Newtonian Fluid – Definition, Examples, Industrial Applications

By BYJU'S Exam Prep

Updated on: September 25th, 2023

A Newtonian Fluid is one whose viscosity is unaffected by shear rate: when all other factors are equal, flow speeds and shear rates have no effect on viscosity. Newtonian fluids include air and water. Some liquids, on the other hand, have viscosities that alter with shear rate. When force is applied to Newtonian liquids, such as water, their viscosity does not change. When force is applied to non-Newtonian fluids like ketchup, their viscosity changes. 

Before we go into the details of a Newtonian fluid, let’s define fluid. Any liquid, gas, or generally any material that, when at rest, cannot withstand a tangential, or shearing, force and undergoes a continual change in shape when subjected to such stress is referred to as a fluid. Flow is a characteristic attribute of fluids that describes the continuous and irreversible change in position of one portion of a material relative to another when it is subjected to shear stress. Let’s take a closer look at Newtonian fluid, the definition of a Newtonian fluid, and Industrial Applications along with appropriate examples.

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What is Newtonian Fluid?

Newtonian Fluid Definition

Newtonian fluid is defined as the fluid which will have viscous stresses generated from its flow that are linearly proportional to the local strain rate, the rate at which its deformation changes over time at every site. The rate of change of the fluid’s velocity vector determines the stress.

Newtonian Fluid Meaning

A fluid is a Newtonian fluid only if the tensors describing viscous stress and strain rate are coupled by a constant viscosity tensor that is independent of the flow’s stress state and velocity. The viscosity tensor is reduced to two real coefficients reflecting the fluid’s resistance to continuous shear deformation and continuous compression or expansion, respectively, when the fluid is also isotropic (mechanical properties are the same in all directions).

The simplest mathematical models of fluid flow that account for viscosity are Newtonian fluids. While no real liquid or gas exactly meets the description, many common liquids and gases, like water and air, can be considered to be Newtonian for practical calculations under normal circumstances. Non-Newtonian fluids, on the other hand, are rather frequent and include oobleck (which stiffens when vigorously sheared) and non-drip paint (which becomes thinner when sheared).

Newton’s Law of Viscosity

The relationship between the shear stress and the shear rate of a fluid subjected to mechanical stress is defined by Newton’s law of viscosity. The viscosity of the Newtonian Fluid is a constant for a given temperature and pressure and is defined as the ratio of shear stress to shear rate. Mathematically,

τ= μ(du/dy)


  • τ is the shear stress;
  • μ is the viscosity, and
  • (du)/(dy) is the shear rate.

Newtonian Fluid Example

Newtonian fluids are single-phase fluids made up of tiny molecules in general (but not always). In the case of Newtonian Fluids Viscosityof the Newtonian fluid is solely a function of the fluid’s condition, particularly its temperature. Here is an example of Newtonian fluid.

  • Water, sugar solutions, glycerin
  • silicone oils, light hydrocarbons, air
  • alcohol, glycerol
  • thin motor oil, and other gases

Industrial Applications of Newtonian Fluid

The simplest mathematical models of fluids that account for viscosity are Newtonian fluids. While no real liquid or gas exactly meets the description, many common liquids and gases, like water and air, can be considered to be Newtonian for practical calculations under normal circumstances. The common applications of Newtonian and Non-Newtonian fluids in the industry are listed below.


Newtonian fluids are ideal for lubrication applications. Even though they alter viscosity as a function of temperature, these fluids do not change viscosity as a function of shear. A good example is motor oil. In a nutshell, it remains fluid at lower temperatures but doesn’t get too thin at higher engine operating temperatures. The important thing is that it does not change viscosity when it is shared between the engine elements. This is critical if it is to perform its function of separating parts and decreasing friction and wear. If the viscosity tumbled as a function of shear, it would be easily dislodged, and the metal components would destroy each other.


Non-Newtonian fluids, on the other hand, are ideal for coating applications. The viscosity of these fluids changes as a function of shear as well as temperature. We can obviously change the viscosity of the coating by heating or cooling it. Modern coatings’ shear-thinning feature, on the other hand, has numerous uses in the application process.

Take, for example, a spraying procedure. The spray gun orifice is known as he ultimate shear device. The addition of shear at this point in the dispense causes the coating to thin. This is advantageous since it is still at a low viscosity when it reaches the part, allowing it to flow out evenly and with a nice surface finish. The fluid will then recover from the nozzle’s shear.

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