Viscosity

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Definition - What does Viscosity mean?

Viscosity is the resistance of a fluid to a change in shape, or to the movement of neighboring portions relative to one another. The reciprocal of the viscosity is called fluidity, a measure of the ease of flow.

Viscosity is a prime factor in determining the forces that must be overcome when fluids are used in lubrication and transported in pipelines. It controls the liquid flow in processes like:

  • Spraying
  • Injection molding
  • Surface coating

Corrosionpedia explains Viscosity

The viscosity of a fluid is an evaluation of its resistance to gradual deformation by shear stress or tensile stress. The viscosity may be thought of as internal friction between the molecules; such friction opposes the development of velocity differences within a fluid. When the intermolecular forces of attraction are strong within a liquid, there is a higher viscosity.

For many fluids the tangential or shearing stress divided by the rate of shear strain is constant for a given fluid at a fixed temperature. This constant is called the dynamic or absolute viscosity, or often simply the viscosity. The dimensions of dynamic viscosity are force times time divided by area. The unit of viscosity is newton-second per square meter. Fluids that behave in this way are called Newtonian fluids.

The viscosity of liquids decreases rapidly with an increase in temperature, whereas the viscosity of gases increases with an increase in temperature. Thus, liquids flow more easily, whereas gases flow more sluggishly upon heating.

Zero viscosity is observed only at very low temperatures, otherwise all fluids have positive viscosity. A liquid whose viscosity is less than that of water is sometimes known as a mobile liquid, while a substance with a viscosity substantially greater than water is called a viscous liquid.

For some applications the kinematic viscosity is more useful than the dynamic viscosity. Kinematic viscosity is the absolute viscosity of a fluid divided by its mass density. The dimensions of kinematic viscosity are area divided by time; the appropriate units are meter squared per second.

Viscosity is measured with various types of viscometers and rheometers. A rheometer is used for those fluids that cannot be defined by a single value of viscosity and therefore require more parameters to be set and measured than is the case for a viscometer.

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