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Surface & Interfacial Tension
Photo
courtesy
of
Mike,
Flickr
Photo
courtesy
of
Joy
Ito,
Flickr
Prepared & Presented by:
Professor Abd Karim Alias
Universiti Sains Malaysia
Surface & Interfacial Tension by Prof. Abd Karim Alias is licensed under a
Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported
License.

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} Surface tension is the property of a liquid in
contact with air that makes it behave as if it
was covered with a thin membrane under
tension.
Surface & Interfacial Tension
Abd Karim Alias, 2013

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} The molecule inside the liquid interact
equally with other molecule, from all sides,
whereas the molecule at the surface is
affected only by the molecule below it ⇒
the molecules exposed to air behave
differently & try to contract to the smallest
possible area (hence spherical shape).
Surface & Interfacial Tension
Abd Karim Alias, 2013

4. 4
} Surface tension (N/m) is defined as the
force acting over the surface of the liquid
per unit length of the surface
perpendicular to the force.
Surface & Interfacial Tension
Abd Karim Alias, 2013

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AIR
LIQUID
Air-liquid interface
Surface & Interfacial Tension
Abd Karim Alias, 2013

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} Surface tension (γs) can also be defined as
the amount of energy (∆E) required to
increase the surface area between a liquid
and a gas (e.g. air and water) by an amount
∆A:
∆E = γs∆A
} Interfacial tension (γi) is defined as the
amount of energy required to increase the
interfacial area between two immiscible
liquids (e.g. oil & water).
Surface & Interfacial Tension
Abd Karim Alias, 2013

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} One very important quality of the
colloids is the large interfacial area
between the dispersed & the
continuous phase
Importance of Large Interfacial Area
Abd Karim Alias, 2013

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We have 20 cm3 of oil in 1
cm radius droplets. Each
has a volume of (4/3.π.r3)
5.5 cm3 and a surface area
of (4.π.r2) 12.5 cm2.
⇒ We can have about 3.6
droplets ∴ we would have
a total area of 45.5 cm2
The same oil is split into 0.1
cm radius droplets, each has
a volume of 0.004 cm3 and a
surface area 0.125 cm2.
⇒ We can have about 5000
droplets ∴ we would have a
total area of 625 cm2!
Importance of Large Interfacial Area
Abd Karim Alias, 2013