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What is Viscosity?-Definition, And Types

August 17, 2022
written by Adeel Abbas

The fluid’s Viscosity is a measure of its resistance to change at a given rate. It matches the informal concept of “thickness” for liquids, for example, syrup has a higher viscosity than water.

What is Viscosity?

The internal frictional force between adjacent layers of fluid is quantified by the Viscosity. When a fluid is forced through a tube, it flows more quickly near the tube’s axis than it does near its walls.

Experiments show that a pressure difference between the two ends of the tube is necessary to sustain the flow. This is due to the fact that a force is needed to overcome the motion between the layers of the fluid. For a tube with a constant rate of flow, the strength of the compensating force is dependent on the fluid’s viscosity.

Viscosity Formula

F=\mu A{\frac {u}{y}}.

Here F


\mu the viscosity of the fluid

A area of each plate

{\frac {u}{y}}rate of shear deformation

Unit of Viscosity

These are some units of viscosity.

SI UnitPascal seconds (Pa·s) or kg·m−1·s−1
CGS UnitPoise (P)

Unit of Kinematic Viscosity

Si Unitsquare meter per second or m2/s
CGS UnitStokes (St)

Types of viscosity

There are two types of viscosity

  • Dynamic Viscosity 
  • Kinematic Viscosity

Dynamic Viscosity 

In materials science and engineering, one is interested in understanding the forces and stresses involved in a material’s change of shape. Hook’s law says that the force experienced by a spring is proportional to the distance displaced from equilibrium, so if the material were a simple spring, the answer would be given.

Elastic stresses are caused by the change of material from a rest state to a rest state. The stresses in other materials can be attributed to the rate of change in the material. These are the types of stresses that are called sclerosing stresses.

In a fluid such as water, the stresses which arise from shearing do not depend on the distance the fluid has been sheared; rather, they depend on how quickly the shearing happens.

Kinematic viscosity

Kinematic viscosity is a measure of a fluid’s internal resistance to flow under gravitational forces. It is determined by measuring the time in seconds, required for a fixed volume of fluid to flow a known distance by gravity through a capillary within a calibrated viscometer at a closely controlled temperature.