What is viscosity ? Explain its causes.

Explanation

Viscosity is defined as 'the property of a fluid to oppose the relative motion between its layers'.
Let us consider a liquid flowing through a horizontal surface with two neighbouring layers. The uppe layer tends to accelerate the lower layer and in turn, the lower layer tends to retard the upper layer. As a result, a backward tangential force is set-up. This tends to destroy the relative motion. This accounts for the viscous behaviour of fluids.