arXiv subjects

Antal Jákli

Publications and source records attributed to Antal Jákli.

13 recordsLinked to original sources

arXiv2026

Shear-mode Direct Piezoelectric Response of Ferroelectric Nematic Liquid Crystals

Piezoelectricity (linear coupling between mechanical deformation and electric signal) was originally observed only in solid crystals. Recently, it was discovered that liquid ferroelectric nematic liquid crystals are also piezoelectric. However, so far only their converse piezoelectric signals (applied voltage-induced mechanical deformation) were measured quantitatively. In this work, we have carried out periodic shear-induced electric current and oscillatory rheology measurements on the two archetypic ferroelectric nematic compounds, RM734 and DIO. From temperature, frequency and strain dependent results of the first and second harmonic current signals together with the results from oscillatory rheometry, we were able to quantitatively determine the shear-mode direct piezoelectric coupling constants. These values are similar for both materials and are compared to results of previous converse piezoelectric measurements. We propose a physical mechanism in which the flow alignment of ferroelectric polarization leads to the direct piezoelectric response.

cond-mat.soft↗

arXiv2025

Low-Power Temperature Control by Chiral Ferroelectric Nematic Liquid Crystal Windows

Low power consumption is critical for smart windows for temperature control and privacy. The recently discovered ferroelectric nematic liquid crystals exhibit strong coupling of the ferroelectric polarization with electric fields, making them promising candidates for energy-efficient electrochromic devices. Here we investigate the electrochromic properties of a room temperature chiral ferroelectric nematic liquid crystal in films with in-plane electrodes, where the electric field is perpendicular to the helical axis. We demonstrate that smart windows based on this material can regulate interior temperatures within a 10 Celsius range using only 50 milliwatt per square meter specific power, achieving fifty percent larger temperature modulation and 50-100 times lower power consumption than polymer dispersed and polymer stabilized liquid crystal windows. These findings suggest that chiral ferroelectric nematic liquid crystals offer a highly efficient approach for smart window applications, potentially surpassing existing electrochromic technologies in energy efficiency and thermal regulation.

cond-mat.soft↗

arXiv2025

Ferroelectric Nematic and Smectic Liquid Crystals with Sub-Molecular Spatial Correlations

A number of highly polar three ring rod-shaped compounds with a terminal thiophene ring have been synthesized and the physical properties of a subset are reported in detail. On cooling from the isotropic fluid, they directly transition to a ferroelectric nematic liquid crystal (NF) phase that shows the strongest spatial correlations corresponding to 1/3 of the molecular length (L/3). The set of thiophene compounds reported here have ferroelectric polarizations about 20% larger than that of usual ferroelectric nematic liquid crystal materials. Such large polarization values are due to the ~20% larger mass densities of these thiophene compounds compared to most of the NF materials with short terminal chains. These unusual properties are consequences of tighter molecular packing due to the lack of flexible terminal chains. Below the NF phase, compounds with a single nitro or two cyano polar groups on the terminal benzene ring exhibit a so far never observed smectic phase with periodicity ~1/3 the molecular length. Based on our experimental results, we propose a model of this phase featuring antipolar packing of the molecules within the layers.

cond-mat.soft↗

arXiv2025

Giant Electro-Viscous Effects in Polar Fluids with Paraelectric-Modulated Antiferroelectric-Ferroelectric Phase Sequence

The recently discovered ferroelectric nematic liquid-crystal material DIO exhibits an antiferroelectric (AF) phase, characterized by a sinusoidally modulated structure between the paraelectric (P) and ferroelectric (F) nematic phases. Although these sinusoidal modulated structures associated with the P-AF-F phase sequence is commonly observed in solid ferroelectrics, their presence in soft matter systems is scarce. This study is aimed at examining the macroscopic properties of DIO, identifying unique rheological properties, such as switching between shear thinning and shear thickening behaviors at certain shear rate in the ferroelectric nematic phase. Additionally, a significant electroviscous effect is observed, with the viscosity increasing by 70 times under an ultra-low electric field of 0.15 V um-1 at the AF-F transition.

