Convection in the presence of a first-order phase change

We report experimental and theoretical results for two-phase convection in a thin horizontal layer of a fluid with a first-order phase change and heat ed from below. A top layer of the nematic phase of a liquid crystal is loca ted above the bottom layer of the isotropic phase of the same. substance. A horizontal field of 1000 G is applied in order to align the director of th e nematic phase. Over some ranges of the thickness of the isotropic phase, and in sufficiently large thermal gradients, the more dense nematic phase c an be stably stratified above the less dense isotropic one, with a stable i nterface between them. Based on the equations of motion derived for this pr oblem by Busse and Schubert [J. Fluid Mech. 46, 801 (1971)], we evaluate th e bifurcation lines between the quiescent and convecting states and the cor responding critical wave vectors as a function of the interface position. W e report experimental measurements based on Nusselt-number determinations f or the locations of the bifurcation lines. They are in good agreement with the theoretical results. We also report approximate determinations of the c ritical wave numbers which are semiquantitatively consistent with the theor y. A great diversity of patterns is observed in the convecting states, incl uding normal and parallel rolls, rolls with defects and disorder, target pa tterns and spirals, and cellular flow with upflow or downflow at the cell c enter. These patterns are discussed in terms of the breaking of the mirror symmetry at the horizontal midplane by the interface, and in terms of the o rienting effects of the magnetic field. [S1063-651X(99)05707-4].