324 research outputs found
Tunability of wire-grid metamaterial immersed into nematic liquid crystal
We propose electrically tunable hybrid metamaterial consisting of special
wire grid immersed into nematic liquid crystal. The plasma-like permittivity of
the structure can be substantially varied due to switching of the liquid
crystal alignment by external voltages applied to the wires. Depending on the
scale of the structure, the effect is available for both microwave and optical
frequency ranges.Comment: 3 page
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
Order parameters of some nematic p, p′ substituted tolanes as determined by polarized Raman scattering
The order parameters 〈P2〉 and 〈P4〉 of three nematic compounds have been determined by polarized Raman scattering. Results obtained with p‐pentyl‐p′‐cyanobiphenyl (5CB) agree well with published data. Next the influence of a strongly polar cyano end group on the order parameters was investigated. This was done by comparing two nematic liquid crystals of the tolane class, namely p‐heptyl‐p′‐cyanotolane and p‐heptyl‐p′‐methoxytolane, which have an almost identical geometrical shape. In spite of the difference in polarity hardly any difference in the order parameters 〈P2〉 and 〈P4〉 of both compounds could be observed. 〈P2〉 and 〈P4〉 of both tolanes show approximately mean field behavior in contrast to the low 〈P4〉 of 5CB
Pretransitional behavior in a water-DDAB-5CB microemulsion close to the demixing transition. Evidence for intermicellar attraction mediated by paranematic fluctuations
We present a study of a water-in-oil microemulsion in which surfactant coated
water nanodroplets are dispersed in the isotropic phase of the thermotropic
liquid crystal 5CB. As the temperature is lowered below the isotropic to
nematic phase transition of pure 5CB, the system displays a demixing transition
leading to a coexistence of a droplet rich isotropic phase with a droplet poor
nematic. The transition is anticipated, in the high T side, by increasing
pretransitional fluctuations in 5CB molecular orientation and in the
nanodroplet concentration. The observed phase behavior supports the notion that
the nanosized droplets, while large enough for their statistical behavior to be
probed via light scattering, are also small enough to act as impurities,
disturbing the local orientational ordering of the liquid crystal and thus
experiencing pretransitional attractive interaction mediated by paranematic
fluctuations. The pretransitional behavior, together with the topology of the
phase diagram, can be understood on the basis of a diluted Lebwohl-Lasher model
which describes the nanodroplets simply as holes in the liquid crystal.Comment: 64 pages, 16 figures, J. Chem. Phys. in pres
Effect of a rigid nonpolar solute on the splay, bend elastic constants and on rotational viscosity coefficient of 4-4^\prime -n-octyl-cyanobiphenyl
The effect of a rigid nonpolar non-mesogenic solute, ``biphenyl'' which is
(C_{6}H_{5}-C_{6}H_{5}), on the splay and bend elastic constants and on the
rotational viscosity coefficient of (4,4^{\prime})-n-octyl-cyano biphenyl (8CB)
is reported. The experiments involve the measurement of voltage dependence of
capacitance of a cell filled with the mixture. Anomalous behavior of both
(K_{11}) and (\Delta \epsilon) near the (N-S_{A}) transition have been
observed.Comment: RevTeX - 8 figures. Accepted to be published in Physical Review
Viscosities of the Gay-Berne nematic liquid crystal
We present molecular dynamics simulation measurements of the viscosities of
the Gay-Berne phenomenological model of liquid crystals in the nematic and
isotropic phases. The temperature dependence of the rotational and shear
viscosities, including the nonmonotonic behavior of one shear viscosity are in
good agreement with experimental data. The bulk viscosities are significantly
larger than the shear viscosities, again in agreement with experiment.Comment: 11 pages, 4 Postscript figures, Revte
Electric-field-induced nematic-cholesteric transition and 3-D director structures in homeotropic cells
We study the phase diagram of director structures in cholesteric liquid
crystals of negative dielectric anisotropy in homeotropic cells of thickness d
which is smaller than the cholesteric pitch p. The basic control parameters are
the frustration ratio d/p and the applied voltage U. Fluorescence Confocal
Polarising Microscopy allows us to directly and unambiguously determine the 3-D
director structures. The results are of importance for potential applications
of the cholesteric structures, such as switchable gratings and eyewear with
tunable transparency based.Comment: Will be published in Physical Review
Continuous Paranematic-to-Nematic Ordering Transitions of Liquid Crystals in Tubular Silica Nanochannels
The optical birefringence of rod-like nematogens (7CB, 8CB), imbibed in
parallel silica channels with 10 nm diameter and 300 micrometer length, is
measured and compared to the thermotropic bulk behavior. The orientational
order of the confined liquid crystals, quantified by the uniaxial nematic
ordering parameter, evolves continuously between paranematic and nematic
states, in contrast to the discontinuous isotropic-to-nematic bulk phase
transitions. A Landau-de Gennes model reveals that the strength of the
orientational ordering fields, imposed by the silica walls, is beyond a
critical threshold, that separates discontinuous from continuous
paranematic-to-nematic behavior. Quenched disorder effects, attributable to
wall irregularities, leave the transition temperatures affected only
marginally, despite the strong ordering fields in the channels.Comment: 4 pages, 3 figures, Physical Review Letters (in press
Electro-Mechanical Fredericks Effects in Nematic Gels
The solid nematic equivalent of the Fredericks transition is found to depend
on a critical field rather than a critical voltage as in the classical case.
This arises because director anchoring is principally to the solid rubbery
matrix of the nematic gel rather than to the sample surfaces. Moreover, above
the threshold field, we find a competition between quartic (soft) and
conventional harmonic elasticity which dictates the director response. By
including a small degree of initial director misorientation, the calculated
field variation of optical anisotropy agrees well with the conoscopy
measurements of Chang et al (Phys.Rev.E56, 595, 1997) of the electro-optical
response of nematic gels.Comment: Latex (revtex style), 5 EPS figures, submitted to PRE, corrections to
discussion of fig.3, cosmetic change
Volume-energy correlations in the slow degrees of freedom of computer-simulated phospholipid membranes
Constant-pressure molecular-dynamics simulations of phospholipid membranes in
the fluid phase reveal strong correlations between equilibrium fluctuations of
volume and energy on the nanosecond time-scale. The existence of strong
volume-energy correlations was previously deduced indirectly by Heimburg from
experiments focusing on the phase transition between the fluid and the ordered
gel phases. The correlations, which are reported here for three different
membranes (DMPC, DMPS-Na, and DMPSH), have volume-energy correlation
coefficients ranging from 0.81 to 0.89. The DMPC membrane was studied at two
temperatures showing that the correlation coefficient increases as the phase
transition is approached
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