Mechanics & Properties of Matter - Part 5

MECHANICS & PROPERTIES OF MATTER - PART 5

VISCOSITY

• The viscosity of a fluid is a measure of its resistance to gradual deformation by shear stress or tensile stress.
• For liquids, it corresponds to the informal concept of “thickness”;

For example:

• honey has a much higher viscositythan water

Flow of liquid through a pipe :

• Let us consider a liquid flowing through a pipe. There are two types of flow namely streamlined flow and tubulent flow.
• If all the particles of the liquid pass across a point with the same velocity, the flow is said to be stream lined. In this flow, a particle follows the same path throughout its motion.
• If the particles pass across a point with different velocities, the flow is turbulent. In this flow, a particle does not follow the same path throughout its motion.
• When a liquid flows slowly and steadily through a pipe, the velocity of the layer of the liquid in contact with the walls of the pipe is zero.
• As we move towards the axis of the tube, the velocity of the layers gradually increases and reaches a maximum value along the axis of the tube.
• In the case of streamlined flow of a river, the velocity is maximum for water on the upper layer (surface) of river.
• The velocity is minimum for water in the bottom most layer.
• When two parallel layers of a liquid are moving with different velocities, they experience tangential forces which tend to retard the faster layer and accelerate the slower layer.
• The unit of coefficient of viscosity is N s m^–2 or Poise.
• The values of coefficient of viscosity are different for different liquids.

Coefficient of viscosity of some fluids 

Fluid	Poise
Glycerine	13.4
Castor Oil	9.86
Olive Oil	0.84
Turpentine	0.015
Water	0.018
Mercury	0.0015
Honey	0.2
Blood	0.0027
Air	0.19 X 10-3

Applications of viscous fluids in daily life

• The motion of falling raindrops is opposed by the viscous force offered by air. Hence the rain drops falls slowly.
• The viscosity of sea water makes the waves subside during a storm.
• The motion of objects in fluids depends upon the viscosity of the fluids. The viscous force of water or air opposes the motion of ships, cars, aeroplane etc., Hence their shapes are streamlined in order to minimise the viscous drag on them.
• Friction reduces the efficiency of a machine by converting mechanical energy into heat energy and causes much wear and tear of the moving parts. Friction is reduced by using lubricants. A lubricant is a substance used to reduce friction. The lubricant forms a thin layer between the two surfaces in contact. It also fills the depressions present in the surfaces of contact and reduces friction considerably. In light machinery, thin oils (e.g., clock oil) with low viscosity are used. In heavy and fast moving machinery solids or thick highly viscous oils (e.g., grease) are used. By adding long chain polymers with lubricating
• oil, its coefficient of viscosity is kept constant even at high temperatures. 

A good lubricant should have the following

• It should be able to spread and fill up the minute depressions in the surfaces.
• It should be chemically inert and should not undergo any decomposition at high
• temperature.
• It should be capable of conducting away the heat produced by friction.
• If the arteries and veins of human body contract and become hard, their diameters decrease. Hence the flow of blood is affected due to the viscosity of blood and the blood pressure increases. This affects the functioning of heart. When the temperature of human body increases during fever, the coefficient of viscosity of blood decreases. This increases the blood circulation and the normal heart functioning is maintained.