Phase behaviour in mixtures of unixial hard particles: biaxiality and confinement
Seminar Room 1, Newton Institute
The nematic biaxial phase has remained a key challenge in the science of liquid crystals since it was first proposed. Recently the first experimental evidence of stable biaxial nematic phases has been obtained in thermotropic liquid crystals of single component biaxial mesogens by Madsen et al., and others. Still elusive however is the possibility of stabilizing biaxial nematic phases in mixtures of uniaxial particles. This avenue has been explored in some detail using theory and computer simulation, but leads one to the conclusion that, at least in the case of mixtures of hard particles, the nematic biaxial phase is thermodynamically unstable with respect to demixing into two uniaxial phases. Theoretical calculations have, however, pointed out that with an appropriate attractive unlike interaction, a homogeneous biaxial nematic phase could be stabilized. Experimental work on mixtures of rod and disc-like molecules has tended to confirm the view that such a system would favou r phase separation, until the recent studies of Apreutesei and Mehl. In this contribution, we use canonical Monte Carlo molecular simulations to study model mixtures of rodlike and disklike molecules interacting through two intermolecular potential models: one incorporating spherically symmetric (isotropic) attractive interactions; another with anisotropic attractive interactions. These models exhibit nematic and smectic biaxial phases. In the final part of the talk, if time allows, I will briefly discuss the changes in the phase behavior that occur when uniaxial disc-like particles are placed in confinement between parallel walls and consider the surface ordering and capillary phenomena in this system.