Definition and distinction between Newtonian and non-Newtonian fluids

1. Newtonian fluid

Under a certain temperature and pressure, the fluid whose viscosity remains constant is a Newtonian fluid, and the flow curve obtained by plotting the shear stress against the shear rate is a straight line passing through the origin (Figure 1).

The definition area distribution of Newtonian fluid and non-Newtonian fluid Figure 1

Most pure solvents, low-viscosity liquids, and many low-molecular solutions and dilute dispersions are approximately Newtonian fluids.

2. Non-Newtonian fluid

Most fluids in practical applications, such as polymer melts, coatings, toothpaste, emulsions, suspensions, etc., the relationship between σ and γ does not conform to the above relationship, that is, the ratio of shear stress to shear rate is no longer a constant, the viscosity of the fluid varies with the shear rate. For these fluids, the apparent viscosity ηa is used to represent the viscosity value of the fluid under a certain shear rate (or shear stress) condition. These fluids are collectively referred to as non-Newtonian fluids.

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