Surface Tension

Surface tension of a liquid is defined as the tangential force per unit length acting at right angles on an imaginary line drawn on the surface of the liquid. Its unit is Newton per Metre.

Understanding Surface Tension

Insects like ants, water-spider are able to walk on the surface of water. Mosquitoes sit and move freely on the surface of stagnant water. When we sprinkle water at the roots of trees and shrubs, the sprinkled water gradually rises to their branches upwards. All these observations can be explained on the basis of a property of liquids called surface tension. When we take a clean glass plate and place a very small amount of mercury on the plane surface, we observe that the mercury assumes the form of a spherical drop. However, when we place large amount of mercury on the plane surface, we observe that now mercury assumes ellipsoidal shape. Similarly, when we place a greased sewing needle carefully on a water surface, the sewing needle makes a small depression in the surface and keeps floating even though the density of the needle is very much greater than that of water. A tumbler is filled to the brim with water. Some nails are put inside the water so that water is displaced upwards. A few more nails are added carefully. It is found that water surface rises well above the edge of the tumbler but water does not overflow. This is because the water surface stretches as water is displaced upwards. If a brush is dipped in water its bristles spread out. If it is taken out the bristles come closer and cling together.

The conclusion from the above observations is that there exists a tension on the surface of a liquid which tends to contract the surface to a minimum area. This property of the liquids is known as surface tension.

Adhesive and Cohesive Forces

Surface Tension is essentially a molecular phenomenon. There are two types of molecular forces of attraction viz. adhesive force and cohesive force. Forces between molecules of different substances are called adhesive forces. The adhesive force is different for different pairs of substances. Gum or glue is an adhesive. The force of attraction between gum and paper is an adhesive force. Forces between molecules of the same substances are called cohesive forces. The cohesive forces are short range forces and therefore they are effective only up to a very small distance. The adhesion of water to glass is stronger than the cohesion of water. On the other hand, the cohesion of mercury is greater than its adhesion to glass. The maximum distance at which the molecules can attract each other is called molecular range. The molecular range is of the order of 10⁻⁸ cm.

How Surface Tension works?

A sphere drawn with the molecule as centre and radius equal to the molecular range is called the sphere of molecular influence. The molecular forces are effective within this sphere of molecular influence. Therefore all the molecules lying within this sphere of molecular influence exert a force of attraction on the molecule at the centre. These molecular forces are responsible for surface tension. On the basis of this, Laplace gave an explanation of the surface tension.

In the above diagram, PQ represents the free surface of a liquid in a container. Let A, B and C represents molecules with their spheres of influence drawn around them. The sphere of influence around the molecule A is well within the free surface PQ. Hence it is equally attracted in all directions by the molecules in the sphere of influence. Therefore the resultant force acting on the molecule A is zero.

In the case of molecule B the sphere of influence is partly outside the liquid surface PQ. The number of molecules in the upper half is less than that in the lower half. Thus the resultant force on B acts in the downward direction.

The molecule C is exactly on the free surface PQ. The sphere of influence around the molecule C is exactly half outside and half inside the liquid. Hence this molecule C is attracted in the downward direction with maximum force. Thus we conclude that the molecules in the surface PQ are pulled downwards due to the resultant cohesive force. This makes the free of the liquid at rest behave like a stretched elastic membrane. This force gives rise to the surface tension of the liquid.

To understand this, we can make a circular wire ring in which a loop of thread is attached as shown in the adjacent diagram. The wire and thread are dipped in a soap solution and taken out gently. We see that a film of the soap solution is formed across the ring. The zig-zag loop of the thread lies on the film. If the film inside the loop of thread is punctured with a needle, then the loop takes the shape of a circle due to surface tension. The surface of the liquid film pulls the thread radially outward as shown by the arrows.

Capillary Action

A glass tube with a very fine uniform bore is called a capillary tube. When a capillary tube is dipped vertically into a liquid contained in beaker, the liquid immediately rises or falls in the tube.

The rise or fall of a liquid in a very narrow capillary tube is given by

Where:

T is the surface tension of the given liquid.

• r is the radius of the capillary tube
• ρ is the density of the liquid
• g is acceleration due to gravity
• θ is the angle of contact for the given pair of solid and liquid

The angle of contact is defined as the angle between the tangent to the liquid surface at the point of contact and the solid surface inside the liquid. The angle can be acute or obtuse. If the angle of contact is acute, the level of liquid inside the capillary tube is higher than that in the beaker. This capillary rise is observed in the case of water. If the angle of contact is obtuse, the level of liquid inside the tube is lower than that in the beaker.

This capillary fall is observed in mercury (θ =140°).

For water in silver tube, θ =90° and h = 0. The level of liquid remains the same.

For pure water and clear glass θ = 0°

Applications of Capillary Action in daily life

• The rise of sap in trees and plants: The Xylem or Bark has such structure that the water rises to reach from roots to leaves via capillary action, although some other theories also persist to explain this.When the bark of a tree (Xylem) is removed in a circular fashion all around near its base, it gradually dries up and dies because water from soil cannot rise to aerial parts.
• The rise of kerosene or oil in the wick of an oil lamp or stove.
• The absorption of ink in a blotting paper.
• Sandy soil is gets drier earlier than clay: The interspaces between the particles of the clay form finer capillaries and water rises to the surface quickly.
• The purpose of applying soap to clothes is to spread it over large area. When soap is dissolved in water the surface tension of water is lowered. Surface tension always opposes the spreading of a liquid. By reducing surface tension we facilitate the liquid to spread over larger surfaces. This is why soap is used for washing.
• For the same reason the paste spreads more freely in the mouth and facilitates cleaning of the mouth.
• When we pour oil on the surface of water it lowers the surface tension of water. Hence the mosquito breed sinks down and perishes.
• In voyage at the high seas, when there are violent waves the sailors pour tins of oil around their boats or ships. Due to oil the surface tension of sea water is reduced thereby the height of water waves is also reduced.
• A pen nib is split at the tip to provide the narrow capillary and the ink is drawn upto the tip continuously.
• When molten lead is allowed to fall through the end of a narrow tube, lead drops assume spherical shape due to surface tension. In factories lead shots are manufactured in this way.
• Rain drops assume spherical shape due to surface tension of water.