Recent results on bent-core liquid crystals
We investigate flexodomains, which are observed in planar layers of
certain nematic liquid crystals, when a dc voltage U above a critical value Uc
is applied across the layer. They are characterized by stationary stripelike
spatial variations of the director in the layer plane with a wave number p(U). Our experiments for different nematics demonstrate
that p(U) varies almost linearly with U for U >Uc.
That is confirmed by a numerical analysis of the full nonlinear equations for
the director field and the induced electric potential. Beyond this numerical
study, we demonstrate that the linearity of p(U)
follows even analytically, when considering a special parameter set first used
by Terent’ev and Pikin [Sov. Phys. JETP 56, 587 (1982)]. Their theoretical
paper serves until now as the standard reference on the nonlinear analysis of
flexodomains, since it has arrived at a linear variation of p(U)
for large U Uc. Unfortunately, the
corresponding analysis suffers from mistakes, which in a combination led to
that result.
W. Pesch, A. Krekhov, N. Éber
and Á. Buka, Nonlinear analysis of flexodomains in nematic liquid crystals, Phys. Rev. E 98,
032702 (2018) [pdf]
The possibilities to induce regular, stationary stripe
patterns with easily controllable wavenumber have been investigated with the
aim to apply them in optical devices as gratings. Several, electric field
induced phenomena have been studied on three members of a homologous series of
a bent core nematic. Pattern morphologies, threshold voltages and wave numbers
have been determined. Temporal evolution and switching dynamics have been
analyzed.
N. Éber, Y. Xiang, and Á. Buka: Bent core nematics as optical gratings, J.
Mol. Liq. http://dx.doi.org/10.1016/j.molliq.2017.09.025
Optical gratings have been
created by flexoelectric domains in a bent-core nematic liquid crystal. A
unique feature of this structure is that its wavelength can be controlled by
the amplitude of the applied voltage, as demonstrated by polarizing microscopy
and light diffraction techniques. In order to understand the reaction of the
system to the voltage change, the dynamics of the switching process has been
studied via digital processing of recorded image sequences. It has been shown
that the characteristics and the switching mechanisms are different, if the
lower voltage level is below or above the threshold of pattern onset. In both
cases, the response to increasing voltage levels is much slower than the
response to decreasing voltage levels.
Ying Xiang, Hong-Zhen Jing, Zhi-Dong Zhang, Wen-Jiang Ye, Ming-Ya Xu,
Everett Wang, Péter Salamon, Nándor
Éber, and Ágnes Buka: Tunable optical grating based on the
flexoelectric effect in a bent-core nematic liquid crystal, Phys. Rev. Appl. 7, 064032 (2017). [pdf]
The primary research goal within contemporary liquid crystalline
display industry is to discover new materials exhibiting phase temperature range
and properties useful for display application. In thiswork,we
studied five binary mixtures composed of different concentrations of bentcore and
calamitic compounds. Pure compounds and selected mixtures were studied by
polarizing optical microscopy, differential scanning calorimetry, and X-ray
diffractometry, as well as by semi-empirical quantum-chemical calculations.
Phase transition types, temperatures, and enthalpies were determined. Molecular
packaging in the nematic mesophase is proposed. All studied mixtures exhibit
the nematic phase of wide temperature range near to the room temperature.
M. Cvetinov, M. Stojanovic, D. Obadovic, I. Ristic, A. Vajda, K.
Fodor-Csorba, and N. Eber: Phase
behavior and molecular self-assembly of some calamitic/bent-core nematic
mixtures, Molecular
Crystals and Liquid Crystals, 630 (1), 28-36 (2016).
[pdf]
We have investigated three binary mixtures composed of selected
banana-shaped dopant in low concentrations and calamitic mesogen in high.
Banana-shaped dopant forms a B7 phase, while the calamitic mesogen exhibit
nematic and smectic SmA and SmC phases. The occurring mesophases have been
identified by their optical textures. At dopant concentrations of 2.2 and 3.1
mol%, there is evident broadening of nematic and smectic SmA temperature ranges
in respect to the pure calamitic compound. Yet, the mixture with dopant
concentration of 7 mol% exhibits narrower temperature ranges of mesophases.
Increasing dopant concentration caused lowering of all phase transitions
temperatures (TI-N, TN-SmA, TSmA-SmC) in all investigated mixtures. Therefore,
mixing classic calamitic compounds with novel bananashaped compound in low
concentrations is viable way to attain useful mesophase range for application
in industry.
