Viskos

Viscosity, referred to as viskos in several languages, is a measure of a fluid’s resistance to flow and to deformation under shear. It affects how easily liquids pour, how they lubricate moving parts, and how they behave in pipes, pumps, and mixers. The property depends on temperature, pressure, and the fluid’s composition.

Two related quantities are commonly used: dynamic viscosity (μ) and kinematic viscosity (ν). Dynamic viscosity measures a fluid’s resistance to shear at a given temperature and is expressed in pascal-second (Pa·s). Kinematic viscosity is the ratio of dynamic viscosity to density (ν = μ/ρ) and has units of square meters per second (m^2/s). The combination of μ and the fluid’s density determines flow behavior in most engineering contexts.

For Newtonian fluids, the shear stress (τ) is proportional to the shear rate (γ̇), with τ = μ·γ̇. Non-Newtonian fluids deviate from this linear relation and can exhibit shear-thinning, shear-thickening, thixotropy, or yield stress, meaning their apparent viscosity changes with shear rate or time.

Temperature and pressure influence viscosity in characteristic ways. For liquids, viscosity generally decreases as temperature rises; for gases, viscosity typically increases with temperature. Empirical models such as the Arrhenius or Andrade equations describe these trends. Viscometers assess μ or ν; common instruments include capillary viscometers (e.g., Ostwald type) and rotational viscometers (e.g., Brookfield), among others.

Common reference values: water at 20°C has about 1.0 mPa·s; honey at room temperature can range from 2 to 10 Pa·s; engine oils vary roughly from 0.1 to 0.5 Pa·s depending on grade; air’s dynamic viscosity is about 1.8×10⁻⁵ Pa·s. These figures illustrate the wide range of viscous behavior across fluids.