84 research outputs found
Stability of Biaxial Nematic Phase for Systems with Variable Molecular Shape Anisotropy
We study the influence of fluctuations in molecular shape on the stability of
the biaxial nematic phase by generalizing the mean field model of Mulder and
Ruijgrok [Physica A {\bf 113}, 145 (1982)]. We limit ourselves to the case when
the molecular shape anisotropy, represented by the alignment tensor, is a
random variable of an annealed type. A prototype of such behavior can be found
in lyotropic systems - a mixture of potassium laurate, 1-decanol, and ,
where distribution of the micellar shape adjusts to actual equilibrium
conditions. Further examples of materials with the biaxial nematic phase, where
molecular shape is subject to fluctuations, are thermotropic materials composed
of flexible trimeric- or tetrapod-like molecular units. Our calculations show
that the Gaussian equilibrium distribution of the variables describing
molecular shape (dispersion force) anisotropy gives rise to new classes of the
phase diagrams, absent in the original model. Depending on properties of the
shape fluctuations, the stability of the biaxial nematic phase can be either
enhanced or depressed, relative to the uniaxial nematic phases. In the former
case the splitting of the Landau point into two triple points with a direct
phase transition line from isotropic to biaxial phase is observed.Comment: 18 pages containing 6 figure
Thermal phase diagrams of columnar liquid crystals
In order to understand the possible sequence of transitions from the
disordered columnar phase to the helical phase in hexa(hexylthio)triphenylene
(HHTT), we study a three-dimensional planar model with octupolar interactions
inscribed on a triangular lattice of columns. We obtain thermal phase diagrams
using a mean-field approximation and Monte Carlo simulations. These two
approaches give similar results, namely, in the quasi one-dimensional regime,
as the temperature is lowered, the columns order with a linear polarization,
whereas helical phases develop at lower temperatures. The helicity patterns of
the helical phases are determined by the exact nature of the frustration in the
system, itself related to the octupolar nature of the molecules.Comment: 12 pages, 9 figures, ReVTe
Evolution of interfaces and expansion in width
Interfaces in a model with a single, real nonconserved order parameter and
purely dissipative evolution equation are considered. We show that a systematic
perturbative approach, called the expansion in width and developed for curved
domain walls, can be generalized to the interfaces. Procedure for calculating
curvature corrections is described. We also derive formulas for local velocity
and local surface tension of the interface. As an example, evolution of
spherical interfaces is discussed, including an estimate of critical size of
small droplets.Comment: Discussion of stability of the interface is added, and the numerical
estimates of width and velocity of the interface in the liquid crystal
example are corrected. 25 pages, Latex2
Competing orders in a magnetic field: spin and charge order in the cuprate superconductors
We describe two-dimensional quantum spin fluctuations in a superconducting
Abrikosov flux lattice induced by a magnetic field applied to a doped Mott
insulator. Complete numerical solutions of a self-consistent large N theory
provide detailed information on the phase diagram and on the spatial structure
of the dynamic spin spectrum. Our results apply to phases with and without
long-range spin density wave order and to the magnetic quantum critical point
separating these phases. We discuss the relationship of our results to a number
of recent neutron scattering measurements on the cuprate superconductors in the
presence of an applied field. We compute the pinning of static charge order by
the vortex cores in the `spin gap' phase where the spin order remains
dynamically fluctuating, and argue that these results apply to recent scanning
tunnelling microscopy (STM) measurements. We show that with a single typical
set of values for the coupling constants, our model describes the field
dependence of the elastic neutron scattering intensities, the absence of
satellite Bragg peaks associated with the vortex lattice in existing neutron
scattering observations, and the spatial extent of charge order in STM
observations. We mention implications of our theory for NMR experiments. We
also present a theoretical discussion of more exotic states that can be built
out of the spin and charge order parameters, including spin nematics and phases
with `exciton fractionalization'.Comment: 36 pages, 33 figures; for a popular introduction, see
http://onsager.physics.yale.edu/superflow.html; (v2) Added reference to new
work of Chen and Ting; (v3) reorganized presentation for improved clarity,
and added new appendix on microscopic origin; (v4) final published version
with minor change
Core charge distribution and self assembly of columnar phases: the case of triphenylenes and azatriphenylenes
<p>Abstract</p> <p>Background</p> <p>The relation betweeen the structure of discotic molecules and columnar properties, a crucial point for the realization of new advanced materials, is still largely unknown. A paradigmatic case is that hexa-alkyl-thio substituted triphenylenes present mesogenic behavior while the corresponding azatriphenylenes, similar in shape and chemical structure, but with a different core charge distribution, do not form any liquid crystalline mesophase. This study is aimed at investigating, with the help of computer simulations techniques, the effects on phase behaviour of changes of the charge distribution in the discotic core.