Property of viscosity:

In laminar flow, a fast moving layer of liquid tends to accelerate the motion of a given layer adjacent and lower to it whereas the slow moving layer adjacent and lower to a given layer tends to retard its motion. Thus, there are two types of forces which act on a layer in opposite directions and consequently the layer of liquid is subjected to a shearing (or tangential) stress. (You will learn about shearing stress in the next section of this unit.) Forces acting on two adjacent layers tend to destroy the relative motion between them. This force is called the dragging force or the viscous force which is the characteristic of a particular liquid. The property by virtue of which a liquid opposes relative motion between its different layers is called viscosity.

All gases and liquids possess the property of viscosity. Gases offer a smaller resistance to flow than liquids do because the viscosity of gases is less than liquids.