Growth below and above the spinodal limit: the cholesteric-nematic front

In classical crystal growth, the solid propagates into its metastable liquid, even in the fast-dynamics regime in which kinetic effects dominate the diffusive effects. In usual experiments, the liquid is always very far from its spinodal limit below which it becomes unstable. For that reason, very little is known about crystal growth just below and above the spinodal limit of the liquid. In order to tackle this problem, we have performed an experiment about the propagation of the cholesteric-nematic front in directional melting. We show experimentally that, in this system, it is possible to reach and pass the spinodal limit. We show the existence of many morphological transitions at increasing velocities which lead to the formation of metastable or unstable phases. A complete phase diagram is drawn up as a function of the front velocity and the sample thickness. In particular, the crossing of the spinodal limit is clearly identified by comparison with the morphologies observed in free growth (i.e., at a homogeneous temperature). We show that this passage is accompanied by a reversal (pi rotation) of the structure in thin samples only. This spectacular effect resembles a "cell-to-dendrite" transition and is chirality induced. The importance of the first-order character of the transition is also emphasized.