OBSERVATION OF FRONT PROPAGATION IN DIRECTIONAL SOLIDIFICATION

We describe how spatial variation in the critical pulling speed for directional solidification is induced by breaking the symmetry about the pulling direction. This spatial variation results in localized nucleation of the Mullins-Sekerka instability. After nucleation, the instability spreads via front propagation. The front propagation is studied as a function of pulling speed, leading to a powerful test of the nature of the bifurcation from the planar to the cellular state. Furthermore, growth in the leading edge of the front is studied in detail, as is the resulting cellular array after the front's passage. We find that front propagation enhances the wavelength selection dramatically compared to the case where the cellular array nucleates everywhere simultaneously.