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The viscosity of a liquid, is its resistance to sideways forces or stretching forces. It is sometimes measured in a Rheometer by the amount of liquid that passes through an orifice in a certain amount of time, or by the amount that flows through a capillary tube[1], or even by measuring the force a spinning outer cylinder induces on an inner cylinder through a fluid between them. [2]

Shear stress on a fluid (caused by sideways force) can be illustrated if we imagine a fluid flowing in a flat bottomed channel, the fluid that touches the bottom will be at rest while the fluid farther away from the bottom will be moving at the velocity of the liquid.[3] This steady change of fluid velocity with height is called a gradient. The 'thicker' (more viscous) the liquid, the farther from the bottom the liquid will attain its full velocity. This is why dust can build up on fan blades, because the air very close to the blade is normally moving at the same speed as the fan blade. The dust won't build up higher than a certain amount because the air movement is stronger at that height and would blow the dust off. The resistance to this shear stress (which makes one level of the fluid move faster than another level) is a way to measure the viscosity.

Extension stress on a fluid is usually a stretching force which is illustrated by land forms which form crevices when it is stretched. Bending a sheet of clay can form cracks and crevices because one side of the sheet of clay is stretched. Normally, stretching of the crust of the earth is the result of extensional stresses. [4] When lava cools, it is kept from shrinking because of the ground underneath it, and so crevices form as the lava tries to shrink. It is as if the ground underneath stretched the lava extensionally.

Jean Louis Marie Poiseuille

Jean Louis Marie Poiseuille [5] was a French scientist who was interested in the flow of blood through small tubes like veins and capillaries. The unit of viscosity named Poise was named after him in order to honor his researches.

Water has a viscosity of about 1 centapoise, milk has a viscosity of 3, and blood about 10. Motor oil has a viscosity of between 140 and 900 centapoise, while honey has a viscosity of 10,000 and ketchup a viscosity of 50,000 centapoise.[6][7]

Sir George Stokes

Sir George Stokes, an evangelical scientist in Cambridge university in England, studied many phenomena including the viscosity of different liquids. His technique involved studying the speed at which small spheres (such as ball bearings of different sizes) fall through liquids. If the liquid is thick enough or viscous enough, the sphere will reach a constant velocity called its terminal velocity. Kinematic viscosity is the name given to how much a liquid resists flow under the force of gravity. Another way of measuring kinematic viscosity is to measure how quickly a fluid will flow out of a vessel through a small tube at the bottom. Different liquids take more or less time to flow out because they are more or less viscous when in motion. The unit of measurement of kinematic viscosity is (centimeter)2/second which are called stokes, in honor of George Stokes. [8] George Stokes was also president for a time of the Victoria Institute which worked to counter evolution, and was vice president of the British and Foreign Bible Society[9]

General properties

Viscosity is very much dependent on temperature as anyone recognizes who has taken honey out of a refrigerator to use. [8] Motor Oils have to be specially formulated to provide sufficient lubrication at low temperatures. It sometimes comes as a shock to learn that water gets much thicker when cold. Water at 0o C (32o F) has a viscosity of 1.79 while water at 20o C (68o F) has a viscosity of 1.00 and water at 40o C (104o F) has a viscosity of .65.[8] This means that water is nearly three times "thicker" when near freezing than when it comes out of a hot water faucet. Hot water flows more easily into tiny crevices to remove dirt than cold water (even apart from the lowering of surface tension by soaps and detergents).

Interestingly, gases, which have viscosities nearly 1000 times less than water, will become more viscous with increasing temperature. So flows of gas at high temperature are different than flows at room temperature [8].

Some materials have viscosities which are non-Newtonian. They flow faster or slower depending on the stresses which affect them. Since they vary with time, they are sometimes said to have a memory[8], since they act as if they remembered their previous state in order to change it.

House paints are non-Newtonian, shear-thinning fluids [8] which flow more easily when brushed or rolled but become more sticky at rest so they drip less than other fluids. Toothpaste also flows more easily when stressed when you squeeze the tube.

Shear-thinning fluids can be classified into one of three general groups. A material that has a viscosity that decreases under shear stress but stays constant over time is said to be pseudoplastic. A material that has a viscosity that decreases under shear stress and then continues to decrease with time is said to be thixotropic. If the transition from high viscosity (or nearly semisolid) to low viscosity (or essentially liquid) takes place only after the shear stress exceeds some minimum value, the material is said to be a bingham plastic.


Some materials thicken when they are worked and are called shear-thickening fluids. A mixture of cornstarch and water is often mentioned, because it moves like a liquid when it moves slowly but becomes rubbery under rapid stress. Properly developed sheer thickening materials could be good for armor because they would harden when struck[8].


  1. Medical Dictionary. Accessed 17 December 2009.
  2. Rheometer, Wikipedia. Accessed 12 February 2011.
  3. Shear stress Wikipedia. Accessed 12 February 2011.
  4. Extensional Seismogenic Stress and Tectonic Movement on the Central Region of the Tibetan Plateau by Jiren Xu and Zhixin Zhao.International Journal of Geophysics. Volume 2009 (2009), Article ID 897424, 8 pages. Accessed 17 December 2009.
  5. Jean Louis Marie Poiseuille Wikipedia. Accessed 12 February 2011.
  6. Viscosity TablesV&P Scientific, Inc. Accessed 12 February 2011
  7. Viscosity The Physics Hypertextbook. Glenn Elert. Accessed 12 February 2011.
  8. 8.0 8.1 8.2 8.3 8.4 8.5 8.6 8.7 Viscosity The Physics Hypertextbook. Glenn Elert. Accessed 12 February 2011.
  9. Sir George Stokes, 1st Baronet Wikipedia. Accessed 12 February 2011.

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