Miroslav Cvetinov, Maja Stojanović, Dušanka Obadović, Aniko Vajda,
Katalin Fodor-Csorba, and Nandor Eber:
Broadening of mesophase temperature
range induced by doping calamitic mesogen with banana-shaped mesogen, AIP Conf. Proc. 1722, 220008
(2016). [pdf]
Phase transitions of different binary mixtures of a bent-core
(10DClPBBC) and a rod-shaped (6OO8) liquid crystal were studied using differential
scanning calorimetry. For the binary mixture with 50:50 weight ratio of
bent-core and rod-shaped molecules, the nematic to smectic transition occurred
below the temperature of 40◦C and crystallization was shifted to sub-ambient temperature. It was
found that crystallization was the phase transition with the lowest apparent
activation energy.
A. Juríková, K. Csach, J. Miškuf, N. Tomašovičová, Z. Mitróová, P. Kopčanský, N. Éber, K.
Fodor-Csorba, and
A. Vajda: DSC study of bent-core and rod-shaped liquid crystal mixtures, Mol. Cryst. Liq. Cryst. 610, 187-192 (2015) [pdf]
We have investigated the
influence of doping with spherical magnetic nanoparticles on the mixture of a
bent-core and a calamitic liquid crystal. Results showed a reduction of the
critical field of the magnetic Fréedericksz transition by more than a factor of
two after the doping. Moreover, we give for the first time experimental
evidence of the theoretically predicted magnetically induced negative shift of
the isotropic to nematic phase transition temperature.
N. Tomašovičová, M. Timko, N. Éber, T. Tóth-Katona, K. Fodor-Csorba, A.
Vajda, V. Gdovinová, X. Chaud, and P. Kopčanský: Magnetically Induced Shift of the Isotropic-Nematic
Phase Transition Temperature in a Mixture of Bent-Core and Calamitic Liquid
Crystals Doped with Magnetic Particles, Liquid
Crystals, 42,
959-963 (2015) [[pdf]
A regular
domain structure consisting of parallel stripes – flexodomains
– have been induced by low frequency (subHz) electric voltage in a bent
core nematic liquid crystal. The wavelength of the pattern is in the range of
1–10 micrometers and thus can conveniently be observed in a polarizing
microscope. It also serves as an optical grating and
produces a regular system of laser diffraction spots. The pattern was found to
emerge and disappear consecutively in each half period of the driving, with the
wavelength of the flexodomains changing periodically
as the ac voltage oscillates. Analyzing the polarization characteristics of the
diffracted light, the polarization of the first order spot was found
perpendicular to that of the incident light, in accordance with a recent
theoretical calculation.
Ming-Ya
Xu, Meng-jie Zhou, Ying Xiang, Péter Salamon, Nándor Éber, and Ágnes Buka:
Domain structures as optical gratings controlled by electric field in a
bent-core nematic. Optics
Express 23(12),
15224
(2015) [pdf]
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In the work phase transitions in bent-core
liquid crystals were studied using differential scanning calorimetry. For the
binary mixture of bent-core molecules with 50 wt% of rod-shaped compound, the
nematic to smectic transition occured below 40oC and the
crystallization temperature shifted to sub-ambient temperatures. The
influence of doping of the bent-core liquid crystals with magnetic
nanoparticles on the kinetics of observed phase transitions was studied. The
phase transition temperatures were shifted depending on the nanoparticle type
and changed with varying cooling rate for all studied liquid crystal samples. |
|
A.
Juríková, K. Csach, J. Miškuf, N. Tomašovičová, Z. Mitróová, V. Závišová, M. Koneracká,
P. Kopčanský, M. Timko, N. Éber, K. Fodor-Csorba, A. Vajda: Thermal Stability
of Bent-Core Liquid Crystals Doped with Magnetic Nanoparticles, Acta Phys.
Polon. A 127(2), 638-640 (2015)
[pdf]
|
Electric-field-induced patterns of diverse morphology have been
observed over a wide frequency range in a recently synthesized bent-core nematic (BCN) liquid crystal. At low
frequencies (up to ∼25 Hz), the BCN exhibited
unusual polarity-dependent patterns. When the
amplitude of the ac field was enhanced, these two time-asymmetrical patterns
turned into time-symmetrical prewavylike stripes.