cond-mat.soft↗

arXiv2024

Transient Negative Capacitance in Ferroelectric Nematic Liquid Crystals

The transient negative capacitance (NC) of solid ferroelectric materials used in field effect transistors can reduce the power dissipation of electronics. Here we show that similar negative capacitance appears in the recently discovered fluid ferroelectric nematic liquid crystal (FNLC) films while switching their ferroelectric polarization. Instead of sidewise motion of domains with opposite direction of polarization of ferroelectric crystals, in FNLCs the polarization vector that at zero field aligns parallel to the substrates, rotates toward the applied electric field. In spite of that difference, the observed switching time per unit electric field values are comparable. Although current FNLCs have much larger electric conductivity than of ferroelectric crystals, they have several advantages: they are organic, lightweight, can be prepared and operated at ambient temperature, the direction of the polarization can be set by simple surface alignment, and they require much less electric field for switching of the polarization. These properties make FNLCs attractive to test them in in flexible organic electrochemical transistor (OECT) devices.

cond-mat.soft↗

arXiv2024

Directional Swimming of B. Subtilis Bacteria Near a Switchable Polar Surface

The dynamics of swimming bacteria depend on the properties of their habitat media. Recently it was shown that the motion of swimming bacteria dispersed directly in a non-toxic water-based lyotropic chromonic liquid crystal can be controlled by the director field of the liquid crystal. Here we investigate whether the macroscopic polar order of a ferroelectric nematic liquid crystal (NF) can be recognized by bacteria B. Subtilis swimming in a water dispersion adjacent to a glassy NF film by surface interactions alone. We show that B. Subtilis tends to move in the direction antiparallel to the spontaneous electric polarization at the NF surface. Their speed was found to be the same with or without a polar NF layer. In contrast to observation on crystal ferroelectric films, the bacteria do not get immobilized. These observations may offer a pathway to creation of polar microinserts to direct bacterial motion in-vivo.

cond-mat.soft↗

arXiv2023

Fluid ferroelectric filaments

Freestanding slender fluid filaments of room temperature ferroelectric nematic liquid crystals are described. They are stabilized either by internal electric fields of bound charges formed due to polarization splay, or by external voltage applied between suspending wires. The phenomenon is similar to those observed in dielectric fluids, such as deionized water, except that in ferroelectric nematic materials the voltages required are 3 orders of magnitudes smaller and the aspect ratio is much higher. The observed ferroelectric fluid threads are not only unique and novel, but also offer measurements of basic physical quantities, such as the ferroelectric polarization and viscosity. Ferroelectric nematic fluid threads may have practical applications in nano-fluidic micron-size logic devices, switches, and relays.

cond-mat.soft↗

arXiv2023

Electric field-induced interfacial instability in a ferroelectric nematic liquid crystal

Studies of sessile droplets and fluid bridges of a ferroelectric nematic liquid crystal in externally applied electric fields are presented. It is found that above a threshold, the interface of the fluid with air undergoes a fingering instability or ramification, resembling to Rayleigh-type instability observed in charged droplets in electric fields or circular drop-type instabilities observed in ferromagnetic liquids in magnetic field. The frequency dependence of the threshold voltage was determined in various geometries. The nematic director and ferroelectric polarization direction was found to point along the tip of the fingers that appear to repel each other, indicating that the ferroelectric polarization is essentially parallel to the director. The results are interpreted in connection to the Rayleigh and circular drop-type instabilities.

cond-mat.soft↗

arXiv2022

Ferroelectric nematic liquid crystal thermo-motor

A thermal gradient-induced circular motion of particles placed on ferroelectric nematic liquid crystal sessile drops is demonstrated and explained. Unlike hurricanes and tornadoes that are the prime examples for thermal motors and where turbulent flows are apparent, here the texture without tracer particles appears completely steady indicating laminar flow. We provide a simple model showing that the tangential arrangement of the ferroelectric polarization combined with the vertical thermal gradient and the pyroelectricity of the fluid drives the rotation of the tracer particles that become electrically charged in the fluid. These observations provide a fascinating example of the unique nature of fluid ferroelectric liquid crystals.

