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Oleg D. Lavrentovich

Publications and source records attributed to Oleg D. Lavrentovich.

At least 37 records · Page 2Linked to original sources
arXiv2024

Dielectric properties of a ferroelectric nematic material: quantitative test of the polarization-capacitance Goldstone mode

The recently discovered ferroelectric nematic (NF) liquid crystals (LC) have been reported to show an extraordinarily large value of the real part of the dielectric constant at low frequencies. However, it was argued by Clark et al in Physical Review Research 6, 013195 (2024) that what was measured was the capacitance of the insulating layer at LC/electrode surface and not that of the liquid crystal. Here we describe the results of dielectric spectroscopy measurements of an NF material in cells with variable thickness of the insulating layers. Our measurements quantitatively verify the model by Clark et al. Additionally, our measurements in cells with bare conducting indium tin oxide surface provide a crude estimate of epsilon perpendicular~100 in the NF phase.

cond-mat.mtrl-sci↗
arXiv2024

Electrically tunable total reflection of light by oblique helicoidal cholesteric

An oblique helicoidal state of a cholesteric liquid crystal (ChOH) is capable of continuous change of the pitch P in response to an applied electric field. Such a structure reflects 50% of the unpolarized light incident along the ChOH axis in the electrically tunable band determined by P/2. Here we demonstrate that at an oblique incidence of light, ChOH reflects 100% of light of any polarization. This singlet band of total reflection is associated with the full pitch P. We also describe the satellite P/2, P/3, and P/4 bands. The P/2 and P/4 bands are triplets while P/3 band is a singlet caused by multiple scatterings at P and P/2. A single ChOH cell acted upon by an electric field tunes all these bands in a very broad spectral range, from ultraviolet to infrared and beyond, thus representing a structural color device with enormous potential for optical and photonic applications.

physics.optics↗
arXiv2024

Confinement and magnetic field effect on chiral ferroelectric nematic in Grandjean-Cano wedge cells

We explore the structure and magnetic field response of edge dislocations in Grandjean-Cano wedge cells filled with chiral mixtures of the ferroelectric nematic mesogen DIO. Upon cooling, the ordering changes from paraelectric in the cholesteric phase N* to antiferroelectric in the smectic SmZ_A* and to ferroelectric in the cholesteric N_F*. Dislocations of the Burgers vector b equal the helicoidal pitch P are stable in all three phases, while dislocations with b=P/2 exist only in the N* and SmZ_A*. The b=P/2 dislocations split into pairs of τ(-1/2) λ(+1/2) disclinations, while the thick dislocations b=P are pairs of nonsingular λ(-1/2) λ(+1/2) disclinations. The polar order makes the τ(-1/2) disclinations unstable in the N_F* phase, as they should be connected to singular walls in the polarization field. We propose a model of transformation of the composite τ(-1/2) line-wall defect into a nonsingular λ(-1/2) disclination, which is paired up with a λ(+1/2) line to form a b=P dislocation. The SmZ_A* behavior in the in-plane magnetic field is different from that of the N_F* and N*: the dislocations show no zigzag instability, and the pitch remains unchanged in the magnetic fields up to 1 T. The behavior is associated with the finite compressibility of smectic layers.

cond-mat.soft↗
arXiv2024

High order nonlinear electrophoresis in a nematic liquid crystal

Electrophoresis is the motion of particles relative to a surrounding fluid driven by a uniform electric field. In conventional electrophoresis, the electrophoretic velocity grows linearly with the applied field. Nonlinear effects with a quadratic speed vs field dependence are gaining research interest since an alternating current field could drive them. Here we report on the giant nonlinearity of electrophoresis in a nematic liquid crystal in which the speed grows with the fourth and sixth powers of the electric field. The mechanism is attributed to the shear thinning of the nematic environment induced by the moving colloid. The observed giant nonlinear effect dramatically enhances the efficiency of electrophoretic transport.

cond-mat.soft↗
arXiv2024

Splay-bend elastic inequalities shape tactoids, toroids, umbilics, and conic section walls in paraelectric, twist-bend, and ferroelectric nematics

