Fluid Mechanics Nov 9th 2014

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Petroleum Engineering Department

Faculty of Engineering

Koya University

Experiment No. : 3

Group: B

Group Members:

1- Sarwar Salam

2- Huner Mahdi

3- Savio Sarmad

4- Sangar Nasih

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Aim of the Experiment:

Determination of the Surface Tension force(ST) in the

Taper Vessel and in the Communicating Tubes also, and

its impact and influence on the applicable examples out

in our real life.

Finding the proportional type in between the distance of

the gaps and the Surface Tension

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Introduction:

The cohesive forces between liquid molecules are responsible

for the phenomenon known as surface tension (ST). The mol-

ecules at the surface do not have the similar neighboring atoms

on all sides and thus they cohere more strongly to those directly

associated with them on the surface. This forms a surface

“FILM” which makes it more difficult to move an object

through the surface than move it when it is completely immersed

(Figure 1). The same situation applies also at the interface of the

two liquids that do not mix together. In this case the term

interfacial tension (IFT) is used. There are several different units

for surface and interfacial tension; typically mN/m (which is

equivalent to dynes/ cm) is used .

(Figure 1)

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Theory:

The phenomenon of surface tension of a liquid can be explained on the

basisof molecular theory.

Consider a beaker containing water or liquid in which A, B and C arethe

molecules of liquid with their spheres of influence.

The molecule A is well inside the liquid. It is equally attracted by the

neighbouring molecules ofliquid. Hence, resultant cohesive force on the

molecule A is zero.

(Figure 2)

Consider the molecule B, just below the free surface of a liquid. The

maximum part of its sphere of influence is in liquid and some part of

sphere of influence is in air. Since the number of molecules of the liquid

in the lower half of the sphere is greater than that in the upper half,the

molecule B experiences resultant downward force.

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As the molecule approaches the surface, this downward cohesive force

on it increases because the number of molecules in the upper half of the

sphere decreases.

The molecule C is just on the free surface of the liquid. It's half of the

sphere of influence is in air and half of the sphere of influence is in

liquid. The upper half of the sphere of influence contains very few

molecules of the air.

So, the downward force, on the molecule becomes maximum. This

applies to all other molecules on the surface of the liquid. This

downward cohesive force per unit area of a liquid, is called its internal,

intrinsic or cohesive Pressure. If a molecule from the interior of the

liquid is brought to the surface, work has to be done against downward

cohesive force and its potential energy is increases. This shows that the

molecules on the surface possess greater potential energy than inside the

liquid.

In nature, a system tends to have minimum potential energy for stable

Equilibrium. Hence the liquid surface in trying to have the least

potential energy, tends to maintain least surface area. This tendency of

the free surface of the liquid at rest to have minimum area is called

surface tension.

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(Figure 3)

Procedure of performing the Taper vessel Experiment:

Fill the tapered vessel with water, as shown in the adjacent illustration.

(Figure 4)

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.

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-TO MODIFIY the experiment….

Add a few drops of washing-up liquid to the water The line of the water

level immediately changes. Emulsifying agents in the washing-up liquid

are able to reduce the surface tension of the water. The surface tension

is dependent on the active substance pairing,

(Figure 5)

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B- Capillary tubes experiment

Aim of the experiment:

The water level in the capillary tubes rises as the tubes narrows.

(Figure 6)

The more the narrow the tube, the higher the level

the liquid is.

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Procedure of performing capillary communicating tubes:

Fill the capillary tubes with water. According to the law of

communicating tubes, the water level should be the same in all tubes.

As a result of the surface tension of the water, however, the water level

raises the narrower the tube. This property is termed the capillary effect

of water.

In visual terms, the surface tension of the water produces an upward

force, which is able to draw the head of water underneath it upwards.

With the same weight of water, the head of water can become higher the

"narrower" it is.

In reality the rise height depends not only on the surface tension of the

liquid, but also on its ability to coat the wall (adhesion). The adhesion

can be seen by the curved surface of the liquid.

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Discussion:

1- ) what is SFF stands for? And what is that?

-Ans.) SFF, is stands for Surface Tension Force. Which is (γ) is the

magnitude F of the force exerted parallel to the surface of a liquid

divided by the length L of the line over which the force acts:

γ =F/L

SI Unit of Surface Tension: N/m

2- ) what do we mean with Capillary tubes? And what

makes it differ from intercommunicating tubes?

Ans.) A tube with a very small diameter, which is called a capillary

tube. The difference between capillary tubes and interconnected tubes

is that the diameter is not limited in the connected tubes while tubes

diameter should very small in capillary tubes.

3- ) give a brief description of Cohesion?

Ans.) Molecules liquid state experience strong intermolecular

attractive forces. When those forces are between like molecules, they

are referred to as Cohesive force, for example the molecules of water

droplets are held together by cohesive forces, and the especially strong

cohesive forces at the surface constitute the Surface Tension.

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4- ) Write some examples of surface tension?

Ans.) -Walking on water

Small insects such as the water strider can walk on water because their weight is

not enough to penetrate the surface.

(Figure 7)

-Floating a needle

If carefully placed on the surface, a 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.

-Soaps and detergents

help the cleaning of clothes by lowering the surface tension of the water

so that it more readily soaks into pores and soiled areas.

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5- ) what is the Surface tension of water?

Ans.) The surface tension of water is 72 dynes/cm at 25°C . It would

take a force of 72 dynes to break a surface film of water 1 cm long. The

surface tension of water decreases significantly with temperature as

shown in the graph. The surface tension arises from the polar nature of

the water molecule.

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.

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References

1- ) http://hyperphysics.phy-

astr.gsu.edu/hbase/surten.html

2- )

http://scipp.ucsc.edu/~haber/ph5B/bubble.pdf

3- ) www.hyperphysics.edu

4- )http://water.usgs.gov/edu/surface-

tension.html

5- )

https://www.google.com/search?q=surface+te

nsion&tbm=isch&imgil=gzNtYkbao0UKzM%253

A%253BeHc51ce9i9gMxM%253Bhttp%25253A

%25252F%25252Fhyperphysics.phy-

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