Rheology

Rheology is the science of deformation and flow of matter. It investigates how materials respond to applied stresses as a function of time, rate, and history. The field spans liquids, suspensions, gels, polymers, food, cosmetics, soils, foams, and other complex materials, and it underpins both processing operations and product performance.

Key concepts include shear stress and shear rate, apparent viscosity, and the distinction between rate-independent (Newtonian) and rate-dependent, non-Newtonian behavior. Non-Newtonian fluids may exhibit shear-thinning or shear-thickening, yield stress, thixotropy, or time-dependent recovery. Viscoelastic materials display both viscous flow and elastic storage, described by moduli in oscillatory measurements.

Measurement techniques include rheometers and related instruments. Rotational rheometers measure torque and angular velocity to obtain flow curves; capillary rheometers assess extrusion through narrow passages; oscillatory rheometry characterizes viscoelastic properties via storage modulus G' and loss modulus G''. Micro-rheology uses tracer particles to probe local behavior.

Constitutive models link stress and deformation. The Newtonian model applies to simple liquids with constant viscosity. Non-Newtonian models include Bingham plastic, Herschel–Bulkley, power-law (Ostwald–de Waele), and Carreau-type relations, among others. These models capture yielding, shear-thinning, and other effects to predict behavior in processing.

Applications span polymer processing, food and beverage, cosmetics, oil recovery, geology, and materials science. Rheology informs formulation, quality control, processing design, and failure analysis by describing how materials respond under practical stresses and time scales.