Viscosity

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/m² = kg/ms) or as kinematic viscosity ν (m²/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 (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 cm²/s = 0.0001 m²/s.

Some dynamic viscosities of Newtonian fluids are listed below:

Gases (at 0 °C):

hydrogen 8.4 × 10^-6 Pa·s

air 17.4 × 10^-6 Pa·s

xenon 21.2 × 10^-6 Pa·s

Liquids (at 20 °C):

ethyl alcohol 0.248 × 10^-3 Pa·s

acetone 0.326 × 10^-3 Pa·s

methanol 0.59 × 10^-3 Pa·s

benzene 0.64 × 10^-3 Pa·s

water 1.025 × 10^-3 Pa·s

nitrobenzol[?] 2.0 × 10^-3 Pa·s

mercury 17.0 × 10^-3 Pa·s

sulfuric acid 30 × 10^-3 Pa·s

olive oil 81 × 10^-3 Pa·s

castor oil 985 × 10^-3 Pa·s

glycerin 1485 × 10^-3 Pa·s

pitch[?] 10⁷ 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 such as honey have a wide range of viscosity.