Elastic constants of splay K_11, twist K_22, and bend K_33 of nematic liquid crystals are often assumed to be equal to each other in order to simplify the theoretical description of complex director fields. Here we present examples of how the disparity of K_11 and K_33 produces effects that cannot be described in a one-constant approximation. In a lyotropic chromonic liquid crystal, nematic droplets coexisting with the isotropic phase change their shape from a simply-connected tactoid to a topologically distinct toroid as a result of temperature or concentration variation. The transformation is caused by the increase of the splay-to-bend ratio K_11/K_33. A phase transition from a conventional nematic to a twist-bend nematic implies that the ratio K_11/K_33 changes from very large to very small. As a result, the defects caused by an externally applied electric field change the deformation mode of optic axis from bend to splay. In the paraelectric-ferroelectric nematic transition, one finds an inverse situation: K_11/K_33 changes from small to large, which shapes the domain walls in the spontaneous electric polarization field as conic sections. The polarization field tends to be solenoidal, or divergence-free, a behavior complementary to irrotational curl-free director textures of a smectic A.

cond-mat.soft↗
arXiv2024

Polar self-organization of ferroelectric nematic liquid crystal molecules on atomically flat Au(111) surface

Understanding nanoscale mechanisms responsible for the recently discovered ferroelectric nematics can be helped by direct visualization of self-assembly of strongly polar molecules. Here we report on scanning tunneling microscopy (STM) studies of monomolecular layers of a ferroelectric nematic liquid crystal on a reconstructed Au(111) surface. The monolayers are obtained by deposition from a solution at ambient conditions. The adsorbed ferroelectric nematic molecules self-assemble into regular rows with tilted orientation, resembling a layered structure of a smectic C. Remarkably, each molecular dipole in this architecture is oriented along the same direction giving rise to polar ferroelectric ordering.

cond-mat.soft↗
arXiv2024

Chiral ground states of ferroelectric liquid crystals

Ferroelectric nematic liquid crystals are formed by achiral molecules with large dipole moments. Its three-dimensional orientational order is universally described as unidirectionally polar. We demonstrate that the ground state of ferroelectric nematic unconstrained by externally imposed alignment directions is chiral, with left- and right-hand twists of polarization. Although the helicoidal deformations and defect walls separating domains of opposite handedness increase the elastic energy, the twists reduce the electrostatic energy and become weaker when the material is doped with ions. The study shows that the polar orientational order of molecules could trigger chirality in the soft matter with no chemically induced chiral centers.

cond-mat.soft↗
arXiv2023

Topological transformations of a nematic drop

Morphogenesis of living systems involves topological shape transformations which are highly unusual in the inanimate world. Here we demonstrate that a droplet of a nematic liquid crystal changes its equilibrium shape from a simply-connected tactoid, which is topologically equivalent to a sphere, to a torus, which is not simply-connected. The topological shape transformation is caused by the interplay of nematic elastic constants, which facilitates splay and bend of molecular orientations in tactoids but hinders splay in the toroids. The elastic anisotropy mechanism might be helpful in understanding topology transformations in morphogenesis and paves the way to control and transform shapes of droplets of liquid crystals and related soft materials.

cond-mat.soft↗
arXiv2023

Ferroelectric nematic liquids with conics

Spontaneous electric polarization of solid ferroelectrics follows aligning directions of crystallographic axes. Domains of differently oriented polarization are separated by domain walls (DWs), which are predominantly flat and run along directions dictated by the bulk translational order and the sample surfaces. Here we explore DWs in a ferroelectric nematic (NF) liquid crystal, which is a fluid with polar long-range orientational order but no crystallographic axes nor facets. We demonstrate that DWs in the absence of bulk and surface aligning axes are shaped as conic sections. The conics bisect the angle between two neighboring polarization fields to avoid electric charges. The remarkable bisecting properties of conic sections, known for millennia, play a central role as intrinsic features of liquid ferroelectrics. The findings could be helpful in designing patterns of electric polarization and space charge.

cond-mat.soft↗
arXiv2022

Toroidal nuclei of columnar lyotropic chromonic liquid crystals coexisting with isotropic phase

