Understanding Viscosity

In this video we take a look at viscosity, a key property in fluid mechanics that describes how easily a fluid will flow. But there’s more too it than that!

We’ll start by defining viscosity using Newton’s Law of Viscosity, that describes the linear relationship between the shear stress in the fluid and the strain rate, and how it relates to the dynamic and kinematic viscosities.

We’ll also explore how viscosity is dependent on temperature – the viscosity of liquids reduces with increasing temperature, but for gases temperature has the opposite effect. We can explain this by looking at what causes viscosity on the molecular level.

Another interesting aspect of viscosity that’s covered in the video is non-Newtonian fluids, like shear thinning or shear thickening fluids, for which the relationship between the shear stress and the strain rate is non-linear.

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You can probably guess that non-Newtonian fluids are the opposite of Newtonian fluids. When shear is applied, the viscosity of non-Newtonian fluids decreases or increases, depending on the fluid. The behavior of the fluid can be described in one of four ways:

  • Dilatant – Viscosity of the fluid increases when shear is applied. For example:
    • Quicksand
    • Cornflour and water
    • Silly putty
  • Pseudoplastic – Pseudoplastic is the opposite of dilatant; the more shear applied, the less viscous it becomes. For example:
    • Ketchup


A Newtonian fluid’s viscosity remains constant, no matter the amount of shear applied for a constant temperature. These fluids have a linear relationship between viscosity and shear stress.


  • Water
  • Mineral oil
  • Gasoline
  • Alcohol