Phase transition in solid-liquid systems is described, combining the Gibbs-Thomson law with the classical Stefan model. Here solid nucleation is the main concern. A model. is proposed, which accounts for the smallness of the capillarity length with respect to the macroscopic scale. Transition from the finer to the coarser length scale is provided by convolution with a Gaussian kernel. This accounts for fast diffusion of latent heat of fusion, and allows one to represent solid nucleation at low undercooling. A new description of so-called mushy regions is thus achieved.