What is a non-Newtonian fluid?

A non-Newtonian fluid is a substance that does not follow Newton's law of viscosity because its viscosity changes when it is subjected to force or stress. Unlike Newtonian fluids, their relation between shear stress and shear rate is not linear, meaning a constant coefficient of viscosity cannot be defined. Common examples include ketchup, blood, starch suspensions, and toothpaste.

What we know

• A non-Newtonian fluid is a fluid that does not follow Newton's law of viscosity and has variable viscosity dependent on stress. [1, 2]
• The viscosity of non-Newtonian fluids typically depends on the shear rate or shear rate history. [1, 2]
• Common household substances like ketchup, toothpaste, shampoo, and starch suspensions exhibit non-Newtonian behavior. [2]
• Oobleck is a mixture of starch and water that demonstrates shear thickening by briefly transforming into a near-solid mass upon sudden impact. [2]

Still open

• Some non-Newtonian fluids with shear-independent viscosity still exhibit normal stress-differences or other complex behaviors, though the precise categorization under varying flow conditions can require advanced tensor-valued constitutive equations.

Where to go next

• the bigger pictureHow does fluid mechanics model forces on fluids?
• a case in pointWhat are everyday examples of non-Newtonian fluids?
• how it workedHow do rheometers measure fluid viscosity?
• compared withHow do Newtonian fluids differ in their stress-rate relation?
• whoWhat role did Sir Isaac Newton play in classical mechanics?
• an unexpected connectionHow do classical mechanics laws apply to both planets and fluids?Classical mechanics describes macroscopic objects ranging from vast planetary orbits to deforming fluids like molten polymers.

Sources

• [1]non-Newtonian fluid (Wikidata Q860589) · CC0 1.0
• [2]Non-Newtonian fluid · CC BY-SA 4.0
• [3]Fluid mechanics · CC BY-SA 4.0
• [4]Classical mechanics · CC BY-SA 4.0
• [5]Orbital mechanics · CC BY-SA 4.0