arXiv subjects

Natan Osterman

Publications and source records attributed to Natan Osterman.

11 recordsLinked to original sources

arXiv2026

Direct visualization of local electric fields in a layer of a ferroelectric nematic liquid

Ferroelectric nematic liquids exhibit complex ferroelectric domains shaped by competing elastic and electrostatic interactions. Using fluorescence and polarizing optical microscopy, we investigate domain formation and evolution under different anchoring conditions. Charged fluorescent ions map the electrostatic potential, revealing that electric fields are localized near domain walls and material surfaces. Upon cooling, domain walls transform from Ising- to N\'eel-type configurations, lowering the electrostatic potential, and reversibly recover upon heating.

cond-mat.soft↗

arXiv2026

Elasticity-Driven Periodic Polarization Patterns in Confined Chiral Ferroelectric Nematic Fluid

Ferroelectric nematic phases are a new class of polar fluids in which spontaneous polarization is directly coupled to the orientational order, providing unique opportunities for creating self-organized materils with spatially modulated electric polarization and nonlinear optical response. Here we report the spontaneous emergence of polarization modulated textures in a chiral ferroelectric nematic material close to the transition to the chiral twist-bend ferroelectric nematic phase. By systematically varying cell thickness and surface anchoring conditions, we map the formation of these modulated states, revealing stripe, square and hexagonal morphologies determined via confinement conditions. These structures are directly translated into periodic modulation of the nonlinear optical response, as evidenced by second-harmonic generation imaging. Comparison with an elasticity based theoretical framework and numerical free energy minimization shows that the instability originates from the softening of the bend elastic constant in the chiral nematic phase as the system approaches the lower-temperature heliconical polar phase. The resulting elastic frustration, combined with confinement, drives the formation of spatially periodic director distortions, highlighting ferroelectric nematic fluids as a promising platform for self-assembled nonlinear optical materials.

cond-mat.soft↗

arXiv2026

Modulated Anti-Ferroelectric Smectic Phases with Orthogonal and Tilted Structures

The discovery of the ferroelectric nematic phase has brought with it a plethora of new polar liquid crystalline phases. One in particular is the anti-ferroelectric smectic A SmA\textsubscript{AF} phase. In this letter we show via observation and analysis of satellite peaks in the X-ray scattering pattern that the structure of the SmA\textsubscript{AF} phase involves a density modulation of $\approx$10-20 nm lateral to the smectic layer normal. Further, we demonstrate a previously undiscovered phase where the anti-ferroelectric order is maintained into a tilted smectic phase demonstrating the robustness of the underlying frustration that leads to the modulated structure. We suggest that the modulations are only in a single dimension and appear parallel to the tilt plane. This new phase also shows a significantly different and complex response to an electric field from other discovered polar LC phases due to the ability to modulate both tilt and polarisation direction.

cond-mat.soft↗

arXiv2025

Antiferroelectric order in nematic liquids: Flexoelectricity vs electrostatics

The recent discovery of ferroelectric nematic liquid crystalline phases marks a major breakthrough in soft matter research. An intermediate phase, often observed between the nonpolar and the ferroelectric nematic phase, shows a distinct antiferroelectric response to electric fields. However, its structure and formation mechanisms remain debated, with flexoelectric and electrostatics effects proposed as competing mechanisms. By controlling the magnitude of electrostatic forces through ion addition in two representative ferroelectric nematic materials, we show that the primary mechanism for the emergence of antiferroelectric order is the flexoelectric coupling between electric polarization and splay deformation of the nematic director. The addition of ions significantly expands the temperature range over which the antiferroelectric phase is observed, with this range increasing with increasing ion concentration. Polarizing optical microscopy studies and second harmonic generation microscopy reveal the splayed structure modulated in two dimensions, while SHG interferometry confirms its antiferroelectric character. We extend the model previously used to describe pretransitional behaviour by incorporating the electrostatic contribution of ions. The model shows qualitative agreement with the experiments, accurately reproducing the phase diagram and temperature-dependent evolution of the modulation period of the observed structure.

