Viskositeetit

Viskositeetit refers to viscosity values, describing a fluid's resistance to flow and deformation under shear. It depends on composition, temperature, and pressure, and affects how easily a liquid moves. In many fluids, viscosity also changes with shear rate, so measurement conditions must be specified when comparing values.

Two commonly used quantities are dynamic (shear) viscosity μ and kinematic viscosity ν. Dynamic viscosity has units of pascal-seconds (Pa·s) and measures internal resistance to shear. Kinematic viscosity, ν = μ/ρ, has units of m^2/s and is often reported in centistokes (cSt) (1 cSt = 1×10^-6 m^2/s). Water at room temperature has μ ≈ 1 mPa·s and ν ≈ 1×10^-6 m^2/s.

Viscosity is measured with viscometers. Capillary viscometers determine flow time through a tube; rotational viscometers measure torque on a spindle; falling-ball viscometers infer μ from ball descent. Temperature control is essential, as viscosity changes with temperature. For non-Newtonian fluids, a single value is not sufficient; specify apparent viscosity at a defined shear rate.

Viscosity typically decreases with temperature, following Arrhenius-like behavior for many liquids. Non-Newtonian fluids—such as polymer solutions and suspensions—exhibit shear-rate dependent viscosity, including shear-thinning or shear-thickening.

Understanding viscosity is crucial in lubrication, coatings, food, cosmetics, pharmaceuticals, and polymer processing, influencing pump design, mixing, pouring, and product stability. Modifiers and temperature control are common tools to achieve desired flow properties.