NichtNewtonschen

NichtNewtonschen, or non-Newtonian fluids, are substances whose flow cannot be described by a single constant viscosity as defined by Newton's law of viscosity. In these fluids the apparent viscosity depends on the shear rate, the time spent under shear, or both. As a result, they can become thinner or thicker when stirred, poured, or subjected to stress.

Common categories include shear-thinning (pseudoplastic) fluids, where viscosity decreases with increasing shear rate (ketchup, many paints); shear-thickening (dilatant) fluids, where viscosity increases with shear rate (suspensions of corn starch in water); and yield-stress fluids (Bingham plastics), which require a minimum stress to start flowing (toothpaste, some clays). Additional behaviors include thixotropy, where viscosity decreases over time under constant shear and recovers at rest, and rheopexy, where viscosity increases with time under shear.

Measurement and modeling rely on rheology. Flow behavior is described by viscosity versus shear rate curves obtained with rheometers. Models such as the power-law (Ostwald–de Waele), Herschel–Bulkley (yield-stress with shear-thinning), and Carreau or Cross equations are used to fit data and predict flow under different conditions.

Applications span industries and everyday life. Non-Newtonian fluids are important in food processing, cosmetics, paints, and plastics; in civil and petroleum engineering for drilling muds and cements; and in biomedicine, where blood exhibits shear-thinning behavior. Understanding their rheology is essential for processing, handling, and designing systems that rely on flow under varying stresses.