Cellular instabilities in directional solidification

We have studied the morphology of the solid-liquid interface during the directional solidification of thin (5-150 μm) samples of a dilute binary mixture (CBr4/∼ 0.1 % Br2) . Above a critical pulling speed, for a given temperature gradient and solute concentration, the interface, initially planar, breaks down into a periodic cellular pattern. The bifurcation from a planar to a cellular solidification front is sub-critical. The critical pulling speed is strongly dependent on the thickness of the sample : the problem cannot be considered two-dimensional. We have also measured wavelengths at, and above the threshold and examined response times. These results are compared to existing theoretical predictions.