30 Amazing Tricks of Surface Tension

SURFACE TENSION

Surface Tension :
It is the property of a liquid by virtue of which it has the tendency to have the area of its free surface minimum as if it were under tension like a stretched elastic membrane.
If a force F acts on an imaginary line of length l then T = F/l = Force/Length
Surface tension of a liquid decreases with the increase of temperature and becomes zero at critical temperature.
Some examples of surface tension:
If a needle is very slowly kept on the surface of water, it floats due to surface tension.
The addition of detergent or soap decreases the surface tension of water and thus increases the cleaning ability.
Bubbles of soap solution are big because addition of soap decreases the surface tension of water.
When kerosene oil is sprinkled on water, its surface tension decreases. As a result the larva of mosquitoes floating on the surface of water die due to sinking.

30 Amazing Tricks of Surface Tension

Warm soup is tasty because at high temperature its surface tension is low and the soup spreads on all parts of the tongue.
The smaller drops of mercury are spherical.
Cohesive force :
The force of attraction between the molecules of same substance is called cohesive force. This force is maximum in solids. This is why solids have a fixed shape. Cohesive force is negligible in case of gases.
Adhesive force : Force of attraction between the molecules of different substances is called adhesive force. Due to adhesive force, one body sticks to other.
If the cohesive force among the molecules of liquid is less than the adhesive force between the liquid and the vessel then that liquid wets the walls of the vessel.
Example: water wets the walls of the vessel.On the other hand if the cohesive force among the molecules of liquid is greater than the adhesive force between the liquid and the vessel then that liquid does not wet the walls of the vessel.Example:mercury does not wet the walls of the vessel.

30 Amazing Tricks of Surface Tension

The adhesive force between ink and paper is more than the cohesive force of the ink so at the time of writing, the ink sticks to the paper and writing is possible.
We can not clean the glass by silk .For this purpose, we use cotton. The reason is that the cohesive force of silk is greater than the adhesive force between water and glass.
The adhesive force between oil and water is less than the cohesive force of the molecules of the water but is more than the cohesive force of the oil so the water drops are spherical on the surface of oil while the oil drops expand on the surface of the water.
Capillarity: If a capillary tube is dipped in a liquid, liquid ascends or descends in this tube. This phenomenon is called capillarity.

30 Amazing Tricks of Surface Tension

The height by which liquid ascends or descends in a capillary tube depends upon the radius of the tube.

The capillarity depends on the nature of liquid and solid both. The liquid which wets the wall of tube rises in the tube and the liquid which does not wet the wall of the tube descends in the tube.

For example, when a glass capillary tube is dipped in water, water rises in the tube and shape of water is concave, similarly when a glass capillary tube is dipped in mercury, mercury decends in the tube and shape is convex.

A piece of blotting paper soaks ink because the pores of the blotting paper serve as capillary tubes.
The oil in the wick of a lamp rises due to capillary action of threads in the wick.
The root hairs of plants draws water from the soil through capillary action.
Action of towel in soaking up water from the body is due to capillary action of cotton in the towel.

30 Amazing Tricks of Surface Tension

Viscous force: The force which opposes the relative motion between different layers of liquid or gases is called viscous force.
Viscosity: It is the property of a liquid by virtue of which it opposes the relative motion between its different layers.
Viscosity is the property of liquids and gases both.
The viscosity of a liquid is due to cohesive force between its molecules.
The viscosity of a gas is due to diffusion of its molecules from one layer to other layer.
Viscosity of gases is much less than that of liquids. There is no viscosity in solids.
Viscosity of an ideal fluid is zero.
With rise in temperature, viscosity of liquids decreases and that for gases increases.
Viscosity of fluid is measured by its coefficient of viscosity.

30 Amazing Tricks of Surface Tension

Critical velocity: The maximum velocity up to which fluid motion is streamline is called critical velocity. Clearly, if the velocity of flow is below critical velocity, flow is streamline and of the velocity is above the critical velocity, flow is turbulent.

If the velocity of flow is less than critical velocity, the rate of flow of fluid depends on viscosity of fluid. If the velocity of fluid is more than critical velocity, the rate of flow depends on the density of fluid. Due to this reason, on eruption of the volcano the lava coming out of its flow very swiftly although it is very dense having large viscosity.