arXiv subjects

Alexandros G. Vanakaras

Publications and source records attributed to Alexandros G. Vanakaras.

22 records · Page 2Linked to original sources

arXiv2009

Biaxial nematics: symmetries, order domains and field-induced phase transitions

We study the symmetry and the spatial uniformity of orientational order of the biaxial nematic phase in the light of recent experimental observations of phase biaxiality in thermotropic bent-core and calamitic-tetramer nematics. We present evidence supporting monoclinic symmetry, instead of the usually assumed orthorhombic. We describe the use of deuterium NMR to differentiate between the possible symmetries. We present the spatial aspects of biaxial order in the context of the cluster model, wherein macroscopic biaxiality can result from the field-induced alignment of biaxial and possibly polar domains. We discuss the implications of different symmetries, in conjunction with the microdomain structure of the biaxial phase, on the alignment of biaxial nematics and on the measurements of biaxial order.

cond-mat.soft↗

arXiv2009

Symmetries and alignment of biaxial nematic liquid crystals

The possible symmetries of the biaxial nematic phase are examined against the implications of the presently available experimental results. Contrary to the widespread notion that biaxial nematics have orthorhombic symmetry, our study shows that a monoclinic ($C_{2h}$) symmetry is more likely to be the case for the recently observed phase biaxiality in thermotropic bent-core and calamitc tetrapode nematic systems. The methodology for differentiating between the possible symmetries of the biaxial nematic phase by NMR and by IR spectroscopy measurements is presented in detail. The manifestations of the different symmetries on the alignment of the biaxial phase are identified and their implications on the measurement and quantification of biaxiality as well as on the potential use of biaxial nematic liquid crystals in electro-optic applications are discussed.

cond-mat.soft↗

arXiv2007

Thermotropic Biaxial Nematics: Spontaneous or Field-Stabilized?

An intermediate nematic phase is proposed for the interpretation of recent experimental results on phase biaxiality in bent-core nematics. The phase is macroscopically uniaxial but consists of microscopic biaxial, and possibly polar, domains. On applying an electric field the phase exhibits substantial macroscopic biaxial ordering resulting from the collective alignment of the domains. A phenomenological theory is developed for the molecular order in this phase and for its transitions to purely uniaxial and to spontaneously biaxial nematic phases.

cond-mat.soft↗

arXiv2005

Molecular Theory of Dendritic Liquid Crystals: Self Organization and Phase Transitions

We formulate the configurational partition function for dendrimers, taking explicit account of their conformations and segmental interactions. Two approximate schemes are presented, one based on the effective dendrimer-dendrimer interactions and the other based on the interactions among the mesogenic units comprising the dendrimers. In the latter scheme, the description of the dendromesogenic system reduces to that of an ensemble of mesogenic dimers. Results of lattice calculations for phase transitions are presented showing that the minimal inclusion of shape anisotropy and of sub-molecular partitioning into chemically distinct parts reproduces the variety of phases and phase sequences observed experimentally and provides insights into the conformational aspect of these transitions.

cond-mat.mtrl-sci↗