I'll take the opportunity to open a small parenthesis on what #rheology aka “the science of the flow”, is: the term, which can be used both for fluids and (some classes of) deformable solids describes the relationship between stress and strain in a continumm.
To wit, for something to flow (or deform), there must be a force applied. The relation between this force and how much (and how quickly) the continuum deforms is what rheology is about.
Rheology deals with two main classes of behavior: #plasticity and #viscosity.
Plastic behavior refers to (permanent) deformations whose magnitude depends on the applied force: smaller forces result in smaller deformations, larger forces in larger deformations. This is typical of solids.
Viscous behavior refers to deformations whose rate depends on the applied force: in this sense deformations can be “infinite” (the distance between two given points can grow arbitrarily), and as long as the force is applied the deformation will grow. #Viscosity determines how strongly the continuum (typically a fluid) resits to the deformation, and thus how quickly (or slowly) it deforms.
And of course you can have hybrid behaviors (viscoplastic, viscoelastic, viscoplastoelastic).
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