At ac frequencies in the middle-frequency range (∼50–3000 Hz), zigzag patterns were
detected whose obliqueness varied with the frequency. Finally, if the
frequency was increased above 3 kHz, the zigzag pattern was replaced by
another, prewavylike pattern, whose threshold voltage depended on the
frequency; however, the wave vector did not. For a more complete
characterization, material parameters such as
elastic constants, dielectric permittivities, and the anisotropy of the
diamagnetic susceptibility were also determined. |
|
Ying Xiang, Meng-jie Zhou, Ming-Ya Xu, Péter Salamon, Nándor Éber, and
Ágnes Buka: Unusual polarity-dependent
patterns in a bent-core nematic liquid crystal under low-frequency ac field, Phys. Rev. E 91,
042501 (2015) [pdf].
In this work, five
mixtures with different concentrations of banana-shaped and calamitic compounds
have been prepared and subsequently studied by polarizing optical microscopy,
differential scanning calorimetry, and X-ray diffraction on non-oriented
samples. The phase sequences and molecular parameters of the binary systems are
presented.
M. Cvetinov, D. Ž. Obadović, M. Stojanović, A. Vajda, K. Fodor-Csorba, N.
Eber, and I. Ristić: Phase behaviors of binary mixtures composed of banana-shaped and
calamitic mesogens, Chin. Phys. B, 23(9), 096402/1-6 (2014). [pdf]
|
Two kinds of electroconvection patterns in an ether-bridged
bent-core nematic liquid crystal material (BCN), which appear in
different frequency ranges, are examined and compared. One is a longitudinal pattern with the stripes parallel to
the orientation of the BCN and with a periodicity of approximately the cell
thickness, occurring in the high-frequency range of several hundreds Hz; the other
one is oblique stripes, which results in a
zigzag pattern, and appears in the low-frequency range of several tens Hz. In
addition, within an intermediate-frequency range, transformations
from oblique to longitudinal and then to normal stripes occur at
increased ac voltages. In particular, we investigated the temperature
behavior of longitudinal and oblique stripes: When the temperature T increases
and approaches the clearing temperature Tc, the contrast of the
domains is enhanced and the frequency range of existence becomes wider, while
the onset voltages increase only moderately instead of diverging, thus
suggesting an isotropic mechanism of pattern formation. |
|
Ying Xiang, Yi-Kun Liu, Ágnes Buka, Nándor Éber, Zhi-Yong Zhang, Ming-Ya Xu, and Everett Wang: Electric-field-induced patterns and their temperature dependence in a
bent-core liquid crystal, Phys.
Rev. E 89,
012502/1-9 (2014) [pdf]
The mesophase
behaviour of binary mixtures of bent-core and calamitic liquid crystals is presented.
The nematogenic 4,6-dichloro-1,3-phenylene bis[4’-(10-undecen-1-yloxy)-1,1’-biphenyl-4-carboxylate]
(I) was the banana-shaped component. As the calamitic compound ethyl 4’-(9-decen-1-yloxy)-1,1’-biphenyl-4-carboxylate
(II), similar to one arm of the bent-core molecule, was used which
exhibits smectic phases in a wide temperature range. A total of six mixtures
with different compositions were prepared and studied by polarising optical
microscopy, differential scanning calorimetry and X-ray diffraction on
non-oriented samples. In the mixtures, a nematic phase is not concomitant with
smectic A phase, and the temperature range of both phases highly depends on the
concentration of the comprising compounds. Lowered melting temperatures have
been observed for all mixtures with respect to that of the pure compounds.
Unforeseen finding is the induction of a monotropic SmC phase in mixtures with
lowest concentration of the bent-core compound. Semi-empirical quantum-chemical
calculations have also been performed. Based on the calculated molecular
conformation, as well as on collected X-ray diffraction data, a model for a
possible self-assembly of the banana-shaped and calamitic compounds is
proposed.
M. Cvetinov, D. Obadović, M. Stojanović, A.
Vajda, N. Éber, K. Fodor-Csorba, I. Ristić: Mesophase behaviour of binary mixtures of bent-core and calamitic
compounds. Liquid Crystals 40(11), 1512-1519
(2013) [pdf]
Bent-core (BC)
liquid crystals possess a tendency to form polar smectic ordering even in their
nematic phase, which has great influence on their flexoelectric properties. A
measuring technique based on flexing sandwich cells is introduced which allows
to determine the flexocoefficients by the direct flexoelectric effect. We
report on giant flexoelectricity found in several bent-core nematics by this
method, as well as by detecting the electric field induced flexing of the cell
(a manifestation of giant converse flexoelectricity). Conventional (indirect)
measurements of flexocoefficients on BC nematics, which might not detect giant
flexoelectricity, are also reviewed. We address the physical origin of giant
flexoelectricity and attempt to explain the apparent discrepancy found between
results obtained by different measurement techniques. Finally, we show that
giant flexoelectricity is observable in BC elastomers too, which may open the
way for practical applications.
