURFACE TENSION
Since r is very small, 3/r can be neglected
compared to h
21. For water, θ is small, therefore cos θ ≈ 1
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SURFACE TENSION
22. Problem: A capillary tube of 0.05cm bore stands
vertically in a wide vessel containing a liquid of surface
tension 30 dyne/cm. The liquid wets the tube and has a
specific gravity 0.8. Calculate the rise of the liquid in the
tube.
Solution: Given that
Ɵ = 0
T = 3.061 cm r = d/2 = (0.05/2) c
= 0.025 cm
T = 30 Dyne/cm
ρ = 0.8
g = 980 cm/s2
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SURFACE TENSION
Cos
g
hr
T
2
23. Problem: Calculate the height to which a liquid will
rise in a capillary tube of radius 0.02 cm when surface
tension is 26x10-3 N/m and density 800 kg m-3. take
angle of contact is zero.
Solution: Given that
Ɵ = 0
r = 0.02 cm
= 0.0002 m
= 0.33 m T = 26x10-3 N/m
ρ = 800 kg m-3
g = 9.80 m/s2
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23
SURFACE TENSION
g
r
TCos
h
Cos
g
hr
T
2
2
24. Problem: A liquid of density 1.05 gm/cc and angle
of contact 20 ̊ has a vertical capillary tube of 2 mm
diameter dipping into it. If the surface tension is 235
dynes/cm, find the rise of the liquid in the capillary
tube.
Solution: Given that
Ɵ = 20
d = 2mm
r = (d/2)=1mm=0.1 cm
= 4.29 cm T = 235 dynes/cm
ρ = 1.05 gm/cc
g = 980 cm/s2
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SURFACE TENSION
g
r
TCos
h
Cos
g
hr
T
2
2
