Viscosity

Viscosity is a physical property of fluids. It shows resistance to flow. In a simple example, water has a low viscosity, as it is "thin". Syrup and tar, on the other hand, have a high viscosity, as they are "thick". A way to test for viscosity is the speed at which the substance runs down a slope. Syrup would reach the bottom very slowly, and water would be much quicker.

Mechanics

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Illustration of viscosity; the violet fluid at the bottom has a higher viscosity than the clear one above

There are two types of viscosity: dynamic viscosity, measured in pascal seconds, and kinematic viscosity, measured in metres squared per second .

Viscosity is used as a way to predict when volcanoes erupt. When the lava comes out very thickly (viscous), there is more chance that it will erupt violently. This is because the lava has a hard time getting out and may burst out when it can. If the lava is thin (low viscosity), then it just flows out like water.

The word viscous comes from the Latin root viscum, meaning sticky.

Viscosity Media

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  Viscosity demonstration. The fluid on the left has a lower relative viscosity than the fluid on the right.

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  Illustration of a planar Couette flow. Since the shearing flow is opposed by friction between adjacent layers of fluid (which are in relative motion), a force is required to sustain the motion of the upper plate. The relative strength of this force is a measure of the fluid's viscosity.

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  In a general parallel flow, the shear stress is proportional to the gradient of the velocity.

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  Viscosity, the slope of each line, varies among materials.

• Video showing three liquids with different viscosities

• Experiment showing the behavior of a viscous fluid with blue dye for visibility

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  Common glass viscosity curves

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  Common logarithm of viscosity against temperature for B2O3, showing two regimes

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  In the University of Queensland pitch drop experiment, pitch has been dripping slowly through a funnel since 1927, at a rate of one drop roughly every decade. In this way the viscosity of pitch has been determined to be approximately 230 billion (2.3×10¹¹) times that of water.

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