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Revision as of 21:51, 20 February 2003 by Looxix~enwiki (talk | contribs) (disambiguate Mercury)(diff) ← Previous revision | Latest revision (diff) | Newer revision → (diff)Viscosity is a property of fluids describing their internal resistance to flow and may be thought of as a measure of fluid friction. Rheology is the field of science that deals with viscosity; viscosity is measured with a viscometer.
If the viscosity of a fluid is constant (neglecting temperature and pressure effects) it is said to be a Newtonian fluid. Non-Newtonian fluids exhibit a variation of viscosity depending on gradients within the flow field, the history that a fluid 'particle' experiences on its flow path, etc. If the viscosity of a fluid depends solely on the gradients within the flow field it is called generalized Newtonian or purely Newtonian.
The viscosity of fluids is either given as absolute or dynamic viscosity η (Pa.s = Ns/m = kg/ms) or as kinematic viscosity ν (m/s). Both terms are related via the fluid density ρ to each other: η = ν * ρ. The old smaller cgs physical unit for dynamic viscosity is poise after Jean Louis Marie Poiseuille (1797-1869): 1 poise = 100 centipoise = 1 g/cms = 0.1 Pa.s. The old unit for kinematic viscosity is stokes (in U.S. called stoke) after George Gabriel Stokes (1819-1903): 1 stokes = 1 cm/s = 0.0001 m/s.
Some dynamic viscosities of Newtonian fluids are listed below:
- ethyl alcohol 0.248 × 10 Pa.s
- acetone 0.326 × 10 Pa.s
- methanol 0.59 × 10 Pa.s
- benzene 0.64 × 10 Pa.s
- water 1.025 × 10 Pa.s
- nitrobenzol 2.0 × 10 Pa.s
- mercury 17.0 × 10 Pa.s
- sulfuric acid 30 × 10 Pa.s
- olive oil 81 × 10 Pa.s
- castor oil 985 × 10 Pa.s
- glycerin 1485 × 10 Pa.s
- pitch 10 Pa.s
Contrary to many assertions, glass is an amorphous solid, not a liquid, and it does not flow, but still we can talk about its viscosity. See the article on glass for more details on this.
Many fluids like honey have a wide range of viscosity.