The internal resistance in the flow of a liquid is called viscosity.
Liquids have the ability to flow, but different liquids have different rates of flow. Some liquids like honey Mobil oil etc. flow slowly and are called viscous liquids while ether, gasoline etc. which flow quickly are called less viscous.
The viscosity of liquid can be understood by considering a liquid in a tube, a liquid in a tube is considered as made up of a series of molecular layer. The layer of the liquid in contact with the walls of the tube remains stationary and the layer in the center of the tube has highest velocity as shown.
Each layer exerts a drag on the next layer and causes resistance to flow.
Factors on Which Viscosity Depends
Size of Molecules
The viscosity of a liquid depends upon the size of its molecules. If the size of the molecules is bigger the viscosity of the liquid is high.
Shape of Molecules
Shape of the molecules affects the viscosity. If the shapes of the molecules are spherical they can move easily but if the shapes of the molecules are irregular such as linear or trigonal then the molecules will move slowly and its viscosity will be high.
If the force of attraction between the molecules of a liquid is greater the viscosity of the liquid is also greater.
Viscosity of a liquid decreases with the increase of temperature.
Units of Viscosity
Viscosity of a liquid is measured in poise, centipoise or millipoise & S.I unit.
1 poise = 1 N.s.m(-2)
1 centipoise = 10(-2) N.s.m(-2)