We study the initial stages of a directional solidification experiment of a mixture by means of a non-variational phase field model with fluctuations. This model does not invoke fluctuation-dissipation theorem to account for the fluctuations statistics.
We devote our attention to the transient regime during which concentration gradients are building and fluctuations act to destabilize the interface. To this end we calculate both the temporally dependent growth rate of each mode and the power spectrum of the interface evolving under the effect of fluctuations. Theoretical predictions are compared to phase field simulations.