Destabilization of a flat nematic-isotropic interface

We present experiments on the directional solidification of a moving nematic-isotropic interface. We study the bifurcation and marginal stability of the interface in a system of 4,4’-n-octylcyanobiphenyl (8CB) with the impurity hexachloroethane ${\mathrm{C}}_2$ ${\mathrm{Cl}}_6$ added. In the velocity, temperature-gradient parameter space, we can trace the marginal stability line, in qualitative agreement with theory. A quantitative analysis shows that three-dimensional effects, such as solute-driven convection and the thickness of the sample, must be considered.