Surface tension and viscosity

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Physical Chemistry
Quaid - e - Azam University Islamabad

Group Members
• Umm -E- Kalsoom.
• Shazia Sultan.
• Muneebah Noor.
• Hira Sulaiman.

Topic
“Surface Tension”
&
“Viscosity”

Viscosity:
Introduction What is Viscosity?
A quantity expressing the magnitude of
internal friction in a fluid, as measured by
the force per unit area resisting uniform flow.
or
Viscosity is a property of the fluid which
opposes the relative motion between the two
surfaces of the fluid that are moving at
different velocities.
The word "viscosity" is derived from
the Latin "viscum", meaning mistletoe

Type
s:About types
Dynamic (or Absolute) Viscosity:
The dynamic viscosity(η) of a fluid is a
measure of the resistance it offers to
relative shearing motion.
F=դ A
ս
ℎ
(դ: Pa-s)
Kinematic Viscosity :
It is defined as the ratio of absolute
viscosity to the density of fluid.
ν= η/ρ m²/s ; ρ= density of
fluid
Viscosity is a quantitative measure
of a fluid’s resistance to flow.

Viscosity Measurements
About measurements
Viscosity is measure by following meters:
Kinematic viscosity measurements:
1) Capillary Viscometer.
2) Bubble Viscometer.
Dynamic viscosity measurements:
1) Rotational Viscometer.
- Rotating Cylinder .
- Cone-on-plate Viscometers.
2) Rheometer.

Exceptions:
Inviscid fluid:
A fluid that has no resistance to
shear stress is known as an ideal or
inviscid fluid.
- Zero viscosity is observed only at very
low temperatures in super fluids.
- A liquid is said to be viscous if its
viscosity is substantially greater than
that of water.
- A liquid is said to be mobile if the
viscosity is noticeably less than water.

Dependence:
It depends upon…. 1.Temperature:
OF LIQUIDS:
Their viscosity decreases with
increase in temperature,
Due to increase in average speed of molecules,
that causes intermolecular forces to becomes
weak so, decreases the viscosity.
OF GASES:
Their viscosity increases with
increase in temperature,
Because the motion of molecules become more
disorganize due to increase in the
intermolecular collisions of gas molecules , that
causes the viscosity to increase.

Dependence:
It depends upon….
2.Intermolecular Forces:
Stronger the intermolecular forces among the
molecules, higher is the viscosity.
For example water is more viscous than
methanol mainly due to extensive hydrogen
bonding.
3.Molecular size:
Liquids whose molecules are small and
compact have low viscosity.
Whereas liquids having large and irregular
shaped molecules tends to get tangled up with
each other. This inhibits the flow of molecules
and leads to high viscosity.

Dependence:
4.Pressure:
Lubricants viscosity increases with
pressure.
For most lubricants this effect is
considerably largest than the other
effects when the pressure is significantly
above atmospheric.
The Barus equation :
It depends upon….

Units:
It’s units are….
S.I Units:
Dynamic Viscosity:
• Kgm-1s-1
• Nm-2s
• Pa-s
Kinematic Viscosity:
• m2·s−1
Non S.I Units are:
• Poise
10 poise= 1 pa-s
• Stokes
1 St = 1 cm2·s−1 = 10−4 m2·s−1.

Applications of Viscosity
About applications
Engineering:
• Selection of lubricants for various
purpose.
- we can choose an optimum range of
viscosity for engine oil.
- for high load and also for speed
operation high viscous lubricants is
required.
• In pumping operation
- for high viscous fluid high power will
require.
- for low viscous fluid low power will
require.
• In making of blend fuel
- less viscous fuels easy to mix.
• In the operation of coating and printing.

About applications
Medical:
• To determine the viscosity of plasma
- To prevent cardiovascular risk and
other disease such as, obesity,
hypertension ,and diabetes.
- To determine the force required to inject
a new drug formulation into a patient.
- It helps in absorbing drugs into the
human body.
• Used in pharmaceutical industries.
- Accurate viscosity values of proteins are
essential in understanding behavior,
designing delivery systems, and
accurately determining particle size of
protein based drugs.

About applications
Industries:
• To determine the viscosity of chemical
structure for ionic liquids to measure:
- impurity of solution.
- chemical structure of solution.
- composition of solution.
• To determine the viscosity of polymer
solution such as, polyamide,
- To control the quality of polyamide
during production.
- To control the quality of the end
product, molar mass and chain length
of the polymer.
• To determine the viscosity of adhesive.
- Low-viscosity adhesives will flow
more readily than high-viscosity ones,
so a high-viscosity adhesive is ideal.

About applications
Industries:
• In food industries to check the viscosity of,
- syrup, to ensure their use as a binders in
the production of food, e. g. of chocolate
cereal mixes or sauces.
- a product so that it can travel efficiently
through pipes and take less time to set or
dry.
- food such as a chewing gum, butter,
margarine, bread, candy, chocolate, to
find best composition ratios when
producing them.

Surface Tension:
Introduction
What is Surface Tension?
“Surface tension is a contractive tendency of
the surface of a liquid that allows it to resist
an external force. Surface tension is an
important property that markedly influences
the ecosystem.”
or
“Surface tension is measured as the energy
required to increase the surface area of a
liquid by a unit of area.”
or
“surface tension is often expressed as an
amount of force exerted in the surface
perpendicular to a line of unit length.”

