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[deleted] t1_j6muy1z wrote

Reply to comment by CrustalTrudger in How viscous is the magma in the mantle? by _whydah_

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dukesdj t1_j6nk0kf wrote

This is not really the definition of fluid as there actually is no strict boundary between what is and is not a solid. Indeed as others have noted but incorrectly commented on, things like the mantle, pitch, and jelly are examples of substances that have a dual nature in both being solid AND liquid.

 

To quote George Batchelor (taken from An Introduction to fluid dynamics), "The distinction between solids and fluids is not a sharp one, since there are many materials which in some respects behave like a solid and in other respects like a fluid." ... "But, even supposing that these two definitions could be made quite precise, it is known that some materials do genuinely have a dual character.".

 

What this really means, and what fluid dynamicists recognise, is trying to constrain a substance/object into being a solid or a fluid has more to do with humans desire to define things in discrete buckets and less about the actual physical world.

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CrustalTrudger t1_j6nmo19 wrote

Thanks for clearly stating what I was trying to express somewhat sloppily. This is largely why in discussions of rheology (for rocks at least), talking about them as either "solid" or "fluid" is uncommon and instead you tend to see them described just as "materials", i.e., when texts introduce useful analogues for thinking about the stress-strain or stress-strain rate relationship (i.e., the various combinations of a sliding frictional block, spring, or dashpot that would produce some sort of equivalent stress-strain or stress-strain rate response) they tend to do so in terms of just materials, e.g., "Maxwell materials" or "Voigt materials" etc. Not all geology texts are good about this though.

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phdoofus t1_j6npg6i wrote

A good (reasonably not bad) example of the rheology of the mantle is Silly Putty.

If you put a ball of it on a desk and hit it with a hammer, it shatters. A good analogy for earthquakes (brittle response)

If you just leave it on the desk it will become nice flat putty pancake (ductile response)

Two different responses to two different stress/strain regimes. Particularly effective in class

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CrustalTrudger t1_j6mx7hi wrote

This is actually kind of a misleading "clarification" though. Pitch is a useful example, i.e., a viscoelastic solid that will deform on long time-scales under its own weight. At room temperature and timescales sufficiently short (i.e., less than a few years), pitch would meet the simple definitions of a "solid", but observed on long enough time scales, it can be observed to flow.

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OlympusMons94 t1_j6ng2lg wrote

Liquids (and solids) are much less compressible than gases, but they are still compressible. Constant volume (incompressibikity) is just (sometimes) a useful simplifying assumption. (In other contexts like sound/seismic wave speed, it would be, well, complicating to say the least, given that would result in an infinite wave speed.)

If you have a tall enough, a column of metal, or even rock, it will deform under the pressure from its own weight. A penny is just very small and light, and deformation is negligible. Like a solid, a liquid will also not deform without without some force being applied, but the type of deformation is different.

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muskytortoise t1_j6o1zlo wrote

That is a common definition, yes, but scientifically liquid is a state of matter. Fluid =/= liquid. Liquid is a phase, fluid is behaviour. You said liquid when you described fluid and while that is correct in common meaning it's unnecessarily confusing people when the difference is described.

https://www.thoughtco.com/definition-of-liquid-604558

https://www.thoughtco.com/definition-of-fluid-604466

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