The physical quantity ‘viscosity’ gives information on how thick a fluid is and how easily it flows. In scientific terms, viscosity is the measure of a fluid’s internal flow resistance. It is the resistance which a fluid shows when being deformed.

In order to determine a fluid’s viscosity, you have to enter the field of viscometry, a subject area of a wider science called rheology. Rheology deals with the flow behavior and deformation of materials.

Imagine all materials as classified on a virtual scale from solid to liquid. Scientists specify solid materials as being elastic and liquids as being viscous. In everyday life, we mostly come across viscoelastic materials. That is, substances which are neither completely elastic, nor entirely viscous. According to a material’s properties, we either classify it as a viscoelastic solid (like e.g. sweet jelly) or as a viscoelastic liquid (like e.g. a yoghurt drink or shower gel).

The specific field of viscometry covers ideally viscous fluids, and – considering certain restrictions – also viscoelastic liquids, i.e. viscous fluids that contain an elastic portion. Fluids which flow easily show a low resistance to deformation. They are low-viscosity fluids. High-viscosity fluids resist deformation. Consequently, they do not flow easily.

What influences flow behavior?

- The substance's molecular structure

- The shear rate

- External conditions, like temperature or pressure

The dimensions of dynamic viscosity are force × time ÷ area. The unit of viscosity, accordingly, is newton-second per square metre, which is usually expressed as pascal-second in SI units.

Source: Official website of Anton-Paar and Britannica.com

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