The document describes experiments conducted to measure surface tension using a tapered vessel, capillary tubes, and surface tension balance. It provides background on surface tension and adhesive forces. The students measure the surface tension of liquids and discuss potential sources of error between measured and theoretical surface tension values, such as temperature fluctuations and human error in reading instruments.

1. Faculty of Engineering Petroleum
Engineering Department
Fluid Mechanics Laboratory, 2nd stage
Experiment Name: Surface Tension
Prepared by: Muhammed Fuad Rashid
Ahmad Jalal Hassan
Muhammad Hassan Aziz
Safwan Tofiq Ameen
Group: A
Supervised by: Mr. Dara & Mr. yonis
Date of Submit: 30/09/2019

2. 2
Contents
1.0. Aim of the experiment......................................................................................................................................3
2.0. Introduction..........................................................................................................................................................4
2.1. Surface tension ..................................................................................................................................................4
2.2. Adhesive Forces .................................................................................................................................................5
2.3. Introduction.......................................................................................................................................................7
2.3.1. taper vessel.................................................................................................................................................7
3.0 Procedure ...............................................................................................................................................................7
3.1. taper vessel........................................................................................................................................................7
2.3.2. capillary pressure ........................................................................................................................................8
3.2. Precedure of capillary pressure..........................................................................................................................8
2.3.3. Surface Tension Balance experiment..........................................................................................................9
3.3. Procedure of Surface Tension Balance experiment............................................................................................9
4.0. Tools used in the experiment...............................................................................................................................10
5.0. Table of Reading..................................................................................................................................................11
6.0. Table of Calculation .............................................................................................................................................11
7.0 Discussion.............................................................................................................................................................12
7.1. Discussion by Ahmad Jalal Hasan.....................................................................................................................12
7.2. Discussion by Muhammad Hassan Aziz............................................................................................................14
7.3. Discussion by Muhammed Fuad Rashid...........................................................................................................15
7.4. Discussion by Safwan Tofiq Ameen..................................................................................................................17
8.0. references............................................................................................................................................................18

3. 3
1.0. Aim of the experiment
1-Taper vessel experiment:
The level of the water rises as the gap gets narrow and small in the taper vessel
2- Capillary tubes experiment:
the level of the capillary tubes are different due to different in their radius
3-Surface Tension Balance experiment:
To measure the value of surface tension of the liquids

4. 4
2.0. Introduction
2.1. Surface tension
It is found that when a liquid is free from the external forces (such as gravity), it always takes
the shape of a spherical drop. It is because, for a given volume, a sphere has the least surface
area.* It means that the surface of every liquid has always a tendency to have least surface
area and in this respect, it behaves like a stretched membrane having a tension in all directions
parallel to the surface. This tension in the surface of a liquid is called surface tension.
Thus, surface tension is that property of a liquid by virtue of which, it behaves like an elastic
stretched membrane with a tendency to contract, so as to occupy a minimum surface area.
Surface tension can be defined in following possible ways:
(i) The property of a liquid on account of which it tends to keep minimum number of
molecules in it's free surface is defined as surface tension.
(ii) The property of a liquid, on account of which it tends to minimize it's free surface area, is
called surface tension.
(iii) The work, required to be done in increasing the free surface of a liquid by unity at constant
temperature, is defined as surface tension.
(iv) The force acting per unit length of an imaginary line drawn on the free liquid surface at
right angles to the line and in the plane of liquid surface, is defined as surface tension.

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2.2. Adhesive Forces
Forces of attraction between a liquid and a solid surface are called adhesive forces. The
difference in strength between cohesive forces and adhesive forces determine the behavior
of a liquid in contact with a solid surface.
 Water does not wet waxed surfaces because the cohesive forces within the drops are
stronger than the adhesive forces between the drops and the wax.
 Water wets glass and spreads out on it because the adhesive forces between the liquid
and the glass are stronger than the cohesive forces within the water.

6. 6
Formation of a Meniscus:
When liquid water is confined in a tube, its surface
(meniscus) has a concave shape because water
wets the surface and creeps up the side.
Mercury does not wet glass - the cohesive
forces within the drops are stronger than
the adhesive forces between the drops
and glass. When liquid mercury is
confined in a tube, its surface (meniscus)
has a convex shape because the cohesive
forces in liquid mercury tend to draw it
into a drop.

