Ask A Scientist Materials Science Archive

name         Carolyn
status       other
grade        other
location     KS

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