non-Newtonian flow

A non-Newtonian fluid is any liquid or semisolid whose viscosity is not a fixed number: it changes depending on how hard you shear it, or how long you keep shearing it. Ketchup that refuses to budge then suddenly gushes, paint that thins under the brush, and a gel that softens when stirred are all non-Newtonian. Most genuinely useful pharmaceutical products fall in this category.

Because the viscosity is not constant, a single number cannot describe these materials; instead you need a rheogram showing how shear stress and shear rate relate across a range, and the curve is not a straight line. The main families are pseudoplastic (shear-thinning), dilatant (shear-thickening), and plastic (flow only after a yield value is overcome). When the effect also depends on the duration of shearing rather than just its rate, the behaviour is called time-dependent, the classic example being thixotropy.

Non-Newtonian behaviour is usually a designed feature, not a defect. A shear-thinning suspension is thick at rest so particles do not settle, yet thins when shaken so it pours and can be swallowed; a topical gel is firm in the tube but spreads easily when rubbed. Formulators deliberately engineer these flow profiles using polymers, gelling agents, and structured dispersions.