rheometry

Rheometry is the experimental field that measures how materials deform and flow under applied stresses or strains. It characterizes viscosity, elasticity, and time-dependent behavior, and how these properties depend on factors such as shear rate, temperature, and sample history. Rheometry is central to rheology, the broader science of flow and deformation.

Measurements are made with a rheometer, an instrument that imposes controlled deformation and records the material response. Rotational or shear rheometers use geometries such as parallel-plate, cone-plate, or concentric-cylinder configurations to apply shear and generate flow curves of viscosity or shear stress versus shear rate. Capillary rheometers assess high-shear behavior by forcing samples through narrow tubes; micro- and extensional rheometers probe small volumes.

Oscillatory rheology applies small-amplitude, sinusoidal deformations to measure viscoelastic properties, yielding the storage modulus G' and loss modulus G'' and the complex viscosity. Steady-shear tests yield viscosity as a function of shear rate; creep and recovery tests quantify time-dependent deformation under a constant stress or strain; temperature and time-temperature superposition expand data across conditions.

Rheometry informs material development and processing across polymers, coatings, foods, cosmetics, pharmaceuticals, and biofluids, as well as geophysics and industrial suspensions. It supports quality control, formulation optimization, and process design by providing material fingerprints and models of flow under processing conditions. Results are reported with temperature, geometry, gap, and the applied stress, strain, or frequency.

Standards and data interpretation often involve ISO and ASTM guidelines, along with model fitting to rheological constitutive equations. Common concepts include shear-thinning or thickening behavior and viscoelastic spectra described by G', G'', and the complex modulus.

---