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Apoorva Mahendra Ranjekar
ICNTAA
SMIT
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This interesting insect can freely run along the surface of a water. It's tiny mass and the
geometry of its legs allow it to be supported by the high surface tension of water.
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Water droplet on a
leaf
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Surface tension, is defined as the force
along a line of unit length, where the
force is parallel to the surface butperpendicular to the line.
Surface tension ghas the dimension of
force per unit length, or energy per unit
area.
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In the depths of a volume of liquid,
each molecule is surrounded on all
sides by other molecules; the forces
between them balance out and the
entire mass is in equilibrium. The
situation is different at the surface of a
liquid. At a liquid-air interface forexample, the molecules at the surface
are being attracted by the surrounding
liquid but not by the air. The forces are
imbalanced and consequently the
liquid behaves as if had a stretched
skin.
This creates some internal pressure
and forces liquid surfaces to contract
to the minimal area.
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Molecules liquid state experience strong
intermolecular attractive forces. When those
forces are between like molecules, they are
referred to as cohesive forces.
When the attractive forces are between unlike
molecules, they are said to be adhesive forces.
Adhesion is any attraction process between
dissimilar molecular species that can potentially
bring them in "direct contact.
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The cohesive forces among the liquid molecules are responsible for this
phenomenon of surface tension.
The attractive forces between molecules in a liquid can be viewed as residual
electrostatic forces and are sometimes called van der Waals forces or van der
Waals bonds.
As a general rule, the greater the proportion of polar groups (e.g O-H groups) in a
molecule the stronger the attractive forces between them. Strong attractive forces
give rise to a high surface tension and a tendency to form discreet droplets on a
surface rather than wet it evenly. The large proportion of O-H groups in water are
responsible for its high surface tension. Alcohols, with their smaller proportion of
O-H groups, have lower surface tensions.
surface tension is always parallel to the surface.
The surface tension is the two-dimensional analog to the pressure.
Water has a highest surface tension than most other liquid.
Foreign molecules in water usually lower the surface tension.
Increased temperature lowers surface tension.
Electrical conditions at interfaces can alter surface tension.
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Surface energy is quantified in terms of the forces
acting on a unit length at the solid-air or the solid-
liquid interface. The units of measurement areexactly the same as for surface tension.
In order to increase the surface area of a mass of
liquid by an amount, A, a quantity of work, gA,
is needed. This work is stored as potential energy. If the surfaces are unequal, the Young-Dupr
equation applies:
W12 = g1 + g2 - g12
where g1 and g2 are the surface energies of thetwo new surfaces, and g12 is the interfacial
tension.
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Where the two surfaces meet, they form a contact angle,
which is the angle the tangent to the surface makes with thesolid surface.
If is greater than 90 the liquid tends to form droplets on
the surface. If is less than 90 the liquid tends to spread out
over the surface and when the liquid forms a thin film,
tends to zero.
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0 indicates vacuum. This is free energy of a unit area of the film
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