A. Jákli, J. Harden and N. Éber, Chapter 3.
Flexoelectricity of bent-core molecules, In eds. Á.
Buka and N. Éber, Flexoelectricity
in Liquid Crystals. Theory, Experiments and Applications, Imperial College Press, London, 2012. pp. 61-99
This article
summarizes the results obtained by various experimental methods on the physical
properties of a bent-core nematic liquid crystal 4-chloro-1,3-phenylene
bis-4-[40-(9-decenyloxy) benzoyloxy] benzoate (ClPbis10BB). The material
exhibits unusual properties in all aspects tested. Its bend flexoelectric
coefficient is 1000 times larger than in calamitics; it is viscoelastic with a
large, shear-rate-dependent viscosity. Its bend and twist elastic constants are
abnormally low; thus the nematic phase can be rendered to be a blue fog phase
with a small amount of chiral dopant. It shows very high flow birefringence and
unusually small leading Landau coefficient. It has two types of isotropic
phases; at lower temperature it is probably tetrahedratic that can be
transferred into the nematic phase with magnetic field. ClPbis10BB has a
frequency-dependent conductivity anisotropy which is characterized by a double
sign inversion. It exhibits various electroconvection (EC) patterns which are
currently not understood in the frame of the standard theory of EC.
Á. Buka, N. Éber, K. Fodor-Csorba, A. Jákli and P.
Salamon: Physical properties of a bent-core nematic liquid crystal and its
mixtures with calamitic molecules. Phase
Transitions 85,
872-887 (2012) [pdf]
The effect of
director pretilt on the twist magnetic Fréedericksz transition of nematics was
investigated in a planar cell. The director configuration was calculated as a
function of magnetic inductance. The dielectric and optical response of the
nematic liquid crystal was numerically modeled. A dielectric measurement method
for determining the elastic constant K22 is presented.
The influence of the conditions for the Mauguin effect is discussed. The
theoretical predictions were confirmed by our experiments. Experimental data
for all elastic constants of a bent-core nematic material are presented and
discussed.
P. Salamon, N. Éber, M. Lehmann, J.T. Gleeson, S.
Sprunt and A. Jákli, Dielectric technique to measure the twist elastic constant
of liquid crystals - The case of a bent-core material. Phys.
Rev. E 85,
061704/1-9 (2012) [pdf]
Dielectric
spectroscopy measurements have been performed on a bent-core nematic liquid
crystal and on its binary mixtures with a calamitic nematic. We have detected
more dispersions in the bent-core compound than in the calamitic one, including
one at an unusually low frequency of a few kilohertz. The dispersions detected
in the mixtures have been identified and the spectra have been split into
contributions of the constituents. In order to connect the dielectric increment
with the molecular dipole moment we have applied a sophisticated conformational
calculation not performed before for a large, flexible mesogen molecule with
numerous polar groups.
P. Salamon, N. Éber, Á. Buka, J.T. Gleeson, S. Sprunt
and A. Jákli: Dielectric properties of
mixtures of a bent-core and a calamitic liquid crystal. Phys.
Rev. E 81,
031711/1-11 (2010) [pdf]
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The
onset of electroconvection in binary mixtures of a
bent-core and a rod-like nematic has been characterized by measuring
the threshold voltage Uc and the critical wave number of
the pattern in a wide range of frequencies f. In the mixtures rich in
bent-core molecules, a "conductive-prewavy2-patternless-prewavy1"
morphological sequence has been detected with
an unusual negative slope of Uc(f)
at high frequencies. This latter scenario seems to be related to the
bent-core component, as it disappears with in creasing the concentration of
rod-like molecules. In addition, one of the parameters most relevant for
electroconvection, the electrical conductivity, has also been varied by ionic
salt doping. It has been found that the above effect of the banana-shaped
molecules on the electroconvection scenarios can be suppressed by the
conductivity. |
|
[S. Tanaka, H. Takezoe, N. Éber, K. Fodor-Csorba, A. Vajda, and Á. Buka:
Electroconvection in nematic mixtures of bent-core and calamitic molecules. Phys. Rev. E 80, 021702 (2009)] [pdf]
We briefly review systematic
and comprehensive studies on several chlorine-substituted bent-core liquid
crystal materials in their nematic phases. The results, in comparison to
rod-shaped molecules, are both extraordinary and technologically significant.
