Stable bistable, metastable

Again related to total energies, and particularly relevant to the hydrogen-shallow-defect pairs that we will discuss, are the phenomena of metastability and bistability (Watkins, 1989). A metastable configuration is one in which the total energy is a local minimum but not a global minimum. The lifetime of the metastable state depends, of course, on the barrier that separates it from the stable configuration. 

The previous results as well as numerical simulations 8 J suggest that a non-equilibrium bistable reactive system may evolve by nucleation process, in agreement with the theory of nucleation-induced transitions [ 9 3and with experimental observations 10 ]], If the whole system is initially in the metastable state a, in such conditions that the lifetime of a is of the order of the observation time, spontaneous local fluctuations may form a microdroplet of the stable state y then the chemical reaction tends to extend its volume, whereas the diffusion tends to reduce it. The deterministic theory shows L J if the radius of the droplet happens to exceed a... 

The time evolution of an inhomogenous bistable reactive system in absence of convection is studied in the birth and death formalism. With the aid of multivariate Master Equations it is shown in simple cases that spatial homegenity can be spontaneously breaken during the passage from metastable to stable state a theory of nucleation is presented, which allows the evaluation of the nucleation rate. 

Rudnicki and Niezgodka [43] used a model to simulate the melting of cocoa butter in DSC samples. They did not study crystallization from the melt, but they looked at how phases of cocoa butter transform from the most metastable to the most stable and finally to the liquid when the sample is heated. They made the following assumptions Transitions between phases are given by a bistable thermodynamic potential. The equilibrium distribution between the phases is described by Boltzmann functions, and process dynamics can be calculated by Arrhenius laws where energy levels of each phase are given by thermodynamics. 

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