arXiv subjects

Oleg D. Lavrentovich

Publications and source records attributed to Oleg D. Lavrentovich.

At least 19 recordsLinked to original sources

arXiv2026

Landau theory applied to antiferroelectric ordering in ferroelectric nematic liquid crystals

The polarization and mass density modulation associated with antiferroelectric ordering in two ferroelectric nematic liquid crystals, a prototypical single compound (referred to as DIO) and a commercial mixture (FNLC919), are studied experimentally as a function of temperature. The wavenumber of the polarization modulation, qA, is determined by small angle X-ray diffraction from the weak, accompanying density wave (wavenumber qS = 2qA) using a conventional laboratory source. Combined results for qS and the saturated value of the polarization are analyzed in terms of Landau theory previously developed to describe the para-/antiferro-/ferroelectric sequence of phase transitions in solid ferroelectrics. The analysis of qS indicates that the polarization modulation can be reasonably well approximated by a simple sinusoid in DIO, whereas in FNLC919 a sharp decrease in qS close to the antiferro- to ferroelectric transition indicates development of a soliton-like profile.

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arXiv2026

Realignment of ferroelectric nematic by photoinduced electric field

Liquid crystal displays (LCDs) employ thin pixels of a paraelectric nematic, confined between two glass plates with transparent indium tin oxide (ITO) electrodes. A voltage applied to the electrodes realigns the molecules, changing the optical appearance of the pixel. Here we demonstrate that a similar molecular realignment can be triggered by low-power light irradiation when the paraelectric nematic in the cell with ITO electrodes is replaced with a ferroelectric nematic (NF). Irradiation creates an intrinsic electric field that is strong enough to realign the NF but too weak to affect the paraelectric nematic. The extraordinary sensitivity of the ITO/NF pair to light irradiation creates a platform for optical control of ferroelectric polarization and applications in photonics, sensing, smart windows, and beyond.

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arXiv2026

Electrically controlled propulsion of skyrmions in chiral nematic

Design of soft matter capable of controllable microscale dynamics is a frontier of modern science. Jiahao Chen et al. demonstrate that an electric field can drive particle-like solitons-skyrmions in a chiral nematic along a preprogrammed trajectory with a variable speed within a two-dimensional plane. The effect is rooted in flexoelectric polarization of a deformed director field.

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arXiv2026

Deformed states in paraelectric and ferroelectric nematic liquid crystals

Ground states of materials with orientational order ranging from solid ferromagnets and ferroelectrics to liquid crystals often contain spatially varying vector-like order parameter caused by inner factors such as the shape of building units or by the geometry of confinement. This review presents examples of how the shapes, chirality, and polarity of molecules and spatial confinement induce deformed equilibrium and polydomain states with parity breaking, splay, bend, and twist-bend deformations of the order parameter in paraelectric and ferroelectric nematic liquid crystals. Parity breaking results either from chirality of the constituent molecules, as a replacement of energetically costly splay and bend in paraelectric nematics, or in response to depolarization field in the ferroelectric nematic. Both paraelectric and ferroelectric nematics exhibit a splay cancellation effect, in which the elastic and electrostatic energies of splay along one direction are reduced by an additional splay along orthogonal directions.

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arXiv2026

Rheological properties and shear-induced structures of ferroelectric nematic liquid crystals

Recently discovered ferroelectric nematic (NF) liquid crystals are fluids with a polar orientational order. The electric polarization vector can be aligned by an electric field and by surface anchoring. Here, we explore how the polarization field and effective viscosity of the NF materials are affected by shear flows. We explore three NF materials, abbreviated RM734, DIO, and a room-temperature FNLC919, all of which exhibit a paraelectric nematic (N) and the NF phase. All materials show an increase of the viscosity upon cooling, with an Arrhenius behavior. In DIO and FNLC919, the antiferroelectric SmZA phase shows a strong dependence of the effective viscosity on the shear rate: this viscosity is lower than the viscosity of the N and NF phases at high shear rates but is much higher when the shear rate is low. The behavior is associated with the layered structure of the SmZA phase. All mesophases exhibit shear-thinning behavior at low shear rates and a nearly Newtonian behavior at higher shear rates. In terms of alignment, we observe three regimes in the N and NF phases: flow-alignment at low shear rates, log-rolling regime with the director and polarization along the vorticity axis at high shear rates, and polydomain structures at intermediate rates. In the flow-aligning regime, the NF polarization does not tilt away from the shear direction, which is in sharp contrast to the flow-induced tilt of the N director. The effect is attributed to the avoidance of splay deformations and associated space charge in the flowing NF. The temperature and shear rate dependencies of the viscosity and the uncovered shear-induced structural effects of NF advance our understanding of these materials and potentially facilitate their applications.

