Viscosity
When two solid surfaces slide over each other, a frictional force acts between them that opposes the relative motion of the bodies. Similarly, when a layer of a liquid slips on another layer the two exert a tangential force on another. This opposes their motion. The property of a liquid by virtue of which, a tangential force acts so as to oppose relative motion between its layers, is called viscosity.
For example, if we were to stir equal amounts of water and glycerine kept inside a beaker, we find glycerine settles down faster than water. This means that the neighbouring layers in glycerine, oppose each other's motion to greater extent when compared to water.The external force required to maintain the relative velocity between different layers of the fluid, is a measure of the viscosity of the fluid. Liquids and gases (i.e., fluids) exhibit viscosity.
Coefficient of viscosity
When fluids flow through pipes, the outermost layer touching the sides of the pipe, have minimum velocity. As one approaches the centre of the tube, the velocity of the innermost (or centre) layer is maximum. Therefore, a velocity gradient develops within the pipe. Mathematically, it is represented by where dv represents the 
difference in velocity of two layers that are separated by a distance.
Velocity profile
Layer 1 travels with maximum velocity and layer 4 travels with minimum velocity.
According to Newton, the tangential force between two layers, say A and B, separated by a distance dx, is directly proportional to the velocity gradient and also to the area of the layer.
where 'h' represents coefficient of viscosity of the fluid. The negative sign indicates that this force opposes the relative motion of the layers.
Definition of coefficient of viscosity
The coefficient of viscosity of a liquid is defined as the tangential viscous force, which maintains a unit velocity gradient between two parallel layers, each of unit area.
Drag and angle of flow
a) The air strikes the wind at a greater angle, and the drag is increased.
b) The air stirred up, or "burbles," and drag is very great.
- The air flows smoothly past the wing and the drag is very small.
- The air strikes the wing at a greater angle and the drag is increased.
- The air is stirred up, or the air 'burbles' and the drag is very great.
Applications of viscosity
- The knowledge of viscosity and its variation with temperature, helps us select a suitable lubricant for a given machine.
- Viscosity of organic liquids such as proteins and cellulose, helps us calculate their molecular weight and determine their shape.
- The knowledge of viscosity helps in studying the circulation of blood through arteries and veins.
- The charge on the electron was determined by Millikan from the knowledge of viscosity.
