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Viscosity is defined as a measure of resistance to flow which arises dues to internal friction between layers of fluid as they slip past one another while liquid flows.

Why can't we say that viscosity is a property of solids as well? Except, it would then be said that solids have infinite viscosity.

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    $\begingroup$ Why, of course you can. It won't even be infinite, just very large. Think of the flowing glaciers or the Pitch drop experiment. $\endgroup$ – Ivan Neretin Sep 13 '16 at 12:11
  • $\begingroup$ And to confuse the issue further, there are non-newtonian fluids, such as mayonnaise (which becomes less viscous on stirring), and "oobleck' (corn-starch in water, which behaves as a solid under pressure). And glasses, solids for all practical purposes, which never actually crystallize... $\endgroup$ – DrMoishe Pippik Sep 13 '16 at 23:18
  • $\begingroup$ @IvanNeretin, That's the definition given in my book( NCERT Chemistry Class11)..so it got me wondering.. At the same time, the Pitch drop experiment , that you've mentioned, is for highly viscous liquids, but ultimately not a solid.Right? $\endgroup$ – Jamil Ahmed Sep 14 '16 at 9:14
  • $\begingroup$ True, bitumen might be considered a highly viscous liquid, but what about ice? It is as solid as solid can be; it is crystalline. And yet glaciers flow to the ocean. $\endgroup$ – Ivan Neretin Sep 14 '16 at 9:41
  • $\begingroup$ @IvanNeretin ,don't know much about the flowing glaciers, but isn't it something like ice flowing on water, instead of viscosity of ice itself $\endgroup$ – Jamil Ahmed Sep 14 '16 at 10:27
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The concept of deformations of solids under loading has a long history. It just isn't called 'viscosity' - instead this is just part of the mechanical properties of solids.

Relevant places to start include:

Wikipedia on Plasticity

Wikipedia on Deformation

The Non-Destructive Testing org on Elastic/Plastic Deformation

In the purely elastic regime, if you pull on a steel bar it will extend under the load, and will return to its original dimensions when the load is removed. If you apply too large of a load, it will plastically deform, so that the dimensions will not be the same when the load is removed. Under some circumstances even fairly small loads over long times can result in permanent deformation.

Physically, these can be a result of:

  1. Dislocation formation and movement in crystals (e.g. slip bands)
  2. Grain boundary motion
  3. Creep (point defect formation and diffusion)
  4. Crazing in plastics (formation of open volumes)
  5. And a number of other, less common, mechanisms.

These topics are typically covered in metallurgy and materials science courses/textbooks. Again, even in amorphous solids, one would not tend to call it viscosity - that is reserved for liquids.

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