viskozit

Viscosity, or viskozit in some languages, is a measure of a fluid’s resistance to gradual deformation by shear or tensile stresses. It reflects how easily a liquid flows and how much internal friction it exhibits when layers slide past one another. Viscosity is most often described in terms of dynamic viscosity (mu) and kinematic viscosity (nu). Dynamic viscosity, measured in pascal-seconds (Pa·s), quantifies the internal friction per unit area. Kinematic viscosity, in square meters per second (m^2/s), equals dynamic viscosity divided by the fluid density.

Common units include the Poise and the centipoise; 1 Pa·s equals 10 Poise, and 1 centipoise (cP) equals 0.001 Pa·s. For reference, water at 20 °C has a dynamic viscosity of about 1.0×10^-3 Pa·s (1 mPa·s) and a kinematic viscosity around 1.0×10^-6 m^2/s (1 cSt).

Viscosity behavior varies among fluids. Newtonian fluids have a constant viscosity independent of the shear rate, examples being water and many mineral oils at a given temperature. Non-Newtonian fluids change viscosity with shear rate or time; examples include honey (shear-thinning), ketchup (shear-thickening), and some paints (Bingham plastics).

Temperature has a strong effect: for liquids, viscosity typically decreases as temperature rises; for gases, viscosity can increase with temperature. Impurities, pressure, and molecular structure also influence viscosity.

Measurement methods include capillary viscometers (such as Ubbelohde), which time the flow through a capillary under gravity, and rotational viscometers or rheometers, which apply controlled shear and measure resistance. Viscosity is a key parameter in designing lubrication systems, chemical processes, food products, polymer processing, and many natural and industrial flows.