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In this article, we will discuss viscosity and its types.

What is Viscosity?

Viscosity is a measure of the fluid's resistance to flow. It explains the internal friction of a flowing fluid. A fluid with a high viscosity is resistant to motion because its molecular composition provides a lot of internal friction. Low viscosity fluid flows easily because its molecular structure results in less friction when it is in motion.

In other words, we can say that "The viscosity of the fluid is a measure of resistance to deformation at a certain rate".

Formula of Viscosity

Viscosity is calculated by the ratio of shear stress to the gradient of velocity in the fluid. If the sphere is dropped into the fluid, we may calculate the viscosity by using the following formula:

η = 2ga2(?p)/gv

Where η (eta) is defined as the viscosity,

?p is the density between the fluid and sphere tested,

a is the radius of the sphere,

v is the velocity of the sphere,

g is the acceleration because of gravity.

The SI unit of viscosity is Pascal seconds (Pa s). As you'll see in this equation, if the spere's speed is lower, the viscosity would be higher.

How to Measure Viscosity?

Viscosity can be measured by a viscometer, a piece of equipment that measures the force necessary to move through a liquid. Or it can be measured with household equipment, such as calculating how fast a liquid can travel on a slight incline after a certain period of time.

Types of Viscosity

There are two different viscosity measures used to describe fluids, dynamics and kinematic viscosities. They define the flow of the fluid in various ways, i.e., how they are measured. They are synonymous if the fluid density is known. Viscosity has two types that are Dynamic and Kinematic Viscosity.

Dynamic Viscosity

Dynamic viscosity determines the shear stress ratio of the fluid to the shear rate. The basic formula of Dynamic viscosity is:

μ = γ/τ

The measures of dynamic viscosity are force × time/area. The SI unit of dynamic viscosity is pascal second (Pa s).

Kinematic Viscosity

Kinematic viscosity determines the ratio between the viscous force and the fluid inertial force. In the equation, when the fluid density is known, it can also be used to convert between dynamic and kinematic viscosity. Kinematic viscosity is similar to mass and heat diffusivity, which is momentum diffusivity. The basic formula of Kinematic Viscosity is:

V = μ/p

Many of them are confused between the two viscosity tests and consider them to be the same. They have many major variations between them. Kinematic viscosity is more important than dynamic viscosity for some applications.


The units of Kinematic and dynamic viscosity are as follows:

  • The kinematic viscosity unit is Stokes, named after Sir George Gabriel Stokes, a British physicist.
  • The stoke is defined as 1cm2/sec.

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