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arXiv2025

Bend Instabilities and Topological Turbulence in Shear-Aligned Living Liquid Crystal

Flagellated microswimmers B. Subtilis dispersed in a nematic phase of a lyotropic chromonic liquid crystal form a living liquid crystal (LLC). The combination of the passive and active components allows us to analyze how the active component transitions from the shear-imposed alignment into topological turbulence. The lateral extension of the experimental cell is 1000 times larger than the 10-micrometer thickness, to avoid the effect of lateral confinement on the dynamic patterns. The surface anchoring is azimuthally degenerate to avoid permanent anisotropy. After the shear cessation, active forces produced by the microswimmers trigger self-amplifying bend undulations. The amplitude of undulations grows with time and then saturates, while the wavelength increases only slightly. Strong bend stresses at the extrema of undulations are released by nucleating disclination pairs, which multiply to produce topological turbulence. The nucleating pairs and the symmetry axes of the +1/2 disclinations are orientationally ordered. In highly active LLCs, this alignment causes a nonmonotonous time dependence of the disclinations' number, as a fast-moving +1/2 disclination of one pair annihilates a -1/2 disclination of the neighboring pair. The spectra of elastic and kinetic energies exhibit distinct wavevector dependencies caused by the energy cost of the deformed passive nematic background. The study demonstrates how applied shears and a passive viscoelastic background affect the dynamics of active matter. Combined with the previously determined viscoelastic properties of the lyotropic chromonic liquid crystal, the results complete a comprehensive description of the experimentally assessable type of active matter.

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arXiv2025

Helix alignment, chevrons, and edge dislocations in twist-bend ferroelectric nematics

We explore surface alignment and edge dislocations in the recently discovered twist-bend ferroelectric nematic, NTBF, in which the vector of spontaneous polarization follows an oblique helicoidal trajectory around a polar twist-bend axis. In a planar cell, the polar axis aligns at some angle to the rubbing direction to mitigate surface electric charge. We demonstrate that the pseudolayers in planar cells form chevron defects, a hallmark defect of one-dimensionally positionally ordered phases, such as smectic A and smectic C. The polar character of the twist-bend axis prevents the cores of NTBF edge dislocations from splitting into semi-integer disclinations, in stark contrast to dislocations in paraelectric and ferroelectric chiral nematics. The tilt of pseudolayers around the defect core allows us to estimate the elastic penetration length as being close to the pitch of NTBF. Compression/dilation stresses around the core modify the heliconical tilt angle of molecules as evidenced by a substantial variation in local birefringence. The climb of dislocations exhibits high mobility, allowing the system to equilibrate the temperature-dependent pitch. The uncovered properties facilitate the development of NTBF materials for electro-optical applications, such as electrically controlled diffraction lattices and structural colors.

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arXiv2025

Polar and apolar light-induced alignment of ferroelectric nematic at photosensitive polymer substrate

Surface alignment of a recently discovered ferroelectric nematic liquid crystal (NF) is usually achieved by buffed polymer films, which produce a unidirectional polar alignment of the spontaneous electric polarization. We demonstrate that photosensitive polymer substrates could provide a wider variety of alignment modes. Namely, a polyvinyl cinnamate polymer film irradiated by linearly polarized ultraviolet (UV) light yields two modes of surface orientation of the NF polarization. (1) A planar apolar mode, in which the equilibrium NF polarization aligns perpendicularly to the polarization of normally impinging UV light; the NF polarization adopts either of the two antiparallel states. (2) A planar polar mode, produced by an additional irradiation with obliquely impinging UV light. In this mode, there is only one stable azimuthal direction of polarization in the plane of the substrate. The two modes differ in their response to an electric field. In the planar apolar mode, polarization can be switched between two states of equal surface energy. In the planar polar mode, the field-perturbed polarization relaxes back to the single photoinduced easy axis once the field is switched off. The versatility of modes and absence of mechanical contact make the photoalignment of the NF attractive for practical applications.

