nonNewtonian

Non-Newtonian refers to a broad class of fluids whose flow behavior cannot be described by a single constant viscosity. In Newtonian fluids, viscosity remains constant and the shear stress is proportional to the shear rate. In non-Newtonian fluids, the relationship between shear stress and shear rate can change with the rate of deformation, time, or both. Because of this, their apparent viscosity varies with how they are mixed, pumped, or stirred.

Non-Newtonian fluids are often categorized as rate-dependent or time-dependent. Rate-dependent fluids change viscosity with shear rate and include shear-thinning (pseudoplastic) materials, whose viscosity decreases with increasing shear, and shear-thickening (dilatant) materials, whose viscosity increases with shear. Some materials exhibit yield stress and are called Bingham plastics; they behave as a solid until the applied stress exceeds a threshold. Time-dependent fluids change viscosity over time under constant shear or rest, including thixotropic liquids (decrease in viscosity over time under constant shear) and rheopectic liquids (increase in viscosity under constant shear). Many materials combine these behaviors and also show elastic or viscoelastic response.

Common examples include ketchup (often shear-thinning), toothpaste and mayonnaise (shear-thinning), paints (shear-thinning and thixotropic), and blood (generally shear-thinning). A classic laboratory example is a cornstarch–water suspension, also known as oobleck, which becomes more solid under sudden impact (shear-thickening).

Applications span industrial processing, food science, cosmetics, and geophysics. Rheology, the study of flow and deformation, characterizes non-Newtonian behavior and informs equipment design, such as pumps and mixers. The term emphasizes the diversity of fluid behavior beyond the ideal Newtonian model.