Nuclei of ordered materials emerging from the isotropic state usually show a shape topologically equivalent to a sphere; the well-known examples are crystals and nematic liquid crystal droplets. In this work, we explore experimentally and theoretically the nuclei of columnar lyotropic chromonic liquid crystal coexisting with the isotropic phase that are toroidal in shape. The geometry of toroids depends strongly on the molecular concentrations and presence of a crowding agent, polyethylene glycol. High concentrations result in thick toroids with small central holes, while low concentrations yield thin toroids with wide holes. The multitude of the observed shapes is explained by the balance of bending elasticity and anisotropic interfacial tension.

cond-mat.soft↗
arXiv2022

Soliton walls paired by polar surface interactions in a ferroelectric nematic liquid crystal

Surface interactions are responsible for many properties of condensed matter, ranging from crystal faceting to the kinetics of phase transitions. Usually, these interactions are polar along the normal to the interface and apolar within the interface. Here, we demonstrate that polar in-plane surface interactions of a ferroelectric nematic NF produce polar monodomains in micron-thin planar cells and stripes of an alternating electric polarization, separated by 180 degree domain walls, in thicker slabs. The surface polarity binds together pairs of these walls, yielding a total polarization rotation by 360 degrees. The polar contribution to the total surface anchoring strength is on the order of 10%. The domain walls involve splay, bend, and twist of the polarization. The structure suggests that the splay elastic constant is larger than the bend modulus. The 360 degree pairs resemble domain walls in cosmology models with biased vacuums and ferromagnets in an external magnetic field.

cond-mat.soft↗
arXiv2022

In-situ measurements of twist and bend elastic constants in oblique helicoidal cholesteric

Unique electro-optical properties of the oblique helicoidal cholesteric (ChOH) stem from its heliconical director structure. An applied electric field preserves the single harmonic modulation of the director while tuning the ChOH period and the corresponding Bragg peak wavelength within a broad spectral range. We use the response of ChOH to the electric field to measure the elastic constants of twist K22 and bend K33 directly in the cholesteric phase. The temperature dependencies of K22 and K33 allow us to determine the range of the electric tunability of the ChOH pitch and the heliconical angle. The data are important for understanding how molecular composition and chirality influence macroscopic elastic properties of the chiral liquid crystals and for the development of ChOH-based optical devices.

cond-mat.soft↗
arXiv2021

Dynamic control of speed and trajectories of active droplets in a nematic environment by electric field and focused laser beam

One objective of active matter science is to unveil principles by which chaotic microscale dynamics could be transformed into useful work. A nematic liquid crystal environment offers a number of possibilities, one of which is a directional motion of an active droplet filled with an aqueous dispersion of swimming bacteria. In this work, using the responsiveness of the nematic to the electric field and light, we demonstrate how to control the direction and speed of active droplets. The dielectric response of nematic to the electric field causes two effects: (i) reorientation of the overall director, and (ii) changing the symmetry of the director configuration around the droplet. The first effect redirects the propulsion direction while the second one changes the speed. A laser beam pointed to the vicinity of the droplet can trigger the desired director symmetry around the droplet, by switching between dipolar and quadrupolar configurations, thus affecting the motility and polarity of propulsion. The dynamic tuning of the direction and speed of active droplets represents a step forward in the development of controllable microswimmers.

cond-mat.soft↗
arXiv2020

Liquid crystal phases with unusual structures and physical properties formed by acute-angle bent-core molecules

Liquid crystals formed by acute-angle bent-core (ABC) molecules with a 1,7 naphthalene central core show an intriguing phase behavior with the nematic phase accompanied by poorly understood additional phases. In this work, we characterize the physical properties of an ABC material, such as birefringence, dielectric permittivities, elastic constants, and surface alignment, and present X-ray diffraction and transmission electron microscopy studies of their ordering. The ABC molecular shape resembling the letter $λ$ yields a very small splay elastic constant in the uniaxial nematic phase and results in the formation of a tetragonal positionally ordered columnar phase consisting of molecular columns with a uniform uniaxial director that can be bent but not splayed.

