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Viscosity is the "thickness" or "thinness" of a fluid; it 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.

Generally, viscosity is measured at 25°C (standard state).

The viscosity of fluids is either given as absolute or dynamic viscosity η (1 Pa·s = 1 N·s/m2 = 1 kg/m·s) or as kinematic viscosity ν (m2/s). Both terms are related via the fluid density ρ to each other: . The old smaller cgs physical unit for dynamic viscosity is poise after Jean Louis Marie Poiseuille (1797-1869): 1 poise = 100 centipoise = 1 g/cm·s = 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.

It is possible to understand the units of viscosity by considering the force required to shear a fluid. If the viscosity is v then a force of v newtons per unit area is required to sustain a unit shear rate (shear rate is measured in m/s per m---or just s-1). Then the units of visosity are just Newtons per square meter per s-1. Put another way: force=viscosity*shear rate*area.

ASTM uses Cps.

Methanol is "thin", having a low viscosity, while vegetable oil is "thick" having a high viscosity.

Some dynamic viscosities of Newtonian fluids are listed below:

Gases (at 0 °C):

air 17.4 × 10-6 Pa·s
hydrogen 8.4 × 10-6 Pa·s
xenon 21.2 × 10-6 Pa·s

Liquids (at 20 °C):
acetone 0.326 × 10-3 Pa·s
benzene 0.64 × 10-3 Pa·s
castor oil 985 × 10-3 Pa·s
ethyl alcohol 0.248 × 10-3 Pa·s
glycerol 1485 × 10-3 Pa·s
methanol 0.59 × 10-3 Pa·s
mercury 17.0 × 10-3 Pa·s
nitrobenzol 2.0 × 10-3 Pa·s
sulfuric acid 30 × 10-3 Pa·s
olive oil 81 × 10-3 Pa·s
pitch 107 Pa·s
water 1.025 × 10-3 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 viscosities.

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