pseudoplastic

Pseudoplastic refers to a class of non-Newtonian fluids whose apparent viscosity decreases with increasing shear rate, a behavior known as shear thinning. In a plot of shear stress versus shear rate, the curve for a pseudoplastic fluid is sublinear, and the apparent viscosity η_a = τ/γ̇ decreases as γ̇ increases. The term is often used interchangeably with shear-thinning, though some materials exhibit additional time-dependent changes in structure (thixotropy) that can accompany pseudoplasticity.

Mathematically, many pseudoplastic fluids are described by the Ostwald-de Waele power-law model: τ = K γ̇^n, where K is the consistency index and n < 1 for pseudoplastic behavior. For n approaching 0, the fluid behaves more like a plastic; for n closer to 1, behavior is closer to Newtonian. The model captures the general trend of decreasing viscosity with higher shear rates, though real materials may deviate.

Molecular and microstructural mechanisms include alignment of elongated molecules or particles under flow, disruption of network structures, and breakage of interactions that resist motion. Common examples of pseudoplastic materials are ketchup, paints, many polymer solutions, toothpaste, and some blood flows. In practice, pseudoplasticity facilitates processing: the material pours or sprays more easily under high shear and recovers viscosity when the shear is reduced.

Measurement is performed with rheometers that apply controlled shear rates and record shear stress, from which the flow curve and apparent viscosity are derived. Pseudoplasticity is an important consideration in food, cosmetics, coatings, and petroleum industries, affecting mixing, pumping, extrusion, and application.