Uniaxial and biaxial nematic liquid crystals

2006 ◽  
Vol 364 (1847) ◽  
pp. 2681-2696 ◽  
Author(s):  
T.J Dingemans ◽  
L.A Madsen ◽  
N.A Zafiropoulos ◽  
Wenbin Lin ◽  
E.T Samulski
Keyword(s):  
Nematic Phase ◽  
Mean Field ◽  
Rotation Frequency ◽  
General Context ◽  
Rotation Rates ◽  
Critical Rotation ◽  
Biaxial Nematics ◽  
Nematic Phases ◽  
Lower Temperature ◽  
Biaxial Nematic

The unusual exhibition of a biaxial nematic phase in nonlinear thermotropic mesogens derived from the 2,5-oxadiazole biphenol (ODBP) core is placed in a general context; the uniaxial nematic phase of the prototypical rod-like mesogen para -quinquephenyl does not follow the classical mean-field behaviour of nematics, thus questioning the utility of such theories for quantitative predictions about biaxial nematics. The nuclear magnetic resonance spectra of labelled probe molecules dissolved in ODBP biaxial nematic phases suggest that a second critical rotation frequency, related to the differences in the transverse diamagnetic susceptibilities of the biaxial nematic, must be exceeded in order to create an aligned two-dimensional powder sample. Efforts to find higher viscosity and lower temperature biaxial nematics (with lower critical rotation rates) to confirm the above conjecture are described. Several chemical modifications of the ODBP mesogenic core are presented.

Role of molecular bend angle and biaxiality in the stabilization of the twist-bend nematic phase

Soft Matter ◽  
2020 ◽  
Vol 16 (18) ◽  
pp. 4350-4357 ◽  
Author(s):  
Wojciech Tomczyk ◽  
Lech Longa
Keyword(s):  
Field Theory ◽  
Nematic Phase ◽  
Mean Field Theory ◽  
Mean Field ◽  
Structural Features ◽  
Bend Angle ◽  
Nematic Phases

Within mean-field theory for V-shaped molecules, we have investigated how the alteration of a molecule's structural features influence the stabilization of modulated and non-modulated nematic phases.

Can multi-biaxial mesogenic mixtures favour biaxial nematics? A computer simulation study

2017 ◽  
Vol 19 (3) ◽  
pp. 2383-2391 ◽  
Author(s):  
Lara Querciagrossa ◽  
Matteo Ricci ◽  
Roberto Berardi ◽  
Claudio Zannoni
Keyword(s):  
Computer Simulation ◽  
Simulation Study ◽  
Nematic Phase ◽  
Computer Simulation Study ◽  
Biaxial Nematics ◽  
Biaxial Nematic ◽  
Biaxial Nematic Phase

A mixture of mesogens with different anisotropies favours the biaxial nematic phase.

scholarly journals Experimental Conditions for the Stabilization of the Lyotropic Biaxial Nematic Mesophase

Crystals ◽  
2019 ◽  
Vol 9 (3) ◽  
pp. 158
Author(s):  
Erol Akpinar ◽  
Antônio Figueiredo Neto
Keyword(s):  
Nematic Phase ◽  
Liquid Crystalline ◽  
Point Of View ◽  
Theoretical Point ◽  
Experimental Conditions ◽  
The Third ◽  
Nematic Phases ◽  
The Stability ◽  
Biaxial Nematic ◽  
Biaxial Nematic Phase

Nematic phases are some of the most common phases among the lyotropic liquid crystalline structures. They have been widely investigated during last decades. In early studies, two uniaxial nematic phases (discotic, ND, and calamitic, NC) were identified. After the discovery of the third one, named biaxial nematic phase (NB) in 1980, however, some controversies in the stability of biaxial nematic phases began and still continue in the literature. From the theoretical point of view, the existence of a biaxial nematic phase is well established. This review aims to bring information about the historical development of those phases considering the early studies and then summarize the recent studies on how to stabilize different nematic phases from the experimental conditions, especially, choosing the suitable constituents of lyotropic mixtures.

scholarly journals First-principles experimental demonstration of ferroelectricity in a thermotropic nematic liquid crystal: Polar domains and striking electro-optics

