This presentation covers concepts such as surface tension, surface energy, liquid drops and bubbles, wetting, capillarity at the elementary school level. Comment down in a box for improvement.

1. SURFACE
TENSION

2. QUESTION 1
Why water droplets and soap bubbles are spherical in shape?

3. How water spiders and water striders can easily walk on water
surface without sinking?
QUESTION 2

4. Despite being denser than water, how steel needle and
paperclip float on water?
QUESTION 3

5. Jumping Sheet
A n e x a m p l e o f s t r e t c h e d
e l a s t i c m e m b r a n e

6. The property by virtue of which the
free surface of a liquid behaves like
a stretched elastic membrane tending
to contract so as to occupy minimum
surface area.
Definition of
Surface
Tension

7. MATHEMATICAL DEFINITION
𝐹 ∝ 𝐿
𝐹𝐿
𝐹 = 𝑇 × 𝐿
𝑇 = 𝐹/𝐿 (𝑁/𝑚)
The force acting per unit length
of an imaginary line drawn on
the free surface.

8. COHESION & ADHESION
Cohesion is the force of attraction between the molecules of
the same substance.
Adhesion is the force of attraction between the molecules
of two different substances.
WATER
Air
BEAKER

9. EXAMPLES OF COHESION
1. Fixed shape and size of solids
2. Two liquid drops merge into one
3. Mercury does not wets the glass
EXAMPLES OF ADHESION
1. Ink sticks to the paper
2. Sunmica sticks to the plywood
3. Water wets the glass

10. MOLECULAR RANGE
Maximum distance upto which a molecule can exert a force of
attraction on other molecules.
For solids and liquids: R = 10 A°

11. SPHERE OF INFLUENCE
Imaginary sphere drawn around a molecule as centre and
molecular range as a radius
R

12. 10 A°
Surface film
𝑃
𝑃′
𝑄
𝑄′
𝐴
𝐵
𝐶
𝐷
MOLECULAR BASIS OF SURFACE TENSION

13. • PQ is the free surface of a liquid.
• P’Q’ is an imaginary plane at a distance equal to
molecular range and parallel to free surface.
• The liquid enclosed between PQ and P’Q’ form a
surface film.
• A molecule below the surface film is surrounded
by similar molecules from all sides. (A & B)
• Therefore, the net force acting on such a molecule
is zero.
• But a molecule within a surface film is surrounded
by air molecules from above and water molecules
from below. (C & D)
• Therefore, the net force is acting downward on
such a molecule.
• Due to the downward force, the molecules of
surface film accelerate towards bulk liquid.
• As a result, the density of surface film decreases,
and pressure becomes negative in that region.
• Negative pressure develops tension force in the
surface film.
• Due to which, free surface of liquid behaves like
stretched elastic membrane and shows property
of surface tension.

14. SURFACE ENERGY
Liquid
Air

15. • The interaction of a molecule with its nearest neighbors leads to a reduction of its
potential energy.
• A molecule at the surface region of a liquid has a smaller no of nearest neighbors.
• Therefore, the potential energy of surface molecules is not decreased as much as
the interior molecules.
• Clearly, the surface molecules possess extra potential energy as compared to the
molecules inside the liquid.
• The potential energy of surface molecules per unit area of the surface is called
surface energy.
𝑆𝐸 = 𝑃𝐸 𝐴 𝐽 𝑚2

16. SURFACE ENERGY

17. • Consider a liquid drop having some surface area.
• If we distort a liquid drop its surface area increases.
• This increase in surface area is caused by the rise of molecules
from the interior to the surface.
• As these molecules reach the surface film, work has to be done
against inward cohesive force.
• This work is stored as the potential energy of the molecules on
the surface.
𝑆𝐸 =
𝑊𝑜𝑟𝑘 𝑑𝑜𝑛𝑒
𝐼𝑛𝑐𝑟𝑒𝑎𝑠𝑒 𝑖𝑛 𝑠𝑢𝑟𝑓𝑎𝑐𝑒 𝑎𝑟𝑒𝑎

18. RELATION BETWEEN T & SE
𝑙
𝑥
𝐹𝑇
𝑇𝑙
𝑇𝑙
𝐹 = 2 × 𝑇𝑙
𝑇 =
𝑊
2𝑙𝑥
=
𝑊
∆𝐴
𝑊 = 𝐹 × 𝑥
𝑊 = 2 × 𝑇𝑙 × 𝑥
𝑊 = 𝑇 × 2𝑙𝑥
𝑇 = 𝑆𝐸

19. EFFECT OF TEMPERATURE ON SURFACE TENSION
Temperature Kinetic energy
Intermolecular
distance
Intermolecular
force
Surface tension
Increases Increases Increases Decreases Decreases
Decreases Decreases Decreases Increases Increases
 The variation of surface tension with temperature is given by
𝑇 = 𝑇𝑜(1 − 𝛼𝑡)
 The temperature at which surface tension of liquid becomes
zero is called the critical temperature of the liquid.

