viscosimetrie

Viscosimetry, or viscosimétrie in French, is the measurement of a fluid’s viscosity, the resistance to gradual deformation by shear or tensile stresses. It covers techniques that quantify dynamic viscosity (η) in pascal-seconds and, in many contexts, kinematic viscosity (ν) in square meters per second, often reported in centistokes (cSt) for liquids. Viscosity is influenced by temperature, composition, and molecular structure, so measurements are typically performed under controlled temperature and shear conditions.

Common methods include capillary viscometry, rotational viscometry, and falling-ball viscometry. Capillary viscometers (for example Ostwald and Ubbelohde) determine viscosity from the flow time of a liquid through a calibrated capillary. Rotational viscometers measure viscosity by imposing a known shear and recording the resulting torque, providing data over a wide range of shear rates. Falling-ball viscometry estimates viscosity from the terminal velocity of a sphere moving through a liquid inside a tube. Modern approaches also employ oscillatory rheometry to characterize viscoelastic behavior.

Key concepts include the distinction between Newtonian fluids, whose viscosity is constant with shear rate, and non-Newtonian fluids, whose viscosity depends on shear history. Temperature control is essential because viscosity decreases with increasing temperature and can change with sample age or composition.

Applications span quality control and research in petroleum, lubricants, food, cosmetics, and polymers. Standards for viscometry are maintained by organizations such as ASTM and ISO, with methods detailing apparatus, procedure, and data interpretation. Limitations include range constraints, sample contamination, air entrainment, and non-Newtonian effects that complicate interpretation.