7. 7
2.3. Introduction
2.3.1. taper vessel
The surface tension is especially noticeable in the range of smaller dimensions. At such
dimensions the surface tension acts so strongly that it is able to raise the water below it.
3.0 Procedure
3.1. taper vessel
* Fill the tapered vessel with water, as shown in the adjacent illustration.

8. 8
2.3.2. capillary pressure
Capillary action is the rise of a liquid that wets a tube up the inside of a small diameter tube
(i.e., a capillary) immersed in the liquid.
The liquid creeps up the inside of the tube (as a result of adhesive forces between the liquid
and the inner walls of the tube) until the adhesive and cohesive forces of the liquid are
balanced by the weight of the liquid.
The smaller the diameter of the tube, the higher the liquid rises. .
3.2. Precedure of capillary pressure
* Fill the capillary tubes with water until you observe the change in head of the water in the
vessels

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2.3.3. Surface Tension Balance experiment
A self-contained instrument for the measurement of surface tension is described. A ground
glass rod is used to pull up a meniscus from a liquid and the adhesion tension is measured. The
readout is given directly in mN m-1 (or dyn cm-1) with an accuracy of measurement of +or-0.1
mN
3.3. Procedure of Surface Tension Balance experiment
This balance enables the student to determine the surface tension by the method of direct
pull. The surface tension forces acting round a frame are measured with this test. The frame
is suspended to touch the surface of the liquid. A light pointer is attached to the torsion wire
of the balance. The pointer moves over a scale. A scale-pan is suspended from the pointer,
which supports a holder in which is fixed a clean microscope slide. A counterweight suitably
adjusts the pointer against the scale. The liquid for which the surface tension is required is
contained in a cleaned beaker (or other suitable container) which is raised until the microscope
slide is partially submerged in the liquid. The beaker containing the liquid is then slowly
lowered and the scale reading of the pointer taken when the microscope slide just breaks away
from the liquid. The microscope slide is now dried off and masses (m) are placed in the scale-
pan until the pointer is depressed to the critical reading obtained above. If the length of the
microscope slide is L, and its breadth is b, the downward pull due to the surface tension of the
liquid at the critical setting is 2σ(L + b) = m g. Hence σ can be evaluated.
SI units: L, b in m, g = 9,81 N kg-1, m in kg gives σ in N.m-1)

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4.0. Tools used in the experiment
taper vessel exp
capilary communicating tubes
Surface Tension balance

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5.0. Table of Reading
6.0. Table of Calculation
No. M(kg) 𝝈calculated (N/m) 𝝈Table (N/m)
1 0.00087 0.0609 0.0721
No. (M)gm
1 0.87

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7.0 Discussion
7.1. Discussion by Ahmad Jalal Hasan
1-What is the different between surface tension measured in lab and theoretical
surface tension?
1-Because the temperature is not stable or we don’t have a standard temperature a standard
temperature.
As a result, surface tension decreases when temperature increases because cohesive forces
decrease with an increase of molecular thermal activity
2-also errors that caused by the instrument or individual human errors such as eye trouble to
read the values in the experiment such as in (surface tension balance) that should be read
carefully to obtain the right values .
In both reasons will lead to bias and different in values of calculated and theoretical
measurements.
2-What is the difference between surface tension and tension?
The main difference between these two is the places where it occurs. Surface tension is
defined to a single liquid surface, whereas the interfacial tension is defined to the interface
of two immiscible liquids. ... Which is a stronger surface, tension or interfacial tension?
3-Why interfacial tension is lower than surface tension?
The surface tension is reduced as some of the water molecules are replaced by the
surfactant molecules and interaction forces between surfactant and water is less than
between two water molecules. ... Surfactants are routinely used in industry to lower the
surface and interfacial tension.

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4-What is the advantages of high surface tension of water?
High surface tension of water is also the reason why rain comes down as a spherical drop.
High surface energy drives the water drop to take a shape with as little surface area as
possible, making a sphere shape most favorable.
5-Does water have a high surface tension?
Because of the relatively high attraction of water molecules to each other through a web of
hydrogen bonds, water has a higher surface tension (72.8 millinewtons per meter at 20 °C)
than most other liquids. Surface tension is an important factor in the phenomenon of
capillarity.
6-What would happen without surface tension?
Without this property, water would be a slimy coating and cells would not have shape. Surface
tension decreases with temperature and salinity. Please take a few minutes to watch this
amusing video to learn more about surface tension of water.
7-How is surface tension used in everyday life?
Surface Tension in Everyday life, The mercury used in the thermometer does not stick to the
wall of the tube because of the surface tension, if it did the measurement of the temperature
wouldn't be correct. The separation of oil and water is also caused by the difference in the
surface tension of the two liquids.
8-Why does sugar increase surface tension?
It has been reported in the literature that sugars such as dextrose and sucrose increase the
surface tension of water. The effect was interpreted as a depletion of the solute molecules
from the water-air interface, The surface tension decreases continuously with increasing
concentration.

