A constitutive equation for the porosity field

This paper deals with the derivation of an evolution equation for the field here denoted by Δn, which represents the difference between the porosity in an arbitrary state of a deformable porous medium and its equilibrium value. The equation is obtained assuming that Δn is a constitutive quantity depending on the history of the fields describing macroscopically a porous material. Moreover, at least in the vicinity of the state of stable equilibrium, the equation obtained for Δn can be regarded as a balance equation liable to various degrees of approximation, so including various models with additional balance laws known in the literature.