Viscosity: Encyclopedia II - Viscosity - Can solids have a viscosity?

Viscosity - Can solids have a viscosity?

It is commonly asserted that amorphous solids, such as glass, have viscosity, arguing on the basis that all solids flow, to some possibly minuscule extent, in response to shear stress. Advocates of such a view hold that the distinction between solids and liquids is unclear and that solids are simply liquids with a very high viscosity, typically greater than 10¹² Pa·s. This position is often adopted by supporters of the widely held idea that apparent glass flow can be observed in old buildings.

However, others argue that solids are, in general, elastic for small stresses while fluids are not. Even if solids flow at higher stresses, they are characterized by their low-stress behavior. Viscosity may be an appropriate characteristic for solids in a plastic regime. The situation becomes somewhat confused as the term viscosity is sometimes used for solid materials, for example Maxwell materials, to describe the relationship between stress and the rate of change of strain, rather than rate of shear.

These distinctions may be largely resolved by considering the constitutive equations of the material in question, which take into account both its viscous and elastic behaviors. Materials for which both their viscosity and their elasticity are important in a particular range of deformation and deformation rate are called viscoelastic. In geology, earth materials that exhibit viscous deformation at least three times greater than their elastic deformation are sometimes called rheids.

One example of solids flowing which has been observed since 1930 is the Pitch drop experiment.

Adapted from the Wikipedia article "Can solids have a viscosity?", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki