DYNAMIC WAVELENGTH SELECTION BY TILT DOMAINS IN THIN-FILM LAMELLAR EUTECTIC GROWTH

Tilt domains are dynamical defects of cellular growth fronts consisting of a group of asymmetric cells traveling laterally along the front. We study the dynamics of these defects in thin-film directional solidification of the CBr4-C2Cl6 eutectic alloy. We show that the sweeping of the growth front by tilt domains, at a given growth velocity V, brings it into a well-defined "dynamically selected" state. Once this state is reached, the tilt domains travel with a constant width. A sudden increase (decrease) of the growth velocity transforms constant-width tilt domains to growing (shrinking) ones. This behavior is in complete agreement with the theoretical predictions made by Coullet et al. [Phys. Rev. Lett. 63, 1954 (1989)] and Caroli et al. [J. Phys. I (Paris) 2, 281 (1992)]. We study the V dependence of the dynamically selected wavelength. It does not follow the V-1/2 law, contrary to the "selected" wavelength of lamellar eutectics in the range of velocities and thermal gradients that we use. Above a velocity V(cr), the dynamically selected basic state is unstable against the lamellar-extinction instability. We describe some dynamical patterns resulting from the interplay between the latter instability and the tilt instability.