Tibor Tóth-Katona

Capacitance changes in ferronematic liquid crystals induced by low magnetic fields

The response in capacitance to low external magnetic fields (up to 0.1 T) of suspensions of spherical magnetic nanoparticles, single-wall carbon nanotubes (SWCNT), SWCNT functionalized with carboxyl group (SWCNT-COOH) and SWCNT functionalized with Fe3O4 nanoparticles in a nematic liquid crystal has been studied experimentally. The volume concentration of nanoparticles was φ1 = 10 −4 and φ2 = 10 . Independent of the type and the volume concentration of the nanoparticles, a linear response to low magnetic fields (far below the magnetic Fréederiksz transition threshold) has been observed, which is not present in the undoped nematic. PACS numbers: 61.30.Gd, 77.84Nh, 75.50.Mm, 75.30.Gw ∗ Corresponding author; tothkatona.tibor@wigner.mta.hu

Prewavy Pattern: A Director-Modulation Structure in Nematic Liquid Crystals

Abstract A periodic structure (the prewavy pattern) found below the onset of electroconvection in a homeotropically aligned nematic liquid crystal is reported. It is characterized by a periodic modulation of the director in the xy-plane, which can be distinguished from electroconvection patterns with modulation in the xz-plane. The prewavy instability is investigated in detail; a phase diagram in the frequency-voltage plane, the voltage dependence of the azimuthal rotation angle, the voltage and frequency dependence of the wavelength of the prewavy pattern, and the director field in the prewavy pattern are provided. This prewavy pattern always evolves into the so-called chevrons above onset of convection. The wavelength of the chevrons and the orientation of their alternating zig and zag rolls depend on the director structure of the prewavy pattern.

Regular dendritic patterns induced by nonlocal time-periodic forcing

The dynamic response of dendritic solidification to spatially homogeneous time-periodic forcing has been studied. Phase-field calculations performed in two dimensions (2D) and experiments on thin (quasi-2D) liquid-crystal layers show that the frequency of dendritic side branching can be tuned by oscillatory pressure or heating. The sensitivity of this phenomenon to the relevant parameters, the frequency and amplitude of the modulation, the initial undercooling and the anisotropies of the interfacial free energy, and molecule attachment kinetics, has been explored. It has been demonstrated that in addition the side-branching mode synchronous with external forcing as emerging from the linear Wentzel-Kramers-Brillouin analysis, modes that oscillate with higher harmonic frequencies are also present with perceptible amplitudes.

Influence of the anisometry of magnetic particles on the isotropic–nematic phase transition

The influence of the shape anisotropy of magnetic particles on the isotropic–nematic phase transition was studied in ferronematics based on the nematic liquid crystal (LC) 4-(trans-4-n-hexylcyclohexyl)-isothiocyanato-benzene (6CHBT). The LC was doped with spherical or rod-like magnetic particles of different size and volume concentrations. The phase transition from isotropic to nematic phase was observed by polarising microscope as well as by capacitance measurements. The influence of the concentration and the shape anisotropy of the magnetic particles on the isotropic–nematic phase transition in LC are demonstrated here. The results are in a good agreement with recent theoretical predictions.

Azo-containing asymmetric bent-core liquid crystals with modulated smectic phases

A new series of azo-containing bent-core liquid crystals derived from 3-hydroxybenzoic acid has been synthesized. Their mesomorphic properties have been characterized by polarizing optical microscopy, differential scanning calorimetry, small-angle X-ray diffraction and electro-optic studies. Almost all the compounds form an enantiotropic modulated smectic (B7 type) phase over relatively broad temperature ranges. Structural modifications, such as the type and length of the terminal chains, the rigidity of wings, and the presence of a Cl-substituent in different positions of the bent core, affect the appearance and temperature range, but not the type of the mesophase of the investigated compounds. Light-induced changes in the texture and phase transition of the mesophase, attributed to the decrease of the order parameter due to trans–cis isomerization, have also been observed.

Structural properties of the ferroelectric liquid crystal mixtures based on the homologous series of alkoxyphenyl alkoxy benzoates

Abstract The temperature dependence of the layer spacings and the average intermolecular distances were studied in smectic A, C, B and nematic phases by X-ray diffraction on non-oriented samples. Two representatives of 4-n-alkoxyphenyl 4′-n-alkoxy benzoates, furthermore their eutectic binary mixture, and ternary ferroelectric mixtures obtained by the chiral additive of (R)-(1-methylheptyl)-1, 1′-4′-1′-terphyenyl-1, 4′-dicarboxilate used in different concentrations were investigated by small and large angle measurements as well.

Flexoelectricity and competition of time scales in electroconvection

An unexpected type of behavior in electroconvection (EC) has been detected in nematic liquid crystals (NLCs) under the condition of comparable time scales of the director relaxation and the period of the driving ac voltage. The studied NLCs exhibit standard EC (s-EC) at the onset of the instability, except one compound in which nonstandard EC (ns-EC) has been detected. In the relevant frequency region, the threshold voltage for conductive s-EC bends down considerably, while for dielectric s-EC it bends up strongly with the decrease of the driving frequency. We show that inclusion of the flexoelectric effect into the theoretical description of conductive s-EC leads to quantitative agreement, while for dielectric s-EC a qualitative agreement is achieved. The frequency dependence of the threshold voltage for ns-EC strongly resembles that of the dielectric s-EC.

Heat diffusion anisotropy in dendritic growth:: phase field simulations and experiments in liquid crystals

An anisotropic heat diffusion coefficient is introduced in order to study some interfacial growth phenomena. This anisotropy has been incorporated in a phase field model which has been studied numerically to reproduce some fundamental solidification situations (needle crystal growth) as well as the dynamics of a nematic–smectic-B interface. As a general result, we find that dendrites grow faster in the lower heat diffusion direction. Simulation results are compared with experiments with remarkable qualitative agreement.

Magnetically induced shift of the isotropic–nematic phase transition temperature in a mixture of bent-core and calamitic liquid crystals doped with magnetic particles

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.

High concentration ferronematics in low magnetic fields

Abstract We investigated experimentally the magneto-optical and dielectric properties of magnetic-nanoparticle-doped nematic liquid crystals (ferronematics). Our studies focus on the effect of the very small orienting bias magnetic field B bias , and that of the nematic director pretilt at the boundary surfaces in our systems sensitive to low magnetic fields. Based on the results we assert that B bias is not necessarily required for a detectable response to low magnetic fields, and that the initial pretilt, as well as the aggregation of the nanoparticles play an important (though not yet explored enough) role.