121 research outputs found
Fourth order nematic elasticity and modulated nematic phases: a poor man's approach
We propose an extension of Frank-Oseen's elastic energy for bulk nematic
liquid crystals which is based on the hypothesis that the fundamental
deformations allowed in nematic liquid crystals are splay, twist and bend. The
extended elastic energy is a fourth order form in the fundamental deformations.
The existence of bulk spontaneous modulated or deformed nematic liquid crystal
ground states is investigated. The analysis is limited to bulk nematic liquid
crystals in the absence of limiting surfaces and/or external fields. The non
deformed ground state is stable only when Frank-Oseen's elastic constants are
positive. In case where at least one of them is negative, the ground state
becomes deformed. The analysis of the stability of the deformed states in the
space of the elastic parameters allows to characterize different types of
deformed nematic phases. Some of them are new nematic phases, for instance a
twist -- splay nematic phase is predicted. Inequalities between second order
elastic constants which govern the stability of the twist--bend and splay--bend
state are obtained
Influence of Homeotropic Anchoring Walls upon Nematic and Smectic Phases
McMillan liquid crystal model sandwiched between strong homeotropic anchoring
walls is studied. Phase transitions between isotropic, nematic, and smectic A
phases are investigated for wide ranges of an interaction parameter and of the
system thickness. It is confirmed that the anchoring walls induce an increase
in transition temperatures, dissappearance of phase transitions, and an
appearance of non-spontaneous nematic phase. The similarity between influence
of anchoring walls and that of external fields is discussed.Comment: 5 pages, 6 figure
Isotropic, Nematic and Smectic A Phase Behaviour in a Fictitious Field
Phase behaviours of liquid crystals under external fields, conjugate to the
nematic order and smectic order, are studied within the framework of mean field
approximation developed by McMillan. It is found that phase diagrams, of
temperature vs interaction parameter of smectic A order, show several
topologically different types caused by the external fields. The influences of
the field conjugate to the smectic A phase, which is fictitious field, are
precisely discussed.Comment: To be published in J. Phys. Soc. Jpn. vol.73 No.
Nanometric pitch in modulated structures of twist-bend nematic liquid crystals
The extended Frank elastic energy density is used to investigate the
existence of a stable periodically modulate structure that appears as a ground
state exhibiting a twist-bend molecular arrangement. For an unbounded sample,
we show that the twist-bend nematic phase is characterized by a
heliconical structure with a pitch in the nano-metric range, in agreement with
experimental results. For a sample of finite thickness, we show that the wave
vector of the stable periodic structure depends not only on the elastic
parameters but also on the anchoring energy, easy axis direction, and the
thickness of the sample.Comment: 11 page
Instability patterns in ultrathin nematic films: comparison between theory and experiment
Motivated by recent experimental observations [U. Delabre et al, Langmuir 24,
3998, 2008] we reconsider an instability of ultrathin nematic films, spread on
liquid substrates. Within a continuum elastic theory of liquid crystals, in the
harmonic approximation, we find an analytical expressions for the critical
thickness as well as for the critical wavenumber, characterizing the onset of
instability towards the stripe phase. Comparing theoretical predictions with
experimental observations, we establish the utility of surface-like term such
as an azimuthal anchoring.Comment: 6 pages, 3 figures, LaTeX macros EPL draft, accepted for publication
in EP
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