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arXiv2025

Twist, splay, and uniform domains in ferroelectric nematic liquid crystals

The newly-discovered ferroelectric nematic liquid crystal exhibits a variety of unique defect phenomena. The depolarization field in the material favors spontaneous spatial variations in polarization, manifesting in diverse forms such as bulk twists and arrangements of alternating polarization domains. The configuration of these domains is governed by a balance between depolarization field reduction and molecular alignment at interfaces. We investigate a ferroelectric nematic confined in a thin cell with apolar surface anchoring, patterned using photoalignment. Under uniform planar alignment, the system forms stripes, while a radial +1 defect pattern results in pie-slice domains. Neighboring domains show either opposite directions of uniform polarization (thin cells) or opposite handedness of the spontaneous twist (thick cells). Our calculations and experiments demonstrate that electrostatic interactions tend to shrink domain size, whereas elastic and surface anchoring effects promote larger domains. In this work, we make predictions and measurements of the domain size as a function of cell thickness, and show that ionic screening suppresses domain formation.

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arXiv2025

Sub-microsecond electric switching of large birefringence in the isotropic phase of ferroelectric nematics

Conventional nonpolar nematic liquid crystals, widely known for their successful display applications, are not well-suited for fast electro-optical switching because of the slow (milliseconds) field-off relaxation of the director. In this work, we demonstrate sub-microsecond field-on and field-off switching of a large (~0.1) birefringence by applying a moderate electric field (30-100) V/μm, to an isotropic film of a ferroelectric nematic material. This highly efficient electro-optical switching is rooted in the recently discovered field-induced isotropic to ferroelectric nematic phase transition [J. Szydlowska et al., Phys. Rev. Lett. 130, 216802 (2023)]. The demonstrated sub-microsecond switching of ferroelectric birefringence induced by field (FBIF) has the potential for applications in fast-switching electro-optic devices, such as phase modulators, light shutters, beam steerers, switchable optical compensators.

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arXiv2025

Liquid crystalline structures formed by sphere-rod amphiphilic molecules in solvents

Self-assembly of amphiphilic molecules is an important phenomenon attracting a broad range of research. In this work, we study the self-assembly of KTOF4 sphere-rod amphiphilic molecules in mixed water-dioxane solvents. The molecules are of a T-shaped geometry, comprised of a hydrophilic spherical Keggin-type cluster attached by a flexible bridge to the center of a hydrophobic rod-like oligodialkylfluorene (OF), which consists of four OF units. Transmission electron microscopy (TEM) uncovers self-assembled spherical structures of KTOF4 in dilute solutions. These spheres are filled with smectic-like layers of KTOF4 separated by layers of the solution. There are two types of layer packings: (i) concentric spheres and (ii) flat layers. The concentric spheres form when the dioxane volume fraction in the solution is 35-50 vol%. The flat layers are formed when the dioxane volume fraction is either below (20 and 30 vol%.) or above (55 and 60 vol%.) the indicated range. The layered structures show no in-plane orientational order and thus resemble thermotropic smectic A liquid crystals and their lyotropic analogs. The layered packings reveal edge and screw dislocations. Evaporation of the solvent produces a bulk birefringent liquid crystal phase with textures resembling the ones of uniaxial nematic liquid crystals. These findings demonstrate that sphere-rod molecules produce a variety of self-assembled structures that are controlled by the solvent properties.

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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.

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arXiv2024

Periodic splay Fréedericksz transitions in a ferroelectric nematic

Electric field-induced splay of molecular orientation, called the Fréedericksz transition, is a fundamental electro-optic phenomenon in nonpolar nematic liquid crystals. In a ferroelectric nematic NF with a spontaneous electric polarization P, the splay is suppressed since it produces bound electric charges. Here, we demonstrate that an alternating current (ac) electric field causes three patterns of NF polarization. At low voltages, P oscillates around the field-free orientation with no stationary deformations. As the voltage increases, the polarization acquires stationary distortions, first splay and twist in a stripe pattern and then splay and bend in a square lattice of +1 and -1 defects. In all patterns, P oscillates around the stationary orientations. The stationary bound charge is reduced by a geometrical splay cancellation mechanism that does not require free ions: the charge created by splay in one plane is reduced by splay of an opposite sign in the orthogonal plane.

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arXiv2024

Effects of Photopatterning Conditions on Azimuthal Surface Anchoring Strength

Spatially varying alignment of liquid crystals is essential for research and applications. One widely used method is based on the photopatterning of thin layers of azo-dye molecules, such as Brilliant Yellow (BY), that serve as an aligning substrate for a liquid crystal. In this study, we examine how photopatterning conditions, such as BY layer thickness (b), light intensity (I), irradiation dose, and age affect the alignment quality and the strength of the azimuthal surface anchoring. The azimuthal surface anchoring coefficient, W, is determined by analyzing the splitting of integer disclinations into half-integer disclinations at pre patterned substrates. The strongest anchoring is achieved for b in the range of 5-8 nm. W increases with the dose, and within the same dose, W increases with I. Aging of a non-irradiated BY coating above 15 days reduces W. Our study also demonstrates that sealed photopatterned cells filled with a conventional nematic preserve their alignment quality for up to four weeks, after which time W decreases. This work suggests the optimization pathways for photoalignment of nematic liquid crystals.