Surface Tension:
Phenomena
•The cohesive forces among liquid molecules
are responsible for the phenomenon of
surface tension.
•In the bulk of the liquid, each molecule is
pulled equally in every direction by
neighboring
liquid molecules, resulting in a net force of
zero.
•The molecules at the surface do not have other
molecules on all sides of them and
therefore are pulled inwards.
•This creates some internal pressure and forces
liquid surfaces to contract to the minimal
area. Surface tension is responsible for the
shape of liquid droplets.

What’s going on at the
surface of a liquid?
Let’s take
a look!

Dependence:
It depends upon…….
1.Intermolecular Forces:
Stronger the intermolecular forces among the
molecules of a liquids, greater is the surface
tension and vice versa.
e.g, the surface tension of water is higher than
many liquids due to strong hydrogen bonding
between water molecules.
2.Nature:
It is different for different liquids due to the
presence of different types of intermolecular
forces.

Dependence:
It depends upon….
3.Temperature:
Surface tension of a liquids decreases with the
increase of temperature , reaching a value of 0 at
the critical temperature.
This is because increased kinetic energy of the
molecules decrease the strength of intermolecular
forces.
4.Solute Concentration:
Solutes can have different effects on surface
tension depending on the nature of the surface and
the solute:
A solute can exist in a different concentration at
the surface of a solvent. This difference varies
from one solute–solvent combination to another.

Physical Units:
It’s units are…..
Units:
Surface tension, is usually represented by a
symbol “γ”,is measured in force per unit
length.
γ = F /d
S.I UNIT:
γ = Nm-1 or Jm-2
CGS UNIT:
γ = dyn cm-1 or erg cm-2

Applications:
It’s use are……..
1.Spherical Shape of Water 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.
• The spherical shape minimizes the
necessary "wall tension" of the surface
layer according to LaPlace's law.
• In the absence of other forces, including
gravity, drops of virtually all liquids
would be approximately spherical.
• a sphere has the smallest possible surface
area to volume ratio

Applications:
It’s use are…….. 2.Walking on water:
Small insects such as the water strider can
walk on water because their weight is not
enough to penetrate the surface.
3. Floating a needle:
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.
4. Soaps and detergents:
Surface tension help the cleaning of clothes
by lowering the surface tension of the water
so that it more readily soaks into pores and
soiled areas.

Applications:
It’s uses are…..
5.Don't touch the tent:
Common tent materials are somewhat rainproof in
that the surface tension of water will bridge the
pores in the finely woven material. But if you
touch the tent material with your finger, you break
the surface tension and the rain will drip through.
6. Clinical test for jaundice:
Normal urine has a surface tension of about 66
dynes/cm but if bile is present (a test for jaundice),
it drops to about 55. In the Hay test, powdered
sulfur is sprinkled on the urine surface. It will
float on normal urine, but sink if the S.T. is
lowered by the bile.

Applications:
7.Washing with cold water:
The major reason for using hot water for
washing is that its surface tension is lower
and it is a better wetting agent. But if the
detergent lowers the surface tension, the
heating may be unnecessary.
8. Surface tension disinfectants:
Disinfectants are usually solutions of low
surface tension. This allow them to spread
out on the cell walls of bacteria and disrupt
them. One such disinfectant, S.T.37, has a
name which points to its low surface tension
compared to the 72 dynes/cm for water.

Applications:
9.Capillary Action:
Capillary action is the result
of adhesion and surface tension. Adhesion of
water to the walls of a vessel will cause an
upward force on the liquid at the edges and
result in a meniscus which turns upward. The
surface tension acts to hold the surface intact,
so instead of just the edges moving upward,
the whole liquid surface is dragged upward.
10.Alveoli of the Lungs:
The oxygen exchange in the lungs takes place
across the membranes of small balloon-like
structures called alveoli attached to the
branches of the bronchial passages. These
alveoli inflate and deflate with inhalation and
exhalation. The behavior of the alveoli is
largely dictated by LaPlace's law and surface
tension.

Applications:
11.Cell Maintenance:
Water particles at the atomic level help keep
the cell membrane from collapsing on itself.
Water is a critical component in cytoplasm
this means that the surface tension of water
allows the cytoplasm to hold-up the cell
membrane. If the pressure is too extreme, ,
then the water breaks down the cell
membrane and the cell "drowns".
12.Transportation:
The high surface tension of water is also
essential for processes such as water and
blood transport in plants and animals
respectively. The concave motion of the
surface of the water allows it to be "pulled
up" by plants from the ground

QUESTIONS?

Reference:
• ‘Engineering Tribology’ by “Gwidon
W. stachowiak” and “Andrew W.
Batchelor”.
• ‘Engineering Tribology’ by “J. A.
Williams”.
• Petrucci, Ralph H., et al. General
Chemistry: Principles and Modern
Applications. Upper Saddle River, NJ:
Prentice Hall, 2007.
• www.wikipedia.org
• www.google.com

thank you

Surface tension and viscosity

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