About the "normal growth" approximation in the dynamical theory of phase transitions

About the "normal growth" approximation in the dynamical theory of phase transitions L. Truskinovsky Nonequilibrium phase transitions can often be modeled by a surface of dis-continuity propagating into a metastable region. The physical hypothesis of "normal growth" presumes a linear relation between the velocity of the phase boundary and the degree of metastability. The phenomenological coefficient, which measures the "mobility" of the phase boundary, can either be taken from experiment or obtained from an appropriate physical model. This linear approximation is equivalent to assuming the surface entropy production (caused by the kinetic dissipation in a transition layer) to be quadratic in a mass flux. In this paper we investigate the possibility of deducing the "normal growth" approximation from the viscosity-capillarity model which incorporates both strain rates and strain gradients into constitutive functions. Since this model is capable of describing fine structure of a "thick" advancing phase boundary, one can derive, rather than postulate, a kinetic relation governing the mobility of the phase boundary and check the validity of the "normal growth" approximation. We show that this approximation is always justified for sufficiently slow phase boundaries and calculate explicitly the mobility coefficient. By using two exact solutions of the structure problem we obtained unrestricted kinetic equations for the cases of piecewise linear and cubic stress-strain relations. As we show, the domain of applicability of the "normal growth" approximation can be infinitely small when the effective viscosity is close to zero or the internal capillary length scale tends to infinity. This singular behavior is related to the existence of two regimes for the propagation of the phase boundary-dissipation dominated and inertia dominated.