Viscosity:

Viscosity of a fluid is defined as a measure of its resistance to flow. The greater the viscosity of the fluid, larger is the force (or pressure) required to maintain the flow. For example, when water flows in a uniform horizontal tube, there is a fall in its pressure along the tube in the direction of flow. This is because work (force × displacement) needs to be done against viscosity. In other words, viscosity of fluids is similar to the frictional force encountered by solids in motion.

Figure: (a) Laminar Flow of Liquid; and (b) Motion of Two Layers P and Q of Liquid Separated by a Distance dx

To understand the effect of viscosity, let us assume a liquid flowing over a horizontal surface (Figure 15) in such a manner that the velocity of each layer is almost the same as that of the adjacent layer. That is, a velocity changes continuously. The velocity of the layer in contact with the surface (bed) is negligibly small and can be taken to be zero. As the distance increases from the bed, the velocity goes on increasing as shown in Figure(a). The velocity of a given layer is proportional to its distance from the stationary layer. The motion of a liquid characterized by these features is called laminar flow and is valid for flow of viscous fluids at low velocities.