cond-mat.soft↗
arXiv2020

Directional self-locomotion of active droplets enabled by nematic environment

Active matter comprised of self-propelled interacting units holds a major promise for extraction of useful work from its seemingly chaotic out-of-equilibrium dynamics. Streamlining active matter to produce work is especially important at microscale, where the viscous forces prevail over inertia and the useful modes of transport require very specific non-reciprocal type of motion. Here we report that microscopic active droplets representing aqueous dispersions of swimming bacteria Bacillus subtilis show unidirectional propulsion when placed in an inactive nematic medium. Random motion of bacteria inside the droplet is rectified into a directional self-locomotion of the droplet by the polar director structure that the droplet itself creates in the surrounding nematic through anisotropic molecular interactions at its surface. Droplets without swimming bacteria show no net displacement. The trajectory of the active droplet can be predesigned as rectilinear or curvilinear by patterning the molecular orientation of the nematic medium. The effect demonstrates that swimming at microscale can be achieved at the expense of broken spatial symmetry of the medium; it can be used in development of micromachines.

cond-mat.soft↗
arXiv2020

Shear-induced polydomain structures of nematic lyotropic chromonic liquid crystal disodium cromoglycate

Lyotropic chromonic liquid crystals (LCLCs) represent aqueous dispersions of organic disk-like molecules that form cylindrical aggregates. Despite the growing interest in these materials, their flow behavior is poorly understood. Here, we explore the effect of shear on dynamic structures of the nematic LCLC, formed by 14wt ${\%}$ water dispersion of disodium cromoglycate (DSCG). We employ in-situ polarizing optical microscopy (POM) and small-angle and wide-angle X-ray scattering (SAXS/WAXS) to obtain independent and complementary information on the director structures over a wide range of shear rates. The DSCG nematic shows a shear-thinning behavior with two shear-thinning regions (Region I at $\dotγ<1\,s^{-1}$ and Region III at $\dotγ>10 s^{-1}$) separated by a pseudo-Newtonian Region II ($1 s^{-1}<\dotγ<10 s^{-1}$). The material is of a tumbling type. In Region I, $\dotγ<1 s^{-1}$, the director realigns along the vorticity axis. An increase of $\dotγ$ above $1 s^{-1}$ triggers nucleation of disclination loops. The disclinations introduce patches of the director that deviates from the vorticity direction and form a polydomain texture. Extension of the domains along the flow and along the vorticity direction decreases with the increase of the shear rate to $10 s^{-1}$. Above $10 s^{-1}$, the domains begin to elongate along the flow. At $\dotγ>100 s^{-1}$, the texture evolves into periodic stripes in which the director is predominantly along the flow with left and right tilts. The period of stripes decreases with an increase of $\dotγ$. The shear-induced transformations are explained by the balance of the elastic and viscous energies. In particular, nucleation of disclinations is associated with an increase of the elastic energy at the walls separating nonsingular domains with different director tilts.

cond-mat.soft↗
arXiv2020

Design of nematic liquid crystals to control microscale dynamics

Dynamics of small particles, both living such as swimming bacteria and inanimate, such as colloidal spheres, has fascinated scientists for centuries. If one could learn how to control and streamline their chaotic motion, that would open technological opportunities in areas such as the transformation of stored or environmental energy into systematic motion, micro-robotics, and transport of matter at the microscale. This overview presents an approach to command microscale dynamics by replacing an isotropic medium such as water with an anisotropic fluid, a nematic liquid crystal. Orientational order leads to new dynamic effects, such as propagation of particle-like solitary waves. Many of these effects are still awaiting their detailed mathematical description. By using plasmonic metamask photoalignment, the nematic director can be patterned into predesigned structures that control dynamics of inanimate particles through the liquid crystal enabled nonlinear electrokinetics. Moreover, plasmonic patterning of liquid crystals allows one to command the dynamics of swimming bacteria, guiding their trajectories, polarity of swimming, and concentration in space. The patterned director design can also be extended to liquid crystal elastomers, in which case the director gradients define the dynamic profile of elastomer coatings. Some of these systems form an experimental playground for the exploration of out-of-equilibrium active matter, in which the levels of activity, degree of orientational order and patterns of alignment can all be controlled independently of each other.

cond-mat.soft↗