20. EFFECT OF IMPURITIES ON SURFACE TENSION
Type Impurity
Intermolecular
Force
Surface Tension
Soluble
Table Sugar Increases Increases
Alcohol & Phenol Decreases Decreases
Common Salt Increases Increases
Soaps & Detergents Decreases Decreases
Insoluble
Oil & Grease
(𝜌 < 𝜌 𝑤)
Decreases Decreases
Tar & Mercury
(𝜌 > 𝜌 𝑤)
Unaffected Unaffected

21. WATER
O
H H
δ−
δ+ δ+

22. TABLE SUGAR

23. ALCOHOL & PHENOL

24. COMMON SALT
Na+
Cl−
Na+
Cl−

25. SOAPS & DETERGENTS
Na+
Na+

26. REDUCTION OF SURFACE TENSION BY OIL

27. WETTING
Wetting refers to the study of how a liquid deposited on a solid substrate
spreads out.
Understanding of wetting enables us to explain why liquids spread
readily on some solids but not on others.

28. CONTACT LINE
The location where the three phases (solid, liquid and
air) meet.

29. 𝜃
CONTACT ANGLE
The angle between tangent to the liquid-air interface at contact line and
the solid-liquid interface.

30. YOUNG’S EQUATION
𝑇𝑠
𝑇𝑙
𝑇𝑠𝑙
𝑇𝑠 = 𝑇𝑠𝑙 + 𝑇𝑙 cos 𝜃
cos 𝜃 =
𝑇𝑠 − 𝑇𝑠𝑙
𝑇𝑙
𝜃

31. CASE-1 OF WETTING
cos 𝜃 =
𝑇𝑠 − 𝑇𝑠𝑙
𝑇𝑙
If 𝑇𝑠 > 𝑇𝑠𝑙 and 𝑇𝑠 − 𝑇𝑠𝑙 < 𝑇𝑙
0 < cos 𝜃 < 1
𝜃 is acute (𝜃 < 90°
)
𝜃
Wetting condition
(Normal water on glass)

32. CASE-2 OF WETTING
cos 𝜃 =
𝑇𝑠 − 𝑇𝑠𝑙
𝑇𝑙
If 𝑇𝑠 < 𝑇𝑠𝑙 and 𝑇𝑠 − 𝑇𝑠𝑙 < 𝑇𝑙
−1 < cos 𝜃 < 0
𝜃 is obtuse (𝜃 > 90°
)
𝜃
Non-wetting condition
(Normal water on wax)

33. CASE-3 OF WETTINGcos 𝜃 =
𝑇𝑠 − 𝑇𝑠𝑙
𝑇𝑙
If 𝑇𝑠 > 𝑇𝑠𝑙 and 𝑇𝑠 − 𝑇𝑠𝑙 > 𝑇𝑙
cos 𝜃 > 1
Contact angle is impossible and
liquid spread over the surface
Perfect-wetting condition
(Distilled water on clean glass)

34.  For given solid-liquid pair, the angle of contact
is constant.
 The value of angle of contact depends upon
nature of liquid and solid in contact.
 It depends upon the medium which exists
above the free surface of liquid.
 The angle of contact changes due to impurity.
 The angle of contact changes with temperature.
Characteristics of
Contact
Angle

35. APPLICATIONS OF WETTING

36. APPLICATIONS OF WETTING

37. APPLICATIONS OF WETTING

38. APPLICATIONS OF WETTING

39. APPLICATIONS OF WETTING

40. EXCESS PRESSURE INSIDE LIQUID DROP
𝑃𝑖 𝑃𝑜
𝑟 𝑟
∆𝑟
∆𝑃 = 𝑃𝑖 − 𝑃𝑜
𝐴1 = 4𝜋𝑟2
𝐴2 = 4𝜋 𝑟 + ∆𝑟 2
𝐴2 = 4𝜋 𝑟2
+ 2𝑟∆𝑟 + ∆𝑟2
𝐴2 = 4𝜋𝑟2
+ 8𝜋𝑟∆𝑟
∆𝐴 = 8𝜋𝑟∆𝑟
𝑊 = ∆𝑃 × 4𝜋𝑟2
× ∆𝑟
𝑊 = 𝑇 × 8𝜋𝑟∆𝑟
∆𝑃 × 4𝜋𝑟2
∆𝑟 = 𝑇 × 8𝜋𝑟∆𝑟
∆𝑃 × 𝑟 = 2𝑇
∆𝑃 =
2𝑇
𝑟

41. EXCESS PRESSURE INSIDE LIQUID DROP
𝑇 × 2𝜋𝑅 = ∆𝑃 × 𝜋𝑅2
𝑇 × 2 = ∆𝑃 × 𝑅
∆𝑃 =
2𝑇
𝑅

42. EXCESS PRESSURE INSIDE SOAP BUBBLE
𝑇 × 2(2𝜋𝑅) = ∆𝑃 × 𝜋𝑅2
𝑇 × 4 = ∆𝑃 × 𝑅
∆𝑃 =
4𝑇
𝑅

43. PRESSURE ACROSS FREE SURFACE
The pressure on concave side is greater than pressure on
convex side.

44. PRESSURE ACROSS SURFACE
𝑃𝐴
𝑃𝐵
𝑃𝐴
𝑃𝑩
𝑅
𝑅
∆𝑃 = 𝑃𝐴 − 𝑃𝐵 =
2𝑇
𝑅
∆𝑃 = 𝑃𝐵 − 𝑃𝐴 =
2𝑇
𝑅

45. Capillarity is the tendency of a liquid to flow in narrow spaces as a result
of interfacial tensions.
CAPILLARITY

46. Wa t e r M e r c u r y
LIQUID SURFACE NEAR THE CONTACT IS CURVED
(𝜃 < 90°
) (𝜃 > 90°
)
(Concave) (Convex)
𝜃
𝜃

47. CONCAVE SURFACE

48. CONVEX SURFACE

49. Capillary Tube
A g l a s s t u b e h a v i n g a
v e r y f i n e b o r e

50. RISE & FALL OF LIQUID IN A CAPILLARY TUBE
Wa t e r M e r c u r y

51. REASON FOR CAPILLARY ACTION
. ...
C A
BD
. ...
C A
BD
𝑃𝐴 = 𝑃𝐶 = 𝑃𝐵
𝑃 𝐷 < 𝑃𝐵
𝑃𝐴 = 𝑃𝐶 = 𝑃𝐵
𝑃 𝐷 > 𝑃𝐵

52. FORMULA FOR HEIGHT OF CAPILLARY ACTION
𝜃
𝑅
𝑟 𝐴
𝐵
ℎ
𝑃𝐴 + 𝜌𝑔ℎ = 𝑃𝐵
𝑃𝐴 = 𝑃𝑎𝑡𝑚 −
2𝑇
𝑅
𝑃𝐵 = 𝑃𝑎𝑡𝑚
𝜌𝑔ℎ + 𝑃𝑎𝑡𝑚 −
2𝑇
𝑅
= 𝑃𝑎𝑡𝑚
𝜌𝑔ℎ =
2𝑇
𝑅
⇒ ℎ =
2𝑇
𝜌𝑔𝑅
ℎ =
2𝑇 cos 𝜃
𝜌𝑔𝑟
&

53. JURIN’S
LAW
ℎ ∝
1
𝑟

54. WHO BEARS THE WEIGHT OF THE RAISED LIQUID?
𝑇𝑆
𝑇𝑆𝑙
𝑚𝑔
𝑇𝑆 × 2𝜋𝑟 − 𝑇𝑆𝑙 × 2𝜋𝑟 = 𝑚𝑔
(𝑇𝑆 − 𝑇𝑆𝑙) × 2𝜋𝑟 = 𝜌 × 𝜋𝑟2
ℎ × 𝑔
(𝑇𝑆 − 𝑇𝑆𝑙) × 2 = 𝜌𝑟ℎ𝑔
ℎ =
2(𝑇𝑆 − 𝑇𝑆𝑙)
𝜌𝑟𝑔
ℎ =
2𝑇𝑙 cos 𝜃
𝜌𝑟𝑔
𝑇𝑆 = 𝑇𝑆𝑙 + 𝑇𝑙 cos 𝜃

55. APPLICATIONS OF CAPILLARITY

56. APPLICATIONS OF CAPILLARITY

57. APPLICATIONS OF CAPILLARITY

58. THANK
YOU