What is Viscosity?

For a newtonian fluid, viscosity is the same as consistency and is the resistance offered by the fluid to deformation. Viscosity will be stated as either absolute or kinematic viscosity. Absolute viscosity is the fundamental viscosity measurement of a fluid and has units of Poises in the cgs system. Kinematic viscosity is equal to absolute viscosity divided by the fluid density and has units of Stokes in the cgs system.

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Viscosity is a measure of the resistance of a fluid that is being deformed by either shear stress or tensile stress. In everyday terms, viscosity is thickness or internal friction. Thus, water is thin, having a lower viscosity, while honey is thick, having a higher viscosity. The less viscous the fluid is, the greater its ease of movement (fluidity). Viscosity describes a fluid’s internal resistance to flow and may be thought of as a measure of fluid friction.

For example, high-viscosity felsic magma will create a tall, steep stratovolcano, because it cannot flow far before it cools, while low-viscosity mafic lava will create a wide, shallow-sloped shield volcano. With the exception of superfluids, all real fluids have some resistance to stress and therefore are viscous.

A fluid that has no resistance to shear stress is known as an ideal fluid or inviscid fluid. In common usage, a liquid with the viscosity less than water is known as a mobile liquid, while a substance with a viscosity substantially greater than water is simply called a viscous liquid.