</p> <p>Results</p> <p>We described the shape and the pair, dispersive and electrostatic, interactions of hexa alkyl triphenylenes by uniaxial Gay-Berne discs with embedded point charges. Gay-Berne parameters were deduced by fitting the dispersive energies obtained from an atomistic molecular dynamics simulation of a small sample of hexa-octyl-thio triphenylene molecules in columnar phase, while a genetic algorithm was used to get a minimal set of point charges that properly reproduces the ab anitio electrostatic potential. We performed Monte Carlo simulations of three molecular models: the pure Gay-Berne disc, used as a reference, the Gay-Berne disc with hexa-thio triphenylene point charges, the Gay-Berne disc with hexa-thio azatriphenylene point charges. The phase diagram of the pure model evidences a rich polymorphism, with isotropic, columnar and crystalline phases at low pressure, and the appearance of nematic phase at higher pressure.</p> <p>Conclusion</p> <p>We found that the intermolecular electrostatic potential among the cores is fundamental in sta-bilizing/destabilizing columnar phases; in particular the triphenylene charge distribution stabilizes the columnar structure, while the azatriphenylene distribution suppresses its formation in favor of the nematic phase. We believe the present model could be successfully employed as the basis for coarse-grained level simulations of a wider class of triphenylene derivatives.</p
Non-Isothermal Model for Nematic Spherulite Growth
A computational study of the growth of two-dimensional nematic spherulites in
an isotropic phase was performed using a Landau-de Gennes type quadrupolar
ensor order parameter model for the first-order isotropic/nematic transition of
5CB (pentyl-cyanobiphenyl). An energy balance, taking anisotropy into account,
was derived and incorporated into the time-dependent model. Growth laws were
determined for two different spherulite morphologies of the form tn, with and
without the inclusion of thermal effects. Results show that incorporation of
the thermal energy balance correctly predicts the transition of the growth law
exponent from the volume driven regime (n=1) to the thermally limited regime
(approaching n=0.5), agreeing well with experimental observations. An
interfacial nemato-dynamic model is used to gain insight into the interactions
that result in the progression of different spherulite growth regimes
Non-Invasive In Vivo Imaging of Calcium Signaling in Mice
Rapid and transient elevations of Ca2+ within cellular microdomains play a critical role in the regulation of many signal transduction pathways. Described here is a genetic approach for non-invasive detection of localized Ca2+ concentration ([Ca2+]) rises in live animals using bioluminescence imaging (BLI). Transgenic mice conditionally expressing the Ca2+-sensitive bioluminescent reporter GFP-aequorin targeted to the mitochondrial matrix were studied in several experimental paradigms. Rapid [Ca2+] rises inside the mitochondrial matrix could be readily detected during single-twitch muscle contractions. Whole body patterns of [Ca2+] were monitored in freely moving mice and during epileptic seizures. Furthermore, variations in mitochondrial [Ca2+] correlated to behavioral components of the sleep/wake cycle were observed during prolonged whole body recordings of newborn mice. This non-invasive imaging technique opens new avenues for the analysis of Ca2+ signaling whenever whole body information in freely moving animals is desired, in particular during behavioral and developmental studies
Liquid crystals and their defects
These lecture notes discuss classical models of liquid crystals, and the
different ways in which defects are described according to the different
models.Comment: CIME lecture course, Cetraro, 201
Trazodone regulates neurotrophic/growth factors, mitogen-activated protein kinases and lactate release in human primary astrocytes
Background: In the central nervous system, glial cells provide metabolic and trophic support to neurons and respond to protracted stress and insults by up-regulating inflammatory processes. Reactive astrocytes and microglia are associated with the pathophysiology of neuronal injury, neurodegenerative diseases and major depression, in both animal models and human brains. Several studies have reported clear anti-inflammatory effects of anti-depressant treatment on astrocytes, especially in models of neurological disorders. Trazodone (TDZ) is a triazolopyridine derivative that is structurally unrelated to other major classes of antidepressants. Although the molecular mechanisms of TDZ in neurons have been investigated, it is unclear whether astrocytes are also a TDZ target. Methods: The effects of TDZ on human astrocytes were investigated in physiological conditions and following inflammatory insult with lipopolysaccharide (LPS) and tumour necrosis factor-aα (TNF-aα). Astrocytes were assessed for their responses to pro-inflammatory mediators and cytokines, and the receptors and signalling pathways involved in TDZ-mediated effects were evaluated. Results: TDZ had no effect on cell proliferation, but it decreased pro-inflammatory mediator release and modulated trophic and transcription factor mRNA expression. Following TDZ treatment, the AKT pathway was activated, whereas extracellular signal-regulated kinase and c-Jun NH2-terminal kinase were inhibited. Most importantly, a 72-h TDZ pre-treatment before inflammatory insult completely reversed the anti-proliferative effects induced by LPS-TNF-aα. The expression or the activity of inflammatory mediators, including interleukin-6, c-Jun NH2-terminal kinase and nuclear factor ΚB, were also reduced. Furthermore, TDZ affected astrocyte metabolic support to neurons by counteracting the inflammation-mediated lactate decrease. Finally, TDZ protected neuronal-like cells against neurotoxicity mediated by activated astrocytes. These effects mainly involved an activation of 5-HT1A and an antagonism at 5-HT2A/C serotonin receptors. Fluoxetine, used in parallel, showed similar final effects nevertheless it activates different receptors/intracellular pathways. Conclusions: Altogether, our results demonstrated that TDZ directly acts on astrocytes by regulating intracellular signalling pathways and increasing specific astrocyte-derived neurotrophic factor expression and lactate release. TDZ may contribute to neuronal support by normalizing trophic and metabolic support during neuroinflammation, which is associated with neurological diseases, including major depression
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