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9-Do solids have surface tension?
Researchers have shown that surface tension on a solid material is unconnected to the energy
required to create a new surface. Consequently, surface tension on a solid does not exist in its
conventional meaning.
7.2. Discussion by Muhammad Hassan Aziz.
1-what is the surface tension?
Surface tension is the expression of resistance which liquids show in response to an increase
In their surface area.
2-why the water level in vessel rises as the gap narrow?
Because surface tension especially noticeable in the range of smaller dimensions and at the
Small dimensions the surface tension acts so strong that it’s able to raise the water below it.
3-does surface tension affect viscosity?
Yes, because with increasing the temperature viscosity increases but surface tension
Decrease, more viscosity the liquid will be more thick and hence lesser the surface tension
Most definitely.
4-. Define critical temperature?
The temperature at which the surface tension is zero.
5-. The temperature at which the surface tension is zero?
Surface tension decreases with the rise of temperature.

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6-. Give some practical applications of surface tension?
(a) A drop of falling liquid is always in spherical shape.
(b) We use oily substances to set out hairs.
(c) We use soaps and detergent for cleaning clothes.
(d) A thin layer of water over the umbrella protects us against light rain.
(e) Capillary action e.g. rising of oil in the wick of a lamp.
(f) Flying insects can walk on water surface without getting their feel wet.
A
7.3. Discussion by Muhammed Fuad Rashid
Q1. What causes surface tension?
In brief, surface tension arises from the strong interactions between water molecules, called
hydrogen bonding. It is this strong interaction which also manifests in the other unusual
property of water, its high boiling point.
In the bulk of a liquid, each water molecule can make an optimal number of H-bonds to other
water molecules. On the surface, however, the interactions with the neighboring molecules
are limited and weaker, resulting in a higher free energy and reduced intermolecular hydrogen
bonding of the molecules. In nature, water has one of the highest surface tensions, 72.8mN/m
(at 20 degrees Celsius), only exceeded by very few liquids, such as mercury, which has a surface
tension of about 480mN/m.
Surface tension of water also manifests as the so-called hydrophobic effect, evident as the lack
of mixing of oil and water.

16. 16
Q2. What causes surface tension in water?
Water has a high surface tension (72.8 millinewtons per meter at 20 °C) compared to that of
most other liquids Because of a hydrogen bonding of surface molecules.. Surface tension is an
important factor in the phenomenon of capillarity.
Q3. How does surface tension of water work?
The property of the surface of a liquid that allows it to resist an external force, due to the
cohesive nature of its molecules. The cohesive forces between liquid molecules are
responsible for the phenomenon known as surface tension.
Q4. What is the unit of surface tension?
Surface tension, usually represented by the symbol γ, is measured in force per unit length.
Its SI unit is newton per meter but the (cgs) unit of dyne per cm is also used.
Q5. What are cohesion and adhesion force?
Cohesion force is the attractive force between like molecules, whereas, the adhesion is the
attractive force between unlike molecules, e.g. attraction between glass slide and the liquid.
Q6. What are the factors effecting surface tension?
(a) Nature of liquid.
(b) Nature of the surface in contact.
(c) Temperature.

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7.4. Discussion by Safwan Tofiq Ameen
1. What is the different between calculated surface tension and theoretical
surface tension?
Because don’t have a standard temperature .
And errors caused by Machine and human error or eye trouble.
surface tension decreases when temperature increases because cohesive forces decrease
with an increase of molecular thermal activity
2. Why is it better to use hot soapy water to wash clothes?
Hot water is a better cleaning agent because the lower surface tension makes it a
better "wetting agent" to get into pores and fissures rather than bridging them with
surface tension. Soaps and detergents further lower the surface tension.
3. What is the difference between {A and B}? use these terms in your answer
{wet, adhesion, cohesion}
B (HG) A(water)
A:have adhesion force , wetting, don’t have cohesion force
B:Hg don’t have adhesion force , nonwetting , have cohesion force

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8.0. references