Viscosity

Viscosity:

When we pour water from a glass, it flows freely and quickly. But when we pour syrup or honey, it flows slowly and sticks to the container. The difference is due to fluid friction. This friction is both within the fluid itself and between the fluid and its surroundings. This property of fluids is called viscosity. Water has low viscosity, whereas syrup or honey has high viscosity. In the diagrams below, note that there is no dragging force in the non-viscous flow, and all layers are moving with the same velocity. 

Viscosity is that property of fluid, by virtue of which, the relative motion between different layers of a fluid experience a dragging force. This force is called the viscous drag. In liquids, the viscous drag is due to short range molecular cohesive forces, and in gases it is due to collisions between fast moving molecules. In both liquids and gases, as long as the relative velocity between the layers is small, the viscous drag is proportional to the relative velocity. However, in a turbulent flow, the viscous drag increases rapidly and is not proportional to relative velocity but proportional to higher powers of relative velocity.

Velocity gradient:

The rate of change of velocity (dv) with distance (dx) measured from a stationary layer is called velocity gradient (dv/dx).

Coefficient of viscosity:

According to Newton’s law of viscosity, for a streamline flow, viscous force (f) acting on any layer is directly proportional to the area (A) of the layer and the velocity gradient (dv/dx) i.e., 

Note: ‘A’ in this expression is not the cross sectional area, it is the area of the layer, parallel to the direction of the flow. The coefficient of viscosity can be defined as the viscous force per unit area per unit velocity gradient. S.I. unit of viscosity is Ns/m2 .