Biaxial Nematics: Symmetry and Hierarchical Domain Structure
Seminar Room 1, Newton Institute
AbstractWe present theoretical and computer simulation studies on the structure of nematic liquid crystals formed by bent-core mesogens (BCM) and by board-like colloids (BLC). The presence of local orientational and/or positional ordering is a key feature for the interpretation of the biaxial nematic ordering observed in these systems.
In the first part we present the full phase diagram, calculated from MC molecular simulations, of sterically interacting BLC, for a range of experimentally accessible molecular dimensions/anisometries of colloids of this shape. New classes of phase transition sequences such as nematic-nematic and, for the first time, a direct transition from a discotic and a biaxial nematic to an orthogonal smectic-A phase have been identified. We demonstrate rigorously the formation of supramolecular entities and explain the observed phase transitions in terms of the "shape anisotropy" of these entropy driven supramolecular assemblies.
In the second part the structure of nematic liquid crystals formed by bent-core mesogens is studied in the context of MC simulations of a simple molecular model that captures the symmetry, shape, and flexibility of achiral BCMs. Our results indicate the formation of (i) clusters exhibiting local smectic order, orthogonal or tilted, with strong in-layer polar correlations and antiferroelectric juxtaposition of successive layers and (ii) large homochiral domains through the helical arrangement of the tilted smectic clusters, while the orthogonal clusters produce achiral (untwisted) nematic states.
The results of our work offers a deeper understanding of the nematic-nematic transitions and, ultimately, of the nematic phase and can serve as a comprehensive guide to experiment, towards the design of anisotropic liquids with the desired functionality, as well as to theory for testing and improving analytical molecular models using simple intermolecular potentials.