A simulation of liquids with different viscosities. The liquid on the left has lower viscosity than the liquid on the right.
Common symbols
η, μ
Derivations from other quantities
μ = G·t
Dimension
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The viscosity of a fluid is a measure of its resistance to deformation at a given rate.[1] For liquids, it corresponds to the informal concept of "thickness": for example, syrup has a higher viscosity than water.[2] Viscosity is defined scientifically as a force multiplied by a time divided by an area. Thus its SI units are newton-seconds per square meter, or pascal-seconds.[1]
Viscosity quantifies the internal frictional force between adjacent layers of fluid that are in relative motion.[1] For instance, when a viscous fluid is forced through a tube, it flows more quickly near the tube's axis than near its walls.[3] Experiments show that some stress (such as a pressure difference between the two ends of the tube) is needed to sustain the flow. This is because a force is required to overcome the friction between the layers of the fluid which are in relative motion. For a tube with a constant rate of flow, the strength of the compensating force is proportional to the fluid's viscosity.
In general, viscosity depends on a fluid's state, such as its temperature, pressure, and rate of deformation. However, the dependence on some of these properties is negligible in certain cases. For example, the viscosity of a Newtonian fluid does not vary significantly with the rate of deformation.
Zero viscosity (no resistance to shear stress) is observed only at very low temperatures in superfluids; otherwise, the second law of thermodynamics requires all fluids to have positive viscosity.[4][5] A fluid that has zero viscosity (non-viscous) is called ideal or inviscid.
^ abc"Viscosity". Encyclopedia Britannica. 26 June 2023. Retrieved 4 August 2023.
^Growing up with Science. Marshall Cavendish. 2006. p. 1928. ISBN 978-0-7614-7521-7.
^E. Dale Martin (1961). A Study of Laminar Compressible Viscous Pipe Flow Accelerated by an Axial Body Force, with Application to Magnetogasdynamics. NASA. p. 7.
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