Viscositat

Viscositat, or viscosity, is a property of fluids that describes their resistance to gradual deformation by shear or tensile stresses. It arises from internal friction between molecular layers as they slide past one another. The dynamic viscosity, denoted μ, is usually measured in pascal-seconds (Pa·s). In practice viscosity is often expressed in poise or centipoise, where 1 Pa·s equals 10 poise and 1 cP equals 0.001 Pa·s.

Kinematic viscosity, ν, is defined as μ divided by the fluid density (ν = μ/ρ) and has units of square metres per second (m^2/s). It characterizes how fast a fluid spreads under gravity when inertia is small relative to viscous forces.

Viscosity generally varies with temperature: for liquids, viscosity decreases as temperature rises; for gases it increases with temperature. Fluids can be Newtonian, with viscosity independent of shear rate, or non-Newtonian, whose viscosity changes with shear rate. Non-Newtonian examples include shear-thinning fluids, where viscosity decreases with shear, and shear-thickening fluids, where it increases.

Viscosity is measured with viscometers, such as capillary viscometers and rotational viscometers, which report dynamic or apparent viscosity depending on test conditions. In engineering practice viscosity controls flow resistance, pressure drop, and heat transfer, and enters dimensionless numbers such as the Reynolds number (Re = ρ v L / μ). This affects lubrication, coating, polymer processing, and many industrial operations.

Common materials illustrate the range: water has low viscosity (about 1 mPa·s at room temperature), while substances like honey or syrup have much higher values. Accurate viscosity data are essential for design, quality control, and modeling of fluid systems.