Showing papers by "John Bechhoefer published in 1999"

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

Abstract: A single material can solidify into a variety of macroscopic morphologies depending on the undercooling. The manner in which one morphology changes into another as the undercooling is varied has received inadequate experimental 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 the isothermal crystallization of the liquid crystal 10 OCB from its smectic-$A$ phase. As the undercooling is varied, we see several sharp transitions in the growth structure, accompanied by singular points in the front velocity 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 (and the morphology changes discontinuously); and second order, which shows pretransitional effects and continuous changes in growth properties.

External-Field-Induced Tricritical Point in a Fluctuation-Driven Nematic–Smectic-ATransition

Abstract: We study theoretically the effect of an external field on the nematic–smectic-A transition close to the tricritical point, where fluctuation effects govern the qualitative behavior of the transition. An external field suppresses nematic-director fluctuations by making them massive. For a fluctuation-driven firstorder transition, we show that an external field can drive the transition second order. In an appropriate liquid-crystal system, we predict the required magnetic field to be of order 10 T. The equivalent electric field is of order 1 V�mm.