Dynamic viscosity (internal friction coefficient)
The dynamic viscosity η (η = "eta") is a measure of the viscosity of a fluid (fluid: liquid, flowing substance).
The higher the viscosity, the thicker (less liquid) the fluid; the lower the viscosity, the thinner (more liquid) it is.
SI unit of dynamic viscosity: [η] = Pascal-second (Pa*s) = N*s/m² = kg/m*s
Factors influencing dynamic viscosity
The dynamic viscosity η depends on the substance and its temperature and is given in dimensions of Pascal-seconds.
- The dynamic viscosity η drops off very quickly for liquids as their temperatures increase.
- Dynamic viscosity η increases for gases as temperature rises.
Relationship between dynamic and kinematic viscosity v
The kinematic viscosity ν (ν = "nu") is the dynamic viscosity of the medium η divided by its density ρ.
Equation: ν = η / ρ
SI unit of kinematic viscosity: [ν] = m²/s
Other units for kinematic viscosity
Informal but frequently used units are Stokes (St) and Centistokes (cSt). These do not correspond to the SI unit system (SI is the abbreviation of the French Système international d'unités):
1 Stoke (St) = 10-4 m²/s = 1 cm²/s
1 cSt = 10-6 m²/s = 1 mm²/*-s
Examples of kinematic and dynamic viscosity
Example table with viscosity values for kinematic and dynamic viscosity
|Liquids||η / mPa*s at 20°C||η / mPa*s at 0°C||ν / mm²/s at 20°C|
|Gases at 0°C; 1,013 hPa||η / μPa*s||ν / mm²/s|
Source: Horst Kuchling: Pocketbook of Physics; Published by Fachbuchverlag Leipzig, 16. Edition 1996; 2.) Lebensmittel- und Bioverfahrenstechnik [Food and Bioprocess Engineering], H.G. Kessler