Many modes of rapid solidification in a liquid crystal

We have investigated the smectic-to-crystal phase transition of the liquid crystal 10 OCB. The smectic plays the role of a viscous liquid, and the system shares essential characteristics with the freezing of polymers, viscous liquids, and liquid metals. The 10 OCB material has the advantage of convenient time scales: ''rapid''-solidification effects occur at freezing velocities of 10-100 mu m/s, versus 10 mu m/h for polymers and 1 m/s for metals. We observe six different solidification modes, each characterized by a distinct micro- and meso-structure. In addition, each mode is correlated with a separate branch of the front-velocity versus undercooling curve. This curve has a discontinuity in the slope at mode transitions. In one case, the velocity curve itself is discontinuous. A suggested morphology-selection hypothesis - ''the fastest mode wins'' - is explicitly ruled out.