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: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
Many fluids such as honey have a wide range of viscosities.
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