Dictionary > Viscosity

# Viscosity

Viscosity refers to the state or condition of being viscous. It is measured as a physical property of a fluid. As a physical property, it determines the internal resistance of the fluid to flow. A fluid’s viscosity is measured by determining its internal resistance to gradual deformation by shear forces or tensile stress. (1) Pitch, which is a dark viscous waxy material derived from tar, could be the most viscous fluid. (1) The viscosity of a fluid is influenced by the following factors: molecular structure, external forces, and ambient conditions. The molecular structure of a fluid influences viscosity in a way that when the molecules are tightly linked, the higher is the resistance to deformation, and therefore it will have less tendency to flow. External forces such as shear forces or tensile stress act upon the fluid and therefore influence the flow of the fluid. The ambient conditions also affect viscosity. For instance, the viscosity of a fluid is lower when the temperature is higher. Therefore, a decreasing ambient temperature will increase the viscosity of a fluid. (2) In fluid dynamics, the term absolute viscosity (or dynamic viscosity) refers to the force per unit area applied tangentially to a fluid, causing the unit rate of displacement of parallel planes separated by a unit distance. Its units in cgs system: poise. It can be determined through kinematic viscosity (also called momentum diffusivity), i.e. the ratio of the absolute viscosity (μ) to the density of the fluid ρ. Its units: stokes.

## Related terms

• Anomalous viscosity
• Apparent viscosity
• Blood viscosity
• Kinematic viscosity
• Magnetic viscosity
• Poiseuilles viscosity coefficient
• Relative viscosity

• Shear
• Fluid
• Work of breathing
• Fluidity
• Poiseuilles law
• Viscance

## References

1. Hammonds, M. (2013). The World’s Slowest Experiment. AustralianScience.com.au. Retrieved from http://www.australianscience.com.au/science-2/the-worlds-slowest-experiment/