cond-mat.soft↗

arXiv2024

Isotropic to nematic transition in alcohol ferrofluids of barium hexaferrite nanoplatelets

Alcohol ferrofluids made of ferrimagnetic barium hexaferrite (BHF) nanoplatelets (NPLs) form a unique example of a dipolar fluid - a liquid magnet. Its formation is induced at a high enough concentration of the NPLs. The key interactions between the NPLs are long-ranged dipolar magnetic and screened anisotropic electrostatic. Herein, we report the results on tuning the isotropic-nematic phase transition (i.e., the NPLs threshold concentration) in 1-butanol ferrofluids of BHF NPLs by affecting the interactions in the ferrofluids. The threshold concentration was determined by polarizing optical microscopy (POM) combined with a system for magnetic field manipulation and was in the range between 4.6 and 6.6 \% (v/v) depending on the ferrofluid. We observed that the threshold concentration decreased for 0.6 \% (v/v) with a larger mean diameter of the NPLs, up to 0.6 \% (v/v) with increased ionic strength of the ferrofluid, and for $\sim$ 2 \% (v/v) with higher saturation magnetization of the NPLs. We showed that by tuning the parameters affecting the ferrofluid's interplatelet interactions, we can alter the threshold concentration for the liquid magnet formation. The results elucidate the importance of a delicate balance between the repulsive screened electrostatic and attractive dipolar magnetic interactions for the liquid magnet formation.

cond-mat.soft↗

arXiv2023

Polarization patterning in ferroelectric nematic liquids

The recently discovered ferroelectric nematic liquids incorporate to the functional combination of fluidity, processability and anisotropic optical properties of nematic liquids, an astonishing range of physical properties derived from the phase polarity. Among them, the remarkably large values of second order optical susceptibility encourage to exploit these new materials for non-linear photonic applications. Here we show that photopatterning of the alignment layer can be used to structure polarization patterns. To do so, we take advantage of the flexoelectric effect and design splay structures that geometrically define the polarization direction. We demonstrate the creation of periodic polarization structures and the possibility of guiding polarization by embedding splay structures in uniform backgrounds. The demonstrated capabilities of polarization patterning, open a promising new route for the design of ferroelectric nematic based photonic structures and their exploitation.

cond-mat.soft↗

arXiv2020

Sculpting stable structures in pure liquids

Pure liquids in thermodynamic equilibrium are structurally homogeneous. In liquid crystals, flow and light pulses are used to create reconfigurable domains with polar order. Moreover, through careful engineering of concerted microfluidic flows and localized opto-thermal fields, it is possible to achieve complete control over the nucleation, growth, and shape of such domains. Experiments, theory, and simulations indicate that the resulting structures can be stabilized indefinitely, provided the liquids are maintained in a controlled non-equilibrium state. The resulting sculpted liquids could find applications in microfluidic devices for selective encapsulation of solutes and particles into optically active compartments that interact with external stimuli.

cond-mat.soft↗

arXiv2018

Director switching dynamics of ferromagnetic nematic liquid crystals

Successful realization of ferromagnetic nematic liquid crystals has opened up the possibility to experimentally study a completely new set of fundamental physical phenomena. In this contribution we present a detailed investigation of some aspects of the static response and the complex dynamics of ferromagnetic liquid crystals under the application of an external magnetic field. Experimental results are then compared with a macroscopic model. Dynamics of the director were measured by optical methods and analyzed in terms of a theoretical macroscopic model. A dissipative cross-coupling coefficient describing the dynamic coupling between the two system order parameters, the magnetization and the nematic director, is needed to explain the results. In this contribution we examine the dependency of this coefficient on material parameters and the saturation magnetization and the liquid crystal host. Despite the complexity of the system, the theoretical description allows for a proper interpretation of the results and is connected to several microscopic aspects of the colloidal suspension.

cond-mat.soft↗

arXiv2018

Dynamic magneto-optic coupling in a ferromagnetic nematic liquid crystal

Hydrodynamics of complex fuids with multiple order parameters is governed by a set of dynamic equations with many material constants, of which only some are easily measurable. We