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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.


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.