Tilt bifurcation and dynamical selection by tilt domains in thin-film lamellar eutectic growth: Experimental evidence of a tilt bifurcation

In thin-film directional solidification of the CBr₄-C₂{Cl}₆$ eutectic alloy, we observe a quasi-instantaneous transition from symmetric states to drifting parity-broken (``tilted'') states as a result of a sudden increase of the pulling velocity. We show that (i) the observed transition is essentially the forward-tilt bifurcation that Kassner and Misbah [Phys. Rev. Lett. 65, 1458 (1990); 66, 522 (1991)] have recently found, numerically, in a model without anisotropy of the solid phases; (ii) crystal anisotropy, in the eutectic system, makes the bifurcation slightly imperfect. In lamellar eutectics, phase diffusion is extremely slow, and permanent spatial modulations of the pattern wavelength are always present, generated at eutectic grain boundaries. We show that the tilt angle, i.e., the degree of parity breaking of the pattern, adjusts itself locally to the wavelength in a very short time, in contrast with the slow phase diffusion.