Introduction
Surface tension is one of the most important and interesting
properties of the fluids. It is the surface tension of the liquid that decides
what shape a small portion of the liquid will take or the extent to which the
liquid can make contact with the other substance.
Any liquid in small quantity in small
will always assume the form of a spherical drop so a liquid must experience
some kind of force to occupy a minimum surface area. This contracting
tendency of a liquid surface is known as the surface tension of liquid.
When
a camel hair brush is dipped into water, the bristles spread out. When the
brush is taken out, the bristles cling together on account of the films of
water between them contracting. This shows that the surface of a liquid behaves
like an elastic membrane under tension with a tendency to contract. This
tension or pull in the surface of a liquid is called surface tension.
Definition of surface tension:
Surface tension is nothing but a cohesive force that keeps the liquid intact. Each molecule in the beaker is
pulled in every direction equally by adjacent molecules. The resulting net
force is zero. At the surface of the beaker, the molecule does not have another
molecule around the sides of it. Therefore elevation of internal pressure
molecules at the surface pulled inwards top layer of the liquid surface of the beaker
is compressed to a minimum area.
Unit of surface tension
Surface tension brings force per unit length its SI Unit is
Newton per metre
(Nm–1) although the more common unit is the cgs unit dyn/cm. (dyne per centimeter.
(Nm–1) although the more common unit is the cgs unit dyn/cm. (dyne per centimeter.
Forms of liquid drops
When a quantity of liquid rests upon
a horizontal solid plate, which it does not wet, the shape of the drop is
determined by surface tension and gravity, surface tension determines the shape
of the drop. For extremely small drops, the surface tension effects are great
and the gravitational effects are small. Hence the shape is spherical.
On
increasing the size of the drop, the effect of gravitation becomes greater and
that of surface tension less. Now the effect of gravitation alone would be to
make the drop spread out so that its centre of gravity may be lowest. Hence, a
large drop of a heavy liquid spreads out when placed on a glass plate.
Therefore a large drop of mercury is always flat.
Capillarity
When
a capillary tube is immersed in water, and then placed vertically with one end in the liquid, observation shows that the
Water rises in the tube to a height above the surface. The narrower the tube,
the greater the height to which the water rises. This phenomenon is known as
capillarity, and it occurs when blotting – paper is used to dry ink. The liquid
rises up the pores of the paper when it is pressed on the ink.
Molecular forces
There are two kinds of molecular forces
i) Adhesive forces
ii)
Cohesive forces
i) Adhesive forces
Forces
of attraction between molecules of different substances are known as adhesive
forces. For instance, the force of attraction between the glass molecules of a
beaker and molecules of water contained in it is an adhesive force. Adhesive
force is different for different pairs of substances.
ii) Cohesive forces
Forces
of attraction between molecules of the same substance are called cohesive
forces. This varies inversely probably as the eighth power of the distance
between two molecules. Hence it is very appreciable when the distance between
two molecules is small. It is the greatest in solids, less in liquids and the
least in gases. Therefore a solid has a definite shape, a liquid has a definite
free surface and a gas has neither.
Application of surface tension:
o Since the surface
tension of oil is smaller than that of water therefore oil spreads on water
this property is used to calm the stormy waves at sea. The surface tension of a soap solution is less so it can
spread over large areas and wash clothes effectually.
o
In
soldering addition of flux reduces the surface tension of molten tin-hence it
spreads.
o Antiseptics like Dettol have low surface tension so that they spread faster.
o
Surface
tension prevents water from passing through the pores of an umbrella.
o
A
duck can float on water as its feathers secrete oil that lowers the
surface tension of the water.
Different Techniques to Calculate Surface Tension:
Drop weight method
A clean and dry glass tube of
suitable diameter is fitted to a glass fennel through a rubber tube provided
with a pinch clip. Water is poured in the funnel and the pinch clip is adjusted
so that water drops are formed at the rate of about five drops per minute. A
small empty beaker is weighed and fifty drops of water are collected in it. The
weight of the beaker is determined again. The difference between the weights gives
the mass of fifty drops.