cond-mat.soft↗

arXiv2018

Distinct differences in the nanoscale behaviors of the twist-bend liquid crystal phase of a flexible linear trimer and homologous dimer

We synthesized the liquid crystal dimer and trimer members of a series of flexible linear oligomers and characterized their microscopic and nanoscopic properties using resonant soft x-ray scattering and a number of other experimental techniques. On the microscopic scale, the twist-bend phases of the dimer and trimer appear essentially identical. However, while the liquid crystal dimer exhibits a temperature-dependent variation of its twist-bend helical pitch varying from 100 - 170 Å on heating, the trimer exhibits an essentially temperature-independent pitch of 66 Å, significantly shorter than those reported for other twist-bend forming materials in the literature. We attribute this to a specific combination of intrinsic conformational bend of the trimer molecules and a sterically favorable intercalation of the trimers over a commensurate fraction (two-thirds) of the molecular length. We develop a geometric model of the twist-bend phase for these materials with the molecules arranging into helical chain structures, and we fully determine their respective geometric parameters.

cond-mat.soft↗

arXiv2017

Spectral Analysis of Piezoelectric Tensors

A third order real tensor is called a piezoelectric-type tensor if it is partially symmetric with respect to its last two indices. The piezoelectric tensor is a piezoelectric-type tensor of dimension three. We introduce C-eigenvalues and C-eigenvectors for piezoelectric-type tensors. Here, "C" names after Curie brothers, who first discovered the piezoelectric effect. We show that C-eigenvalues always exist, they are invariant under orthonormal transformations, and for a piezoelectric-type tensor, the largest C-eigenvalue and its C-eigenvectors form the best rank-one piezoelectric-type approximation of that tensor. This means that for the piezoelectric tensor, its largest C-eigenvalue determines the highest piezoelectric coupling constant. We further show that for the piezoelectric tensor, the largest C-eigenvalue corresponds to the electric displacement vector with the largest $2$-norm in the piezoelectric effect under unit uniaxial stress, and the strain tensor with the largest $2$-norm in the converse piezoelectric effect under unit electric field vector. Thus, C-eigenvalues and C-eigenvectors have concrete physical meanings in piezoelectric effect and converse piezoelectric effect. Finally, we apply C-eigenvalues and associated C-eigenvectors for various piezoelectric crystals with different symmetries.

math-ph↗

arXiv2016

Investigation of supramolecular architectures of bent-shaped pyridine derivatives: from a three-ring crystalline compound towards five-ring mesogens

In searching for novel photoactive liquid crystals, we have synthesized a series of five-ring pyridine-based bent-core compounds bearing different substituents at the peripheral phenyl rings (CH3O, Cl and NO2). Their mesomorphic behaviour has been investigated by polarizing optical microscopy, differential scanning calorimetry and X-ray scattering, and then compared with the unsubstituted parent compound. The introduction of the methoxy groups at the peripheral phenyl rings of the bent core results in a non-mesomorphic compound, whereas the chloro- and nitro-substituted compounds form enantiotropic B1-like phases. Significant changes of the textures and transition temperatures of the mesophase have been observed under UV light. The presented investigation of the mesomorphic properties of the synthesized compounds, coupled with analysis of the molecular packing of the related three-ring compounds, will help to design self-organized molecules suitable for UV indicators.

physics.chem-ph↗

arXiv2013

Nematic twist-bend phase with nanoscale modulation of molecular orientation

A state of matter in which molecules show a long-range orientational order and no positional order is called a nematic liquid crystal. The best known and most widely used (for example, in modern displays) is the uniaxial nematic, with the rod-like molecules aligned along a single axis, called the director. When the molecules are chiral, the director twists in space, drawing a right-angle helicoid and remaining perpendicular to the helix axis; the structure is called a chiral nematic. In this work, using transmission electron and optical microscopy, we experimentally demonstrate a new nematic order, formed by achiral molecules, in which the director follows an oblique helicoid, maintaining a constant oblique angle with the helix axis and experiencing twist and bend. The oblique helicoids have a nanoscale pitch. The new twist-bend nematic represents a structural link between the uniaxial nematic (no tilt) and a chiral nematic (helicoids with right-angle tilt).

cond-mat.soft↗