§ DICTIONARY · CONCEPT
Newtonian fluid
A fluid whose shear stress is strictly proportional to its velocity gradient, with viscosity independent of shear rate.
§ 01
Definition
A Newtonian fluid obeys τ = η·du/dy exactly, with η a constant that depends on temperature and pressure but not on the shear rate itself. Water, air, most gases, olive oil, honey, and liquid metals are all Newtonian across their normal operating ranges.
Non-Newtonian fluids don't. Ketchup and toothpaste are shear-thinning (yield stress plus pseudoplasticity); cornstarch slurry is shear-thickening (viscosity rises under stress, letting you literally run across a pool of it); polymer melts and blood are both viscoelastic. Non-Newtonian fluid mechanics is a substantially larger and harder subject.