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Name: Carolyn
Status: Other
Grade:  Other
Location: KS
Country: USA
Date: June 2005


Question:
Would you please explain how a fluid could have both elastic and viscous properties? What are the definitions of viscous and elastic? Thanks!



Replies:
Viscous flow is the resistance movement of a mass of substance (usually a liquid). Motor oil, glycerin, and water are examples. Elasticity is the tendency of a mass of substance in some initial shape to return to that shape after it has been deformed in some way. Jell-o, and to a lesser extent honey are examples. Some rubbery materials are both viscous and elastic. That is, if you deform the material slowly, it flows; but if you deform (or attempt to deform) the substance rapidly it returns to its original shape when the deformation is stopped. Some paints show this type of behavior.

Vince Calder


In simple terms elasticity means that the material (like rubber) can stretch and still remain coherent or together. In certain applications elasticity also means that when stretching or compressing a material and the force that stretches or compresses the material is removed, (again like rubber) the material snaps back to its original shape.

Viscosity on the other hand means that a material resists flow. A substance with a high viscosity (like honey) does not flow easily, whereas one with low viscosity (like water) flows relatively easily.

Thus, one can be surprised that there are materials that are both viscous (flows with some resistance) and elastic (stretches, flows, but snaps back to its original shape). But you do not need to look very far to encounter such visco-elastic material. Most polymers (plastics) are viscoeleastic. I especially like "silly putty" (and if you do not remember or have not encountered Silly Putty - then you should go to the nearest toy store and get yourself one ). Silly Putty will bounce - which means it is elastic (it deforms upon impact, but regains its shape), but it is also viscous (stretch away a small chunk and that chunk will pull out a small stream under its own weight; or simply roll the silly putty into a nice ball and leave it on your desk, after a while it flattens a bit - again under its own weight).

Greg (Roberto Gregorius)



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