Morphology transitions in diffusion- and kinetics-limited solidification of a liquid crystal

A single material can solidify into a variety of macroscopic morphologies d epending on the undercooling. The manner in which one morphology changes in to another as the undercooling is varied has received inadequate experiment al attention, particularly for cases where the undercooling is large. There are two main possibilities: there can be distinct transitions, in analogy with equilibrium phase transitions, or there can be a continuous crossover, where one morphology gradually transforms into the next. We have studied t he isothermal crystallization of the liquid crystal 10 OCB from its smectic -A phase. As the undercooling is varied, we see several sharp transitions i n the growth structure, accompanied by singular points in the front velocit y curve. We identify three types of morphology transitions: strongly first order, where the front velocity is discontinuous: weakly first order, where the velocity curve, but not its derivative, is continuous land the morphol ogy changes discontinuously); and second order, which shows pretransitional effects and continuous changes in growth properties.