Overview
Rheology is the study of how materials deform and flow when forces are applied. It treats the flow behavior of fluids and the slow, often time-dependent deformation of soft solids. Typical subjects include simple liquids, gels, pastes, polymer melts and biological fluids such as blood. The name derives from a root in Greek meaning "to flow," reflecting the field's focus on motion and deformation rather than only static stresses.
Key concepts
Central ideas are stress, strain, shear rate and viscosity — a measure of resistance to flow. Materials are classed as Newtonian when viscosity is constant, or non-Newtonian when apparent viscosity changes with shear rate or time. Time-dependent phenomena include thixotropy (structure breaks down under shear) and rheopexy (structure builds up). Many materials show viscoelasticity, storing energy like an elastic solid while dissipating energy like a viscous fluid.
Models and measurement
Constitutive models provide simplified descriptions: Newtonian, Bingham, power-law (Ostwald–de Waele) and linear viscoelastic models such as Maxwell and Kelvin–Voigt are widely used. Measurements employ rheometers and viscometers in modes ranging from rotational and capillary to oscillatory tests. Reported units for dynamic viscosity include pascal-seconds (Pa·s) or poise in some contexts.
History and scope
Rheology developed in the 20th century as researchers sought a unified language for liquids and deformable solids. It combines continuum mechanics, materials science and experimental methods to tackle everyday and industrial problems. The field interfaces with chemistry, biology, geology and engineering when material microstructure influences macroscopic flow.
Applications and practical use
Rheological properties guide formulation and processing across many industries: coatings and paints are designed to spread without sagging, foods are tuned for desirable texture, and polymers are processed reliably in extrusion and molding. In geoscience and civil engineering, rheology helps predict soil and mud behavior. In medicine and physiology, blood rheology informs circulatory studies and device design. Rheological testing is a routine quality-control and research tool to ensure consistent performance.
Distinctions and considerations
Rheology complements classical mechanics by focusing on constitutive behaviour and deformation history. Accurate interpretation requires attention to test conditions (temperature, shear history and timescale) because many complex fluids behave differently under changing conditions. Practical work often combines rheological data with microstructural or chemical information to understand and control material performance.