Specifically:
a) Electrohydrodynamic
instabilities provide unique patterns including well defined, periodic stripes
and optically isotropic structures.
b) Rheological
measurements using different probe techniques (dynamic light scattering, pulsed
magnetic field, electrorotation) reveal that the ratio of the flow and
rotational viscosities are over two orders of magnitudes larger in bentcore
than in calamitic materials which proves that the molecule shape and not its
size is responsible for this behaviour.
c) Giant
flexoelectric response, a measured by dynamic light scattering and by directly
probing the induced current when the material is subject to oscillatory bend
deformation, turns out to be more than three orders of magnitude larger than in
calamitics and 50 times larger than molecular shape considerations alone would
predict. The magnitude of this effect renders these materials as promising
candidates for efficient conversion between mechanical and electrical energy.
d) The converse of
this effect when the bent-core material sandwiched between plastic substrates 4
times thicker than the liquid crystal material provided displacements in the
range of 100nm that is sensitive to the polarity of the applied field thus
suggesting applications as beam steering and precision motion controls.
A. Jákli, M. Chambers, J.
Harden, M. Madhabi, R. Teeling, J. Kim, Q. Li, G.G.
Nair, N. Éber, K. Fodor-Csorba, J.T. Gleeson, S.
Sprunt: Extraordinary properties of nematic phases of
bent-core liquid crystals. In Emerging Liquid Crystal Technologies III, San Jose, January 20-24, 2008, L.C.
Chien (Ed.), Proc. SPIE Vol. 6911,
691105/1-10 (Jan. 29, 2008) [pdf]
Two
series of binary mixtures composed of bent shaped and rod like molecules are reported.
The first star shaped bent core molecules were synthesized and used as a
component of binary mixtures. The chiral rod like compounds having
commensurable length with the arms of the bent core compounds have been chosen
for these mixtures. The resulted compositions show various thermotropic liquid
crystalline phases that are characteristic to both types of liquid crystalline
materials. In case of mixing the rod like molecules to the bent core compound
the B2, B7 and induced B1 phases have been observed. While using the
star-shaped bent core and chiral rod like compounds in mixture, the
paraelectric smectic A, ferroelectric smectic C* and orthogonal hexatic smectic
B phases were preferred. The appearing mesophases were investigated by
differential scanning calorimetry, polarizing optical microscopy and X-ray
diffraction methods.
A. Bubnov, V.
Hamplova, M. Kaspar, A. Vajda, M. Garic, D.Z. Obadovic, N. Éber, Fodor-Csorba
K: Thermal analysis of binary liquid crystalline mixtures: system of bent core
and calamitic molecules. J. Therm. Anal.
and Calorimetry, 90(2), 431-441 (2007) [pdf]
Flexoelectricity
is a coupling between orientational deformation and electric polarization. We
present a direct method for measuring the flexoelectric coefficients of nematic
liquid crystals (NLCs) via the electric current produced by periodic mechanical
flexing of the NLC’s bounding surfaces. This method is suitable for measuring
the response of bent-core liquid crystals, which are expected to demonstrate a
much larger flexoelectric effect than traditional, calamitic liquid crystals.
Our results reveal that not only is the bend flexoelectric coefficient of
bent-core NLCs gigantic (more than 3 orders of magnitude larger than in calamitics)
but also it is much larger than would be expected from microscopic models based
on molecular geometry. Thus, bent-core nematic materials can form the basis of
a technological breakthrough for conversion between mechanical and electrical
energy.
J. Harden, B. Mbanga, N. Éber, K. Fodor-Csorba, S. Sprunt, J.T. Gleeson, A.
Jákli: Giant flexoelectricity of bent-core nematic liquid
crystals. Phys. Rev. Lett., 97, 157802/1-4 (2006)
[pdf]
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We characterize three nonstandard electrohydro-dynamic instabilities in nematic liquid crystals composed of bent-core molecules. In addition to their shape, another important attribute of this material is that the anisotropy in the electrical conductivity changes sign as the frequency of the applied electric field changes. These instabilities do not appear to fit within the standard model for electroconvection. The first instability creates a pattern with stripes parallel to the initial director orientation, with a wavelength about equal to the separation of the cell plates. The next is the previously reported prewavy instability. The third instability is optically and dynamically identical to the prewavy instability, but is distinguished by different threshold behavior. |
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[D.
B. Wiant, J. T. Gleeson, N. Éber, K. Fodor-Csorba, A. Jákli, T. Toth-Katona:
Non-Standard Electroconvection in a Bent Core Nematic. Phys. Rev. E 72, 041712 (2005)] [pdf]