reology

Reology, often spelled rheology, is the science of deformation and flow of matter under applied forces. It examines how materials such as liquids, suspensions, polymers, gels, and pastes respond to stress. Key concepts include viscosity, describing resistance to flow; viscoelasticity, the combination of elastic and viscous response; and non-Newtonian behavior, where viscosity depends on shear rate or history. Time-dependent phenomena such as thixotropy and rheopexy are studied.

Measurement and methods: Rheology uses devices called rheometers to apply controlled stress or strain and measure response. Rotational rheometers probe shear flow; capillary rheometers test high-shear flow; dynamic mechanical analysis (DMA) characterizes viscoelastic moduli via oscillatory deformation. Outputs include shear viscosity, storage modulus G', loss modulus G'', and complex viscosity.

Constitutive models: The Newtonian model assumes constant viscosity independent of stress. Non-Newtonian models include Bingham plastic (yield), Herschel–Bulkley (yield plus power-law), and simple power-law fluids. Linear viscoelastic models such as Maxwell and Kelvin–Voigt describe time-dependent stress, while generalized models capture frequency behavior.

Applications: Rheology informs formulation and processing of polymers, coatings, food, cosmetics, and pharmaceuticals; geophysics uses rheology to study geological fluids; biophysics examines blood and tissue mechanics; and materials science characterizes gels and pastes.

Origin and terminology: The term rheology derives from Greek rheein, to flow. Reology appears as an alternative spelling in some texts, but rheology is the standard term; many sources treat reology as synonymous.