What is Viscosity?

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  • Originally Written By: Tricia Ellis-Christensen
  • Revised By: Phil Riddel
  • Edited By: O. Wallace
  • Last Modified Date: 18 November 2015
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Viscosity is a scientific term that describes the resistance to flow of a fluid. The fluid can be a liquid or a gas, but the term is more commonly associated with liquids. As a simple example, syrup has a much higher viscosity than water: more force is required to move a spoon through a jar of syrup than in a jar of water because the syrup is more resistant to flowing around the spoon. This resistance is due to the friction produced by the fluid’s molecules and affects both the extent to which a fluid will oppose the movement of an object through it and the pressure required to make a fluid move through a tube or pipe. Viscosity is affected by a number of factors, including the size and shape of the molecules, the interactions between them, and temperature.


The viscosity of a liquid can be measured in a number of ways by devices called viscometers. These can either measure the time it takes for a fluid to move a particular distance through a tube or the time taken for an object with a given size and density to fall through the liquid of interest. The SI unit of measure for this is the pascal-second, with the pascal being the unit of pressure. This quality is therefore measured in terms of pressure and time, so that, under a given pressure, a viscous liquid will take more time to move a given distance than a less viscous one.


Factors Affecting Viscosity

As a rule, fluids with larger, more complex, molecules will have higher viscosities. This is particularly true for the long, chain-like molecules that are found in polymers and the heavier hydrocarbon compounds. These molecules tend to become entangled with one another, impeding their movement.

Another important factor is the way the molecules interact with one another. Polar compounds can form hydrogen bonds that link separate molecules together, increasing the overall resistance to flow and movement. Although water is a polar molecule, it has a low viscosity due to the fact that its molecules are small. The most viscous liquids tend to be those with long molecules that have noticeable polarity, such as glycerin and propylene glycol.

Temperature has a major effect on viscosity — so much so that measurements of this quality for fluids are always given with temperatures. In liquids, it decreases with temperature, as can be seen if syrup or honey is heated. This is because the molecules are moving about more, and therefore spend less time in contact with one another. In contrast, resistance to movement in gases increases with temperature. This is because, as the molecules move faster, there are more collisions between them, which reduces the ability to flow.

Importance for Industry

Crude oil is often piped long distances across regions with varying temperatures, and the rate of flow in response to pressure varies accordingly. Oil flowing through Alaska is more viscous than oil in pipelines at the Persian Gulf, due to differing ground temperatures, and consequently more pressure needs to be applied to keep it flowing. To address the issue of force needed to deliver oil through piping, sensors in some pipes measure the viscosity of the fluid and determine if greater or lesser pressure must be added to keep the flow of oil constant and steady.

Naturally, motor oil is also subject to changing viscosity when heated by an engine. Oil that becomes too thin from the engine’s heat will not work properly. To solve this problem, polymers are added to the oil to keep friction rates constant under higher temperatures.

Relevance to Volcanism

The viscosity of magma, or hot, molten rock under the Earth’s surface, is an important factor in the study of volcanoes. Runny lava tends to result in more frequent but less violent eruptions, as it flows easily up from magma chambers and out of the volcano. It also allows dissolved gas to bubble out more easily. Thicker magma tends to trap this gas at high pressure, and more force is required to eject the lava from the volcano, allowing great pressure to build up over time. When this type of volcano does erupt, it does so explosively, often with catastrophic consequences.


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Discuss this Article

Post 34

It's because the molecules are separated when heat is applied, so it is less viscous and runs smoother.

Post 25

Does oil have a point where it will become a solid or not?

Post 24

Why do different liquids flow more easily than others?

Post 21

@talalwa1mtm: Please help me derive the analog of Hagen-Poiseuille equation for a power law fluid.

Post 16

gasoline is a fluid.

Post 15

what are some high viscosity and low viscosity fluids?

Post 12

Thank you ever so much wisegeek!

Post 11

Wow, whew! Thanks wise geek! This is what i need for my science fair!

Post 9

you could look at the reynolds number and how it describes a fluid.

Post 8

how come some liquids have more viscosity then other liquids? gerardo lopez

Post 6

good explanation.

Post 5

why does the viscosity of liquid decrease when temperature increases, whereas the viscosity of gas increases with temperature?

Post 3

derive the analog of Hagen-Poiseuille equation for ostwalde-de-waele model (power law)

Post 2

What is the criterion of deciding whether the liquid has low, medium or high viscosity?

Post 1

but why are some liquids more viscous than others is it because they have different solidifying points? or is it their atoms being closer. Or something completely different

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