viszkozitását

Viscosity is a property of liquids that describes their resistance to flow. Dynamic viscosity, denoted by η, is defined as the ratio of shear stress to shear rate in a flowing fluid and is measured in pascal-seconds (Pa·s). Kinematic viscosity, ν, is η divided by density ρ and is measured in square meters per second (m^2/s); the historical unit Stokes (St) equals 1 cm^2/s, or 1×10^-4 m^2/s.

Fluids can be Newtonian, with constant viscosity independent of shear rate, or non-Newtonian, where viscosity changes with shear rate, time, or stress. Shear-thinning fluids (pseudoplastic) decrease viscosity with increasing shear; shear-thickening fluids (dilatant) increase viscosity with increasing shear; some exhibit yield stress or thixotropy, where viscosity depends on the history of deformation.

Viscosity depends strongly on temperature; for most liquids, viscosity decreases as temperature rises. Pressure effects are typically smaller under ordinary conditions. Composition, presence of polymers, suspended particles, or dissolved substances can markedly alter viscous behavior and lead to complex, non-Newtonian responses.

Measurement and standards: Viscosity is determined with viscometers or rheometers. Capillary viscometers (such as the Ubbelohde type) and rotational viscometers are common methods. Reported values depend on temperature and, for non-Newtonian fluids, on shear rate. Standards include ASTM D445 and ISO 3104 for dynamic viscosity and related measurements.

Applications: Viscosity is critical in lubrication, engine oils, hydraulics, food and cosmetic formulations, chemical processing, and geophysical contexts such as magma flow. Typical reference values include water around 1 mPa·s at 20°C; honey often tens of Pa·s; mineral oils around 0.1–0.2 Pa·s at elevated temperatures.