Glossary term
Rheology
The study of flow and deformation of materials, especially complex fluids and soft solids.
Definition
termRheology is the study of how materials flow, yield, relax, and deform under applied stress or strain.
Rheology describes materials whose behaviour cannot be captured by a single constant viscosity or elastic modulus. It is used for polymers, slurries, paints, foods, gels, blood, drilling muds, pastes, molten metals, adhesives, and many process fluids where shear rate, temperature, time history, solids loading, and microstructure strongly affect flow and deformation.
Rheology extends fluid mechanics and solid mechanics to materials whose response depends on rate, time, structure, or previous loading. A Newtonian liquid has a constant viscosity at a given temperature. Many engineering materials do not: they may shear-thin, shear-thicken, yield only above a critical stress, recover elastically after flow, age at rest, or lose structure under mixing.
Important behaviours include apparent viscosity, yield stress, viscoelasticity, creep, stress relaxation, thixotropy, normal stress differences, gelation, and strain hardening. These behaviours determine whether a material can be pumped, sprayed, extruded, mixed, coated, cast, printed, stored, or cleaned from equipment.
Measurement
Rheological data are usually obtained with rotational rheometers, capillary rheometers, oscillatory tests, squeeze-flow tests, or process-line measurements. The test geometry and protocol matter. A viscosity value without shear rate, temperature, sample preparation, rest time, solids content, and measurement method is often unusable.
Oscillatory tests separate storage modulus G' and loss modulus G'' to describe elastic and viscous response. Flow curves report shear stress or apparent viscosity versus shear rate. Yield-stress measurements require special care because wall slip, evaporation, particle settling, and pre-shear history can dominate the result.
Engineering use
Rheology affects pump sizing, pressure drop, heat transfer, reactor mixing, valve selection, coating thickness, additive manufacturing, sedimentation, product texture, and quality control. For non-Newtonian fluids, a pressure-drop calculation based on water-like behaviour can underpredict motor power, dead zones, residence time variation, or cleaning difficulty.
Common mistakes
A common mistake is quoting “the viscosity” of a non-Newtonian material as if it were a fixed property. Another is measuring a sample after a mixing or rest history that does not match the process. A strong rheology review states composition, temperature, shear-rate range, preconditioning, instrument geometry, slip correction, time dependence, and the process condition the measurement is meant to represent.