Newtonian fluids have a constant viscosity (flow rate). Viscosity, a measure of a fluid's ability to resist gradual deformation by shear or tensile stress, in other words, a property that measures how easily a fluid moves: if the fluid has a high viscosity, the fluid will move more slowly; if the fluid has a low viscosity, Then it will move faster. Example: A fluid like water, unlike a fluid like honey or syrup, has a lower viscosity.
For Newtonian fluids, changing the force applied to the fluids does not change their viscosity. Viscosity remains constant as the applied force varies.
The viscosity of a non-Newtonian fluid changes according to the amount of force applied to the fluid. Viscosity changes with applied force.
Typically, the viscosity of non-Newtonian fluids depends on the shear rate or history of shear rates, or they have a viscosity that is independent of shear but still exhibit normal stress differentials or other non-Newtonian behavior.
In science, pressure means exerting force on the body. The result of this stress is described as strain.
In Newtonian fluids, the relationship between shear stress and shear rate is linear through the origin and the constant of proportionality is the coefficient of viscosity.
In non-Newtonian fluids, the relationship between shear stress and shear rate is different and can even be time-dependent (time-dependent viscosity) - a constant viscosity coefficient cannot be defined for non-Newtonian fluids as it might be for Newtonian fluid.
In the case of non-Newtonian fluids, the concept of viscosity, which is commonly used in fluid mechanics to characterize the shear properties of fluids, is not quite adequate. Instead, it is recommended to study them in terms of several other rheological properties related to stress and strain rate tensors under many different flow conditions measured using different devices or rheometers.
Comparing the viscoelastic, non-Newtonian and Newtonian properties, we can observe that:
| Viscoelasticity | Kelvin material, Maxwell material | "Parallel" linear combination of elastic and viscous effects | some lubricant, whipped stuffing, silly putty |
| Viscosity over time | Rheology | Apparent viscosity increases with stress duration | printing ink, gypsum powder |
| Thixotropic | Apparent viscosity decreases with stress duration | Yogurt, xanthan gum solutions, iron oxide hydrogels, gelatin gels, pectin gels, synovial fluid, hydrogenated castor oil, some clays, suspensions of carbon black in molten tire rubber, some drilling muds, many paints, many colloids suspension | |
| time-independent viscosity | Shear thickener (thinner) | Apparent viscosity increases with increasing stress | Suspension of cornstarch in water (Oobleck), sand in water |
| Shear thinning (pseudoplasticity) | Apparent viscosity decreases with increasing stress | Nail polish, whipped cream, ketchup, syrup, pulp in water, latex paint, ice, blood, some silicone oil, and some silicone coating | |
| Generalized Newtonian fluid | Viscosity is constant. The stress depends on the normal and shear strain rates and the pressure applied on it | Plasma, Custard, Water |
Understanding the behavior of non-Newtonian fluids has important applications:
• Disaster prevention - planning where to build a house or building based on the soil of the terrain
• Security - create body armor/armor that behaves like a non-Newtonian fluid for police or military use
• Medicine - technology to learn more about our bodies and improve health