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How are Newtonian fluids different from non-Newtonian fluids?

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differential manometer, Torricellian barometer, and siphon — fluid mechanics Figure 1: Schematic representations of (A) a differential manometer, (B) a Torricellian barometer, and (C) a siphon.

In fluid mechanics, the distinction between Newtonian and non-Newtonian fluids is based on how their viscosity—or internal resistance to flow—responds to shear stress and time.12

Newtonian Fluids

Newtonian fluids are defined by a constant viscosity that remains unchanged regardless of the rate of shear or the duration of the applied stress. For these fluids, the relationship between the shear stress and the rate of shear strain is linear. This behavior is characteristic of simple liquids and gases, where the internal friction is predictable and follows the mathematical description first formulated by Sir Isaac Newton.123

Non-Newtonian Fluids

In contrast, the viscosity of non-Newtonian fluids is not constant. Their behavior differs in two primary ways:1
* Shear-dependent: The viscosity changes depending on the rate at which the fluid is sheared.1
* Time-dependent: The viscosity changes over time even if the rate of shear remains constant.1

These fluids often contain complex structures, such as long-chain molecules that can become entangled, causing the fluid's resistance to flow to reflect its recent deformation history rather than just the instantaneous stress. An example includes thixotropic fluids, which become thinner and less viscous the longer they are stirred.31