Phase separation under ultraslow cooling: Onset of nucleation

We discuss the interplay between a slow continuous drift of temperature, which induces continuous phase separation, and the nonlinear diffusion term in the phi(4)-model for phase separation of a binary mixture. This leads to a bound for the stability of diffusive demixing. It is demonstrated that the same findings apply in other models, except for slight modifications of the bound. In practice stable diffusive demixing can only be achieved when special precautions are taken in experiments on real mixtures. Therefore, the recent observations on complex dynamical behavior in such systems should be considered as a new challenge for understanding generic features of phase-separating systems.