Under graduate level physics project - SURFACE TENSION

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 so as 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 as 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.

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 stomy 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.