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 25C (standard state).

The viscosity of fluids is either given as absolute or dynamic viscosity η (1 Pa·s = 1 Ns/m2 = 1 kg/ms) 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/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.

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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