211 research outputs found
Reconfigurable knots and links in chiral nematic colloids
Tying knots and linking microscopic loops of polymers, macromolecules, or
defect lines in complex materials is a challenging task for material
scientists. We demonstrate the knotting of microscopic topological defect lines
in chiral nematic liquid crystal colloids into knots and links of arbitrary
complexity by using laser tweezers as a micromanipulation tool. All knots and
links with up to six crossings, including the Hopf link, the Star of David and
the Borromean rings are demonstrated, stabilizing colloidal particles into an
unusual soft matter. The knots in chiral nematic colloids are classified by the
quantized self-linking number, a direct measure of the geometric, or Berry's,
phase. Forming arbitrary microscopic knots and links in chiral nematic colloids
is a demonstration of how relevant the topology can be for the material
engineering of soft matter.Comment: 6 pages, 3 figure
Comparison of Non-stationary Sinusoid Estimation Methods Using Reassignment and Derivatives
(Abstract to follow
Flexoelectricity and piezoelectricity - reason for rich variety of phases in antiferroelectric liquid crystals
The free energy of antiferroelectric liquid crystal which takes into account
polar order explicitly is presented. Steric, van der Waals, piezoelectric and
flexoelectric interactions to the nearest layers and dipolar electrostatic
interactions to the nearest and to the next nearest layers induce indirect tilt
interactions with chiral and achiral properties, which extend to the third and
to the fourth nearest layers. Chiral indirect interactions between tilts can be
large and induce helicoidal modulations even in systems with negligible chiral
van der Waals interactions. If indirect chiral interactions compete with chiral
van der Waals interactions, the helix unwinding is possible. Although strength
of microscopic interactions change monotonically with decreasing temperature,
effective interlayer interactions change nonmonotonically and give rise to
nonmonotouous change of modulation period through various phases. Increased
enatiomeric excess i.e. increased chirality changes the phase sequence.Comment: 4 pages, 1 figur
Vector beams generated by microlasers based on topological liquid-crystal structures
Structured light with designable intensity, polarization and phase fields is
today of high relevance, with application ranging from imaging, metrology,
optical trapping, ultracold atoms, classical and quantum communications and
memory. Specifically, vortex and vector beams can be generated directly in the
laser cavity, however, a controllable, geometrically simple and easy to
manufacture laser microcavity that generates structured light on demand,
especially tailored polarization, is still an open challenge. Here we show that
tunable laser vector beams can be generated from self-assembled liquid-crystal
(LC) micro-structures with topological defects inside a thin Fabry-P\'erot
microcavity. The LC superstructure provides complex three dimensional
birefringent refractive index profiles with order parameter singularities. The
topology of the LC structures is transferred into the topology of the light
polarization. The oriented fluorescent dye emission dipoles enable the
selection of optical modes with a particular polarization, as enabled by the
birefringence profile in the laser cavity. The proposed lasers have no
principal limitation for realizing structured light with arbitrarily tailored
intensity and polarization fields
Generation of Multiple Circular Walls on a Thin Film of Nematic Liquid Crystal by Laser Scanning
We found that multiple circular walls (MCW) can be generated on a thin film
of a nematic liquid crystal through a spiral scanning of a focused IR laser.
The ratios between radii of adjacent rings of MCW were almost constant. These
constant ratios can be explained theoretically by minimization of the Frank
elastic free energy of nematic medium. The director field on a MCW exhibits
chiral symmetry-breaking although the elastic free energies of both chiral MCWs
are degenerated, i.e., the director on a MCW can rotate clockwise or
counterclockwise along the radial direction.Comment: 10 pages, 5 figures. Submitted to Chemical Physics Letters 2nd
Editio
Switching dynamics of surface stabilized ferroelectric liquid crystal cells: effects of anchoring energy asymmetry
We study both theoretically and experimentally switching dynamics in surface
stabilized ferroelectric liquid crystal cells with asymmetric boundary
conditions. In these cells the bounding surfaces are treated differently to
produce asymmetry in their anchoring properties. Our electro-optic measurements
of the switching voltage thresholds that are determined by the peaks of the
reversal polarization current reveal the frequency dependent shift of the
hysteresis loop. We examine the predictions of the uniform dynamical model with
the anchoring energy taken into account. It is found that the asymmetry effects
are dominated by the polar contribution to the anchoring energy. Frequency
dependence of the voltage thresholds is studied by analyzing the properties of
time-periodic solutions to the dynamical equation (cycles). For this purpose,
we apply the method that uses the parameterized half-period mappings for the
approximate model and relate the cycles to the fixed points of the composition
of two half-period mappings. The cycles are found to be unstable and can only
be formed when the driving frequency is lower than its critical value. The
polar anchoring parameter is estimated by making a comparison between the
results of modelling and the experimental data for the shift vs frequency
curve. For a double-well potential considered as a deformation of the
Rapini-Papoular potential, the branch of stable cycles emerges in the low
frequency region separated by the gap from the high frequency interval for
unstable cycles.Comment: 35 pages, 15 figure
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Morphology of polymer networks formed in the chiral and non chiral phases of an anti-ferroelectric liquid crystal
We introduced photo-polymer networks into the various liquid crystalline phases of the antiferroelectric liquid crystal AS612 and studied the effects of these networks by measuring the temperature dependence of the Bragg wavelengths selectively reflected. After polymerization, the decrease in Bragg wavelengths with respect to the original values is consistent with a shorter helical pitch due to polymer network shrinkage. Also, by removing the liquid crystalline material, we are able to image the residual polymer network using scanning electron microscopy and polarized light microscopy. The polymer strands are a few microns thick and the networks show both chiral and non-chiral features
Front propagation into unstable and metastable states in Smectic C* liquid crystals: linear and nonlinear marginal stability analysis
We discuss the front propagation in ferroelectric chiral smectics (SmC*)
subjected to electric and magnetic fields applied parallel to smectic layers.
The reversal of the electric field induces the motion of domain walls or fronts
that propagate into either an unstable or a metastable state. In both regimes,
the front velocity is calculated exactly. Depending on the field, the speed of
a front propagating into the unstable state is given either by the so-called
linear marginal stability velocity or by the nonlinear marginal stability
expression. The cross-over between these two regimes can be tuned by a magnetic
field. The influence of initial conditions on the velocity selection problem
can also be studied in such experiments. SmC therefore offers a unique
opportunity to study different aspects of front propagation in an experimental
system
Melting of 2D liquid crystal colloidal structure
Using video microscopy, we investigated melting of a two-dimensional colloidal system, formed by glycerol droplets at the free surface of a nematic liquid crystalline layer. Analyzing different structure correlation functions, we conclude that melting occurs through an intermediate hexatic phase, as predicted by the Kosterlitz-Thouless-Halperin-Nelson-Young(KTHNY) theory. However, the temperature range of the intermediate phase is rather narrow, <1°C, and the characteristic critical power law decays of the correlation functions are not fully developed. We conclude that the melting of our 2D systems qualitatively occurs according to KTHNY, although quantitative details of the transition scenario may partly depend on the details of interparticle interaction
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