Topic coverd :- Surface tension Interfacial tension Methods to measure surface and interfacial tension

1. SURFACE
TENSION
By-Sneha Pandey)
(B.pharm, IInd year)

2. CONTENTS
● Surface tension
● Interfacial tension
● Measurement of surface and interfacial
tension

3. Surface Tension
Surface tension is the property of liquid and has the
tendency to contract hence liquid occupy the minimum
surface area and surface of liquid is under tension due to
force of attraction.
Due to force of attraction of liquid molecules the free
surface of liquid behaves like elastic membrane or rubber
sheet. And particular kind of tension produced at the
surface of liquid.

4. Example to understand the property of surface
tension.
● The small insect is easily walking on surface of liquid.
● Small drop of water and mercury are in spherical shape.
● A small niddle easily floats on surface of water due to surface
tension.

5. ● insect is easily
walking on surface
of liquid.
● mercury are in
spherical shape
● niddle easily floats
on surface of water
due to surface
tension.

6. Surface Tension
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 I of the lin,then surface
tension is given by, T = F/2l.
Its unit is N m-1

7. INTERFACIAL TENSION
Force per unit length that exist at interface between
two immisicible liquid phases know as interface.
Interfacial tension is less than surface tension because
the two liquid phases forms an interface are greater
than when a liquid and gas phase exist together.
It is useful in :spreading, emulsification and analyzing
fluid reforming.

9. Measurement of surface
and interfacial tension
● Capillary rise method
● Drop weight method
● Drop count method
● Wilhelmy plate method
● Ring detachment method

10. Capillary Rise Method
Principle: When a thin glass tube is placed in between
liquid, liquid rises up in the capillary tube upto certain
height.
It is because adhesive force between capillary and liquid is
more than the cohesive force between molecules of liquid.
Due to surface tension liquid rises but some gravitational
force is also acting on liquid which pulls liquid downward.
When both forces are equal liquid is in equilibrium and
stable in that situation.

11. Upward force:-
F=2πr.rCosQ —------- 1
Where, 2πr =
circumference of capillary
tube.
rCosQ= surface tension
and angle of contact.

12. Downward force:-
f= mgh+ w —----------2
Where, mgh= potential energy with respect to gravitational force.
W= weight of liquid
We know that, density (d) =m/v
m= d*v
m= d* πr2 —---------3

13. Put value of eq(3) in eq (2)
f= dπr2gh + w
Now liquid is in equilibrium means both forces are equal.
So, upward force= downward force
2πr.rCosQ = dπr2gh*w ( Q= 0 , CosQ= 1)
For water, S.t = 1dghr+ w/2 S.t = surface tension
h= height of liq d= density
g= gravitational force r = radius of capillary w= weight of liq

14. Drop Count Method
In this method, we find out surface tension by comparing the
liquid.
Firstly take known liquid, whose surface tension is known
Then fill stalogmometer with that liquid up to point A, whose
surface tension is known.
Now release liquid dropwise from capillary until liquid reached
at point B, and continuously count the drop .
Now, do the same with other whose surface tension have to
find out.

15. So, by comparing both we find out surface tension.
We know that
W= 2πrS.t
where, 2πrS.t = circumference of capillary tube
For water
W1= 2πrS.t1n1 n1= number of drop of water
S. t1= w1/2πrn1 —-------1

16. For test liquid:-
W2= 2πrS.t2*n2
S. t2= W2/2πrn2 —----2 n2= number of drops of test liquid
Now, we know that
W= mg
W=dvg
Putting value of W in equation(1) and (2)
S.t1=d1*vg/2πrn1 —---B v is same because we are using
same capillary

17. S.t2=d2*vg/2πrn —---------C
Dividing equation (B) by (C )
S.t1/ S.t2 = d1*n1/d2* n2
Drop Weight Method
It is same as drop count method, in which we use same capillary
or Stalagmometer.
In this we weight the drop rather then counting the drop.

18. For water:- W1= 2πrS.t1
S.t1 = W1/ 2π r —-------1
For test liquid:- W2= 2πrS.t1
S.t1= W1/ 2πr —----------2
On comparing equation (1) and (2)
S.t1/ S.t2= W1 / W2

19. Welhemy Plate Method
Used to measure both surface and interfacial tension.
Firstly we put the rectangular plate into liquid, whose
surface tension has to be find out.
Now, surface tension is applied on plate which push plate
downward in the liquid.
The force we apply to deattach the container from
rectangular rid is equal to the surface tension.

20. WL = reading on the balance prior to the deatachment.
W= weight of the plate in air
S.t = F/ LCosQ where, F = surface tension*perimeter
WL-W = 2(L+T) S.t
S.t = WL - W / 2(L+T)
S.t = surface tension Q= angel of contact
F= force applied to deatach rectangular ring
L=length of rectangular plate T= thickness of plate

21. Wilhelmy Plate Method

22. Du Nouy Method (Ring
deatachment method)
The liquid is taken in the
Container and the position
Of the container can be adjusted
So that the ring just touches the
Surface of the liquid.

23. Through the torsion wire a pull ‘P’ is exerted on the ring. So,
that the ring deatches from the surface of the liquid. Some of
the liquid on the surface rises above it's level.
The force required to lift the the ring from the surface is
recorded on a calibrated dial reading in dynes, which is directly
proportional to surface tension.
Upward Pull:-
Responsible for deatachment of ring.
Dial reading in dyes P= W w= force in terms of weight

24. Downward Pull:-
Weight of liquid that adhered to the ring act as downward
force.
This pull is equal to mg and is also due to the surface tension of
the liquid which act on the circumference of the ring.
This downward force tries to balance the upward force.
The liquid film lifted above the ring has two surfaces outer and
inner.

25. Net force of downward pull is given by
b= S.t 2πr×2
2πr= circumference of ring
2= two surfaces, liquid film rises above has two surfaces
outer and inner
When the upward pull of the ring exceeds the downward
weight of the liquid, the ring detaches ( provide liquid makes
zero angle of contact with the ring at equilibrium).

26. Upward pull= downward pull
Dial reading (P) = S.t 4 πr
S.t = P/ 4πr
S.t= P/ 2π ( r1-r2)
P= dial reading
S.t = surface tension
r1= inner radius r2 = ourter radius