presented to : Dr | Hamdy El-Kady Physical Chemistry Course 2016-2017 prepared By : Muhammad Mamdouh Abdulsalam Faculty Of Petroleum Engineering, Suez University

1. Prepared By :
Mohamed Mamdouh Mahmoud Abdul Salam
" First Year student " – " Petroleum Department "
Section NO. 4
SURFACE
TENSION Dr . Hamdy El-Kady
Report

2. Mohamed Mamdouh Mahmoud Abdul Salam Petroleum Department1
Surface Tension
Very short Notes
This is a property for liquids and arises from intermolecular forces of
attraction . you can notice that a molecule in the interior of the liquid is
attracted equally from all directions by the molecules around it, while
a molecule at the surface of the liquid is attracted only sideways and
toward the interior. (figure 1) shows the two types of attractions as
discussed. The forces on the sides being counterbalanced the surface
molecule is pulled only inward the liquid . so the molecules have
tendency to go into the bulk of the liquid. Now the surface of the liquid
is under tension and in order to have the minimum number of
molecules at the surface of the liquid so it contracts to have the smallest
possible area. Then this is the reason why the drops of a liquid in air is
assumed to be spherical, because for a given volume the spherical shape
has the smallest possible area.
This property of liquids helps some insects (Water striders) which are
usually denser than water to walk on the surface of the water as shown
in (figure 3).this also could explain the capillary action which will be
discussed in more details.
Figure 1 – Surface tension is a result of the
net inward pull on the surface molecules.
Figure 2 – makes this drop of water to
have this spherical shape in air .
Figure 3 – makes this drop of water to
have this spherical shape in air .

3. Mohamed Mamdouh Mahmoud Abdul Salam Petroleum Department2
Explanation of how (Water striders) could walk on water (figure 3):
This is just when the object in this case the water strider isn't able to
get wet and the weight is small enough to be carried by the forces
arising from surface tension. The legs of water strides can't get wet (as
if it is covered with wax). Which means no attraction between the
molecules of water and the molecules of the leg.so when the leg pushes
down the water, the surface tension of the water only tries to recover its
flatness from its deformation due to the leg. This behavior of the water
pushes the water strider upward so it can stand on the surface as long
as its mass is small so that the water can support. The surface of the
water is acting like an elastic film.
Definition : (Surface Tension ( γ ) )
It is the force in dynes acting along the surface of the liquid at right
angle to any line (section) 1 cm in length.
Units of Surface Tension :
from the previous definition we could deduce the unit of surface tension
which is (dyne per centimeter) dyne.cm-1
in the CGS system and
(Newton per meter) N.m-1
in the SI system.
Notes : 1 dyne.cm-1
= 1 m N.m-1
Force per unit length is equivalent 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.
The Effect of temperature on surface tension:
By increasing the temperature (T) of a liquid results in increasing the
kinetic energy of the molecules (KE). [KE ∝ 𝐓] and thereby decreasing
the intermolecular forces . so the inward forces acting on the molecules
of the first layer of the liquid decrease. in other words, by increasing
temperature the surface tension decreases.

4. Mohamed Mamdouh Mahmoud Abdul Salam Petroleum Department3
Determination
of surface
tension
Capillary-rise
Method
Drop Formation
Method
The following diagram shows two Methods of determination the
surface tension.
1-Capillary-rise Method :
In this method a capillary tube with radius r is inserted
vertically into a liquid, the liquid rises to height h the surface
tension (γ) is acting along the inner circumference exactly
supports the weight of the liquid column.
 From the definition of the surface tension discussed previously
θ:is the angle between the tangent line to the meniscus surface and the tube
γ:is the surface tension of the liquid
r:is the radius of the capillary tube
then the total force (upward force) = 𝟐𝝅𝒓𝜸 𝐜𝐨𝐬 𝜽
θ for most liquids equals zero and 𝐜𝐨𝐬 𝜽 = 𝟏
the total downward force (Mass x Gravity) = 𝝅𝒓 𝟐
𝒉𝒅𝒈
where d : is the density of the liquid
g : is equal to (980 cm/s)
upward force = downward force
𝟐𝝅𝒓𝜸 = 𝝅𝒓 𝟐
𝒉𝒅𝒈
𝛄 =
𝐡𝐫𝐝𝐠
𝟐

5. Mohamed Mamdouh Mahmoud Abdul Salam Petroleum Department4
2- Drop Formation Method:
In this method a drop of water is allowed to form at the lower end of a
capillary tube as shown in the opposite figures.
. the drop of water is supported by two forces, the first one is the
Surface tension force, the second is its weight which pulls it down-
ward. When the two forces are balanced the drop breaks thus at
the point of breaking.
𝒎𝒈 = 𝟐𝝅𝒓𝜸
Where m : the mass of the drop
r : the radius of the tube
γ : the tension surface
Experimental Steps
The apparatus is filled with the liquid with the unknown surface tension, then a
20 drops of water is allowed to fall in a weighing bottle and weighed. Thus the
of one drop is found. The apparatus is cleaned and dried and then filled with
another liquid, say(Water) and weight one drop of this liquid with the same
method .
Thus from the previous equation :
𝒎 𝟏 𝒈 = 𝟐𝝅𝒓𝜸 𝟏
𝒎 𝟐 𝒈 = 𝟐𝝅𝒓𝜸 𝟐
𝜸 𝟏
𝜸 𝟐
=
𝒎 𝟏
𝒎 𝟐
Knowing the surface tension of the reference liquid from tables, that
of the liquid under study can be found.

6. Mohamed Mamdouh Mahmoud Abdul Salam Petroleum Department5
Determinatio
n of surface
tension
Maximum bubble
pressure method
Ring-detachment
method
The following diagram shows other two Methods of determination the
surface tension.
1- Ring-detachment method
In this method the force required to detach a platinum ring from the liquid
surface is Measured [ the measured force(F) is Exactly equal to the downward
pull to surface tension (γ) acting along the circumference of the ring ]. Note that
the surface tension force is acting along twice the length of
the circumference of the ring. In this experiment, we are
using a device called (du Nouy Tensiometer).it is shown in
figure with its essential parts.One end of the torsion wire is
fixed while the other is attached to a knob carrying a
pointer. The pointer moves on a fixed scale. The scale is
previously calibrated by taking different weights on the
beam and noting the scale reading when it is lifted from the horizontal position.
The liquid whose surface tension is to be determined is
placed in a watch glass so that the Ptring just touches its
surface. The knob of the torsion wire is then slowly turned
till the ring is just detached from the surface. The reading
shown by the pointer on the scale gives the force The
surface tension is then calculated from the Following
Equations.
F=4 π rγ
𝛄 =
𝑭
𝟒𝛑𝐫

7. Mohamed Mamdouh Mahmoud Abdul Salam Petroleum Department6
2- Maximum bubble pressure method
In this method air-pressure is applied slowly through a capillary tube dipping in the
experimental liquid
A bubble is formed at the end of the capillary. Slowly
the bubble grows and becomes hemispherical. Then
it breaks away when the pressure recorded by the
manometer is noted. This is the maximum pressure
required tomake a bubble at the end of the capillary.
At the moment of breaking, the forces due to
maximum pressure P equals that of the opposing hydrostatic pressure
Ph and the surface tension γ at the circumference of the capillary. Thus
where r= radius of capillary; d= density of the liquid; h = depth of liquid. Knowing the
value of P, h, d and r, γ can be found.