2020 ◽  
pp. 202002290 ◽  
Author(s):  
Xi Chen ◽  
Eva Korblova ◽  
Dengpan Dong ◽  
Xiaoyu Wei ◽  
Renfan Shao ◽  
...  
Keyword(s):  
Applied Field ◽  
Nematic Phase ◽  
Mean Field ◽  
Electric Polarization ◽  
Dynamics Simulation ◽  
Glassy Material ◽  
Average Value ◽  
Polarization Density ◽  
Polar Orientation ◽  
Lower Temperature

We report the experimental determination of the structure and response to applied electric field of the lower-temperature nematic phase of the previously reported calamitic compound 4-[(4-nitrophenoxy)carbonyl]phenyl2,4-dimethoxybenzoate (RM734). We exploit its electro-optics to visualize the appearance, in the absence of applied field, of a permanent electric polarization density, manifested as a spontaneously broken symmetry in distinct domains of opposite polar orientation. Polarization reversal is mediated by field-induced domain wall movement, making this phase ferroelectric, a 3D uniaxial nematic having a spontaneous, reorientable polarization locally parallel to the director. This polarization density saturates at a low temperature value of ∼6 µC/cm2, the largest ever measured for a fluid or glassy material. This polarization is comparable to that of solid state ferroelectrics and is close to the average value obtained by assuming perfect, polar alignment of molecular dipoles in the nematic. We find a host of spectacular optical and hydrodynamic effects driven by ultralow applied field (E ∼ 1 V/cm), produced by the coupling of the large polarization to nematic birefringence and flow. Electrostatic self-interaction of the polarization charge renders the transition from the nematic phase mean field-like and weakly first order and controls the director field structure of the ferroelectric phase. Atomistic molecular dynamics simulation reveals short-range polar molecular interactions that favor ferroelectric ordering, including a tendency for head-to-tail association into polar, chain-like assemblies having polar lateral correlations. These results indicate a significant potential for transformative, new nematic physics, chemistry, and applications based on the enhanced understanding, development, and exploitation of molecular electrostatic interaction.

scholarly journals Molecular field theory for biaxial nematics formed from liquid crystal dimers and inhibited by the twist-bend nematic

2017 ◽  
Vol 19 (43) ◽  
pp. 29321-29332 ◽  
Author(s):  
T. B. T. To ◽  
T. J. Sluckin ◽  
G. R. Luckhurst
Keyword(s):  
Field Theory ◽  
Liquid Crystal ◽  
Molecular Field ◽  
Molecular Field Theory ◽  
Biaxial Nematics ◽  
Nematic Phases ◽  
Biaxial Nematic

Theory shows the ability of flexible dimers to adopt bent and linear shapes facilitates the formation of biaxial nematic phases.

RECENT DEVELOPMENTS IN BIAXIAL LIQUID CRYSTALS: AN NMR PERSPECTIVE

2010 ◽  
Vol 24 (24) ◽  
pp. 4641-4682 ◽  
Author(s):  
RONALD Y. DONG
Keyword(s):  
Liquid Crystals ◽  
Nematic Phase ◽  
Orientational Order ◽  
Order Parameters ◽  
Smectic C ◽  
Recent Developments ◽  
Nmr Techniques ◽  
Biaxial Liquid Crystals ◽  
Nematic Phases ◽  
Biaxial Nematic

A survey of recent studies of biaxial liquid crystals (LCs), whose nematic and/or smectic-A phases do not possess optical uniaxiality (viz., more than one optical axis exists), is given in this review. In particular, we emphasize on how Nuclear Magnetic Resonance (NMR) spectroscopy can help to advance the understanding of phase biaxiality in general, and to examine recent debates on the existence of biaxial nematic phase reported in low molecular mass bent-core or V-shaped mesogens. A general discussion of orientational order parameters is detailed, particularly in smectic-C (SmC) and biaxial nematic phases. How these orientational order parameters can be determined by various techniques such as NMR, IR absorbance and Raman scattering studies, will be mentioned. Recent X-ray observations of SmC clusters in the nematic phase of V-shaped mesogens are highlighted and contrasted with probable theory. Moreover, deuterium and carbon-13 NMR techniques are briefly reviewed, and their possible utilization to identify phase biaxiality in these biaxial LC systems is explored.

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