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What factors change viscosity, other than temperature?

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Título:
What factors change viscosity, other than temperature?
Tema :
Viscosidad
Pregunta: 

What factors change viscosity, other than temperature?

Answer: 

An introduction from Mohamed Salah Hassan
We can say that viscosity is the resistance a material has to change in form. This property can be thought of as an internal friction.

To get a good feel for viscosity, I suggest that we first remember laminar flow. If a fluid or gas is flowing over a surface, the molecules next to the surface (the ones clinging to the walls) have zero speed. As we get farther away from the surface the speed increases. This difference in speed is a friction in the fluid or gas. It is the friction of molecules being pushed past each other. You can imagine that the strength with which the molecules cling together will be proportional to the friction. This strength is called viscosity. Thus, viscosity determines the amount of friction, which in turn determines the amount of energy absorbed by the flow.

The viscosity of a fluid is basically a measure of how sticky it is. Water has a fairly low viscosity; things like shampoo or syrup have higher viscosities. Viscosity also depends on temperature - engine oil, for instance, is much less viscous at high temperatures than it is in a cold engine in the middle of winter.

For fluids flowing through pipes, the viscosity produces a resistive force. This resistance can basically be thought of as a frictional force acting between parts of the fluid that are traveling at different speeds. The fluid very close to the pipe walls, for instance, travels more slowly than the fluid in the very center of the pipe.

Robert Harries says
The viscosity of a pure fluid changes most with temperature. Pressure has a small effect (much less than temperature) on the viscosity of a gas and the effect of pressure on a liquid is extremely small.

Bob Holicek adds:
Other factors can come into play when considering multiphase liquids ¿ a mixture of liquid, solid and gas. Such mixtures are commonly found in crude oil flowing up an oil well ¿ part of the oil often turns to gas as pressure reduces and the mixture might also include water, pieces of rock, wax, and tar. In this case, fraction of each phase will affect the viscosity of the mixture.

Foams - mixtures of gas and liquid - and emulsions - mixtures of liquids - usually have a higher apparent viscosity that the either individual phase on its own.

For polymers (large organic molecule formed by combining many smaller molecules e.g. plastics), viscosity is generally higher for polymers with higher molecular weights - bigger molecules leads to higher viscosity.

A dilute solution of a polymer in a solvent, for example water, can exhibit power-law behavior- it may have high viscosity under low shear, but low viscosity under high shear. The water itself is Newtonian, but the introduction of the polymer in concentrations (by weight) as low as 0.2 % can have a large affect on its rheological behavior.

Karl-Heinz Marx says:
Some media show a change of viscosity depending on the speed or the force you use, it may increase (the faster you move the more viscositiy) or decrease (thixotropic media). For more information see these pages about rheology (the study of unusual flow) from Central Chemical Consulting (Australia). Also look at two experiments in the SEED Science Lab Ketchup: Thick or Thin? and Cornstarch and Water: Is it Liquid or Solid?

Greg Kubala adds:
Besides temperature, viscosity of the same fluid may vary with sheer stress and pressure. See the web site from Eric Weisstein's World of Science which provides information on the math'physics behind this dependence.


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