Capillary rise method
The
capillary tube is cleaned well and dried before use. Through one of the holes
of the two-holed rubber cork, the capillary tube is inserted a knitting needle
is passed through the other hole. The rubber cork is fixed to a retort clamp
such that the needle and the tube are held vertically side by side. The given
liquid is taken in a beaker and kept well below the rubber cork. The capillary
tube is adjusted to be immersed in the liquid. The rubber tubing attached to
the top and of the capillary tube is pressed and relished repeatedly. The
liquid rises in the tube and completely wets the tube when the rise of the level of
liquid is complete, the knitting needle is so adjusted that the tip just
touches the liquid surface.
The microscope is focused to see the
liquid level clearly. Its horizontal cross-wise is made tangential to the
liquid meniscus and the reading on the vertical scale of the microscope is
taken. The beaker of liquid is removed gently without disturbing the tube or
the knitting needle. Now, the microscope is focused until the tip of the needle
just touches the horizontal crosswise. The reading of the vertical scale gives
the height (h) of the liquid rising in the tube. The experiment is repeated
several times by adjusting the levels of liquid in the breaker to be different.
The
radius of the capillary tube may be found by microscope by holding the tube in the
rubber cork in a horizontal position on founding one end of the tube, the bore
of the tube is observed as shown. Both horizontal and vertical diameters are
measured and its mean value is obtained. Using the other end of the tube,
measurements for the diameter can be repeated.
Searl’s torsion balance method:
It is one of the simplest and
quickest methods to determine the surface tension of liquids.
The
Searl’s tension apparatus consists of a rigid rod, fixed to a fine torsion wire.
The rod terminates in a pointer, moving over a vertical scale at one end and
carrying a sliding weight at the other.
A vessel containing the experimental liquid is placed on an
adjustable, table that can be raised up or lowered down as desired. To start
with, the vessel is raised up until the wireframe dips into it or until this
lower edge of the wireframe or the plate, lies exactly a level with the
liquid surface. A film of the liquid is thus formed in the wireframe
exacting a downward pull on the frame due to the surface tension. The vessel is
removed from the under, the frame or the glass plate. The liquid film disappears and with it also the downward pull, on the frame or the plate and
hence the rod returns back to its initial position.
Weights are placed on the scale pan till the pointer end of
the rod is again deflected downward to the same extent. This downward pull due to
weight is the same as the downward pull due to surface tension.
Jaeger’s method:
We know that the excess pressure
inside an air bubble in a liquid is equal to 2T/r. By measuring this pressure
needed to produce a bubble and radius, the surface tension (T) of the liquid
can be easily determined. The apparatus consists of a long thin glass table AB
with its lower portion ending (B) in a fine jet of about 0.2 to 0.5mm in
diameter. The experimental liquid is contained in a vessel, about 4 to 5 cm the
tube AB is placed inside the liquid.
Due
to capillary action, some liquid rises up into the tube and some air is now
forced into the tube. The bubble is formed there. The radius of the bubble is
equal to the lower portion ending B. The pressure inside will be indicated by the
difference of levels (H) in the two limbs of the manometer.
Quincke’s Method:
This method consists of a glass top,
two knitting needles and a microscope. The glass top is adjusted to be perfectly
horizontal using a sprit level. A large drop of mercury is formed on the glass
on top. One of the knitting needles is held vertically by a stand such that its tip
just touches the upper surface of the mercury. The second needle is adjusted to
be vertical by the side of the drop, to make it just touch the glass
top. The reading (B) of the vertical, scale was noted. The microscope is next adjusted suitably in succession to make the
horizontal crosswise coincide with the tip of the needle touching the glass
top and the surface of the mercury drop. Let the reading on the vertical scale
be A and C respectively. The difference between the reading A and C gives the
weight (H) of the drop.
Rayleigh’s jet method:
A liquid jet issuing horizontally out of an
orifice shows a strange recurrence of forms in its surface phenomenon in which
surface tension plays its own part. We find the initial formative stage, its
various parts are in motion for each other its form oscillating
this way and that about a mean spherical one with its lateral dimensions
measured along a particular direction exhibiting a cyclic change.
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