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arXiv2024

Conic sections in ferroelectric nematics: experiments and mathematical modeling

Domain structure of a fluid ferroelectric nematic is dramatically different from the domain structure of solid ferroelectrics since it is not restricted by rectilinear crystallographic axes and planar surface facets. We demonstrate that thin films of a ferroelectric nematic seeded by colloidal inclusions produce domain walls in the shape of conics such as a parabola. These conics reduce the bound charge within the domains and at the domain walls. An adequate description of the domain structures requires one to analyze the electrostatic energy, which is a challenging task. Instead, we demonstrate that a good approximation to the experimentally observed polydomain textures is obtained when the divergence of spontaneous polarization - which causes the bound charge is heavily penalized by assuming that the elastic constant of splay in the Oseen-Frank energy is much larger than those for twist and bend. The model takes advantage of the fact that the polarization vector is essentially parallel to the nematic director throughout the sample.

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arXiv2024

Microsecond electro-optic switching in the nematic phase of a ferroelectric nematic liquid crystal

Nematic liquid crystals exhibit nanosecond electro-optic response to an applied electric field which modifies the degree of orientational order without realigning the molecular orientation. However, this nanosecond electrically-modified order parameter (NEMOP) effect requires high driving fields, on the order of 100 V/um for a modest birefringence change of 0.01. In this work, we demonstrate that a nematic phase of the recently discovered ferroelectric nematic materials exhibits a robust and fast electro-optic response. Namely, a relatively weak field of 20 V/um changes the birefringence by 0.04 with field-on and-off times around 1 us. This microsecond electrically modified order parameter (MEMOP) effect shows a greatly improved figure of merit when compared to other electro-optical switching modes in liquid crystals, including the conventional Frederiks effect, and has a potential for applications in fast electro-optical devices such as phase modulators, optical shutters, displays, and beam steerers.

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arXiv2024

Trapping of isotropic droplets by disclinations in nematic liquid crystals controlled by surface anchoring and elastic constant disparity

Linear defects such as dislocations and disclinations in ordered materials attract foreign particles since they replace strong elastic distortions at the defect cores. In this work, we explore the behavior of isotropic droplets nucleating at singular disclinations in a nematic liquid crystal, predesigned by surface photopatterning. Experiments show that in the biphasic nematic-isotropic region, although the droplets are attracted to the disclination cores, their centers of mass shift away from the core centers as the temperature increases. The shift is not random, being deterministically defined by the surrounding director field. The effect is explained by the balance of interfacial anchoring and bulk elasticity. An agreement with the experiment can be achieved only if the model accounts for the disparity of the nematic elastic constants; the so-called one-constant approximation, often used in the theoretical analysis of liquid crystals, produces qualitatively wrong predictions. In particular, the experimentally observed shift towards the bend region around a +1/2 disclination core can be explained only when the bend constant is larger than the splay constant. The described dependence of the precise location of a foreign inclusion at defect cores on the elastic and surface anchoring properties can be used in the rational design of microscale architectures.

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arXiv2024

Dynamic control of active droplets using light-responsive chiral liquid crystal environment

Microscopic active droplets are of interest since they can be used to transport matter from one point to another. The challenge is to control the trajectory. In this work, we demonstrate an approach to control the direction of active droplet propulsion by a photoresponsive cholesteric liquid crystal environment. The active droplet represents a water dispersion of bacterial B. subtilis microswimmers. When placed in a cholesteric, a surfactant-stabilized active droplet distorts the local director field, producing a point defect-hedgehog, which breaks the fore-aft symmetry. The chaotic motion of the bacteria inside the droplet is rectified into directional motion by the asymmetric director field outside the droplet. When the pitch of the cholesteric is altered by visible light irradiation, the asymmetry axis and thus the droplet trajectory realign along a new direction. Droplets realign counterclockwise on exposure to light of 535 nm, and clockwise on exposure to light of 450 nm, as dictated by the photoinduced change in the handedness of the cholesteric. The strategy allows for a non-contact dynamic control of active droplets trajectories and demonstrates the advantage of orientationally ordered media in control of active matter over their isotropic counterparts.

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