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[?]. NonNewtonian 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^{2} = kg/ms) or as kinematic viscosity ν (m^{2}/s). Both terms are related via the fluid density ρ to each other: <math>\eta = \nu \cdot \rho</math>. The old smaller cgs physical unit for dynamic viscosity is poise[?] after Jean Louis Marie Poiseuille (17971869): 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 (18191903): 1 stokes = 1 cm^{2}/s = 0.0001 m^{2}/s.
Some dynamic viscosities of Newtonian fluids are listed below:
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 such as honey have a wide range of viscosity.
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