On diffusive creep and viscous flow

The relation between viscous and diffusive deformations is discussed, and their nonequivalence is emphasized. Molecular theories relating self diffusivity to viscosity, as well as the macroscopic behavior of materials deforming by diffusive creep and by viscous flow, are reviewed. It is concluded that while molecular theories can properly relate viscosity to diffusivity, viscous flow and diffusive creep are not macroscopically equivalent. A "viscosity" obtained by equating a diffusive creep flow to a viscous flow will in general depend upon the state of stress and will therefore not be a material property. The differences between the flow types are illustrated with two examples, the spheroidization of a nearly spherical particle, and the effective viscosity of a suspension of particles in viscous liquid.