Viscosity

 Viscosity is commonly known as the thickness of fluids. It describes a fluids resistance to flow and can be thought of as a measure of fluid friction. Water is thin and has lower viscosity and vegetable oil is thicker and has a higher viscosity. All real fluids, except superfluids, have some resistance to flow, but a fluid that has no resistance is known as an ideal fluid or inviscid fluid.

Newton's Theory

Flowing layers usually move at different speeds and the fluid's viscosity raises from the sheer stress between the layers that oppose any applied force. Isaac Newton came up with the idea that for straight, parallel and uniform flow the sheer stress between layers is proportional to the relative motion of the layers.

  Measuring Viscosity

Viscosity can be measured with various types of instruments, usually at 20 degrees celcius. Some fluids have the same viscosity over a wide range of shear rates. Fluids without a constant viscosity are called Non-Newtonian fluids.

  Viscosity in Liquids

The additional forces in molecules become important in liquids. This leads to more contibution to the shear stress thoug the exact mechanics of this are still controversial. So, in liquids viscosity is independant of pressure (except at very high pressure) and viscosity tends to fall as temperature increases.

  Viscosity in Gases

In gases, viscosity rises from the molecular diffusion that transports momentum between layers of flow. Accurate prediction is of the behavior of gaseous viscosity is allowed through the kinetic theory of gases. The theory is applicable when viscosity is independent of pressure and when viscosity increases as temperature increases.

  Can Solids Have a Viscosity?

If on the basis that all solids flow a little bit in response to shear stress then yes, objects known as Amorphous solids, such as glass, may be considered to have a viscosity. This has led some people to the view that solids are simply liquids with a very high viscosity.

Others argue that most solids are elastic for small stresses while fluids are not. They are charactized by their low-stress behavoir even if solids flow at higher stresses. Viscosity may a characteristic for solids with a plastic regime. This can become somewhat confusing because viscosity is somtimes used for solids materials. This may be resolved by considering the equation in question, which take in both the elastic and viscous behaviors of the material.  

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