This document discusses methods for determining surface tension, including the drop weight method. It begins by defining surface tension and explaining that it is caused by cohesive forces between liquid molecules being stronger than adhesive forces between liquid and air molecules. It then describes several methods to measure surface tension, including the capillary method, ring detachment method, and various drop methods. The drop weight method specifically measures surface tension based on the weight and radius of drops detached from a tube under constant conditions. Factors affecting surface tension and applications are also discussed.

1. DETERMINATION OF THE
SURFACE TENSION
COEFFICIENT BY THE DROP
DISCONTINUOUS METHOD
BY – ADITI BARMAN

2. WHAT IS SURFACE TENSION ?
• Surface tension is defined as the tension of the
surface film of a liquid caused by the attraction of
the particles in the surface layer by the bulk of
the liquid , which tends to minimize surface area .
• It is due to the phenomenon of surface tension
that the drops of water tend to assume a
spherical shape to attain minimum surface area .

3. • At liquid – air interfaces , surface tension results from the greater attraction of liquid molecules to each
other ( due to cohesion ) than to molecules in the air ( due to adhesion ) .
• Surface tension has the dimension of force per unit length or energy per unit area . The two are
equivalent , but when referring to energy per unit area , it is common to use the term surface energy ,
which is more general term in the sense that it applies also to solids .
• In material science, surface tension is used for either surface stress or surface free energy.
• SI unit of Surface Tension is (N/m) or (J/m²)
• Its dimension is [M⁰ L¹ T ‐ ² ]
• Angle of contact : The angle measured from the side of the liquid , between the tangent to the solid
surface inside the liquid and tangent to the free liquid surface at the point of contact between solid
and liquid surfaces .

4. HOW CAN WE EXPLAIN SURFACE TENSION ?
If we put some water in one tube and
some mercury in another one , the
water surface take a convex shape
and the Mercury surface rake a
concave shape .

5. EXAMPLES OF SURFACE TENSION
WALKING ON WATER :
Small insects such as
the water strider can
walk on water because
their weight is not
enough to penetrate
the surface
FLOATING NEEDLE : A carefully
placed 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
SURFACE TENSION AND
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

6. • The tension in the surface : It is the force per unit length that must be applied to the surface so
as to counterbalance the net inward pull . It has the units of dyne/cm as shown in the fig.
Fig : The tension in a surface

7. INTERFACIAL TENSION
• It is the force per unit length
existing at the interface between
two immiscible liquid phases
and has the unit of dynes/cm
• Ordinarily it is less than surface
tension because the adhesive
forces between liquid phases
forming an interface are greater
than when a liquid and a gas
phase exist together
• It follows that if two liquids are
completely miscible , no
interfacial tension exists
between them .
Fig : Interfacial tension of reactive, liquid interfaces
and its consequences.

8. Furthermore , there are two important terms related to forces :
• First, COHESIVE FORCES are the intermolecular forces which cause a tendency in liquids to resist
separation . These attractive forces exist between molecules of the same substance
WHILE
• Second, ADHESIVE FORCES are the attractive forces between unlike molecules. They are caused by
forces acting between two substances, such as mechanical forces ( sticking together) and electrostatic
forces ( attraction due to opposing charges).

9. FACTORS AFFECTING SURFACE TENSION
• Impurities present in a liquid appreciably affect surface tension . A highly soluble substance like salt
increases the surface tension whereas sparingly soluble substances like soap decreases the surface
tension .
• 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 .
• EFFECT OF TEMPERATURE
• Oxygen in the atmosphere is known to decrease the surface tension of various substances.
• With increase in Temperature, Surface Tension decreases .
• At critical temperature , Surface Tension is Zero.
• Critical temperature of water is 3744K.
• Surface tension increases with impurity.

10. Fig : Surface Tension versus Temperature

11. • Surface tension mainly depends upon the
force of attraction between the particles
within the given solid or liquid in contact with
it .
• Liquids with strong intermolecular forces
have higher surface tension than liquids with
weak intermolecular forces .
• Adding a surfactant to a coating or detergent
lowers the surface tension of the liquid so it
will flow more , covering the entirety of the
surface .
• Ripples are some kind of capillary wave which
is created by the stress on water . The motion
of a ripple is governed by surface tension .
Fig : surface tension in a water droplet.
Fig : Ripples

12. APPLICATION OF SURFACE TENSION
• Surface tension of soap solution is less , it can spread over large areas and wash clothes more
effectively, since the dirt particles stick to the soap molecules .
• In soldering , addition of flux reduces the surface tension of molten tin . Hence , it spreads .
• Antiseptics like dettol have low surface tension , so they spread faster .
• Surface Tension prevents water from passing through pores of an umbrella .
• A duck is able to float on water as its feathers secrete oil that lowers the surface tension of water .

13. METHODS TO MEASURE SURFACE TENSION
• Various methods can be used for determination of the surface and interface tensions .
• Some of these methods determine the surface tension only , and some are used for
determination of both surface and interface tensions
• The main methods used are
1. Capillary method
2. Wilhelmy plate method
3. Ring detachment method
4. Drop method
• Simple dropper method
• Donnan pipette method
• Pendant drop method
• Sessile drop method
5. Oscillating jet method
6. Maximum bubble pressure method

14. CAPILLARY METHOD
In this method when inversed tube (capillary tube ) in a liquid , the tube will be risen up to a certain
distance by liquid , it depends on :
• Surface tension of liquid (increasurface tension leading to increase height of liquid ) and
• On the cross section area of the tube ( increase area leads to increase height ) for the cross section
area at any point upward force resulting from the surface tension of the liquid on the circumference .

15. RING DETACHMENT
METHOD
The principle of the
instrument depend on the
fact that the force necessary
to detach a platinum ring
immersed at the surface or
interface is proportional to
the surface or interfacial
tension .
Fig : Surface Tension measured by ring detachment
method

16. DETERMINATION OF SURFACE TENSION OF LIQUID BY DROP WEIGHT METHOD ( drop discontinuous)
The surface tension of the liquid is related to the weight of a drop of that liquid which falls freely from the
end of the tube by the expression :
y = (MG ×F ) / R
y = surface tension (gm.cm/sec²=dyne/cm)
M = mass of one drop ,
R = radius ,
F= correction factor
G= 980cm/sec² or 9.8 m/sec²
PROCEDURE
1. Check that the glass tip is very clean and free from any defect particularly around the edges .
2. Allow the drop (e.g. Water) to detach slowly from the tip & collect 10-15 drops in a beaker under
constant conditions ( constant temperature ) .

17. 3. Finally measure the radius of tip.
It is important that the drop has
been correctly formed & detached
and the rate of detachment should
not exceed 1 drop in 2 sec , and
vibration must be guarded against
as well as check the end of the tip is
horizontal .

18. MODIFICATION OF DROP WEIGHT METHOD (drop number method )
It may be performed by counting the numbers of drop (n) by certain volume (0.5ml) under
conditions similar to that pescribed previously . A comparision with liquid of known surface tension
must be similarly treated by using the same tube under the same condition .
The surface tension can be calculated by the following formula : v1 = (n2p1/n1p2) v2
Here ,
v1 =surface tension of test liquid
v2 =surface tension of water
p1&ρ2 =Density of test liquid &water
n1=no. of drops of test liquid
n2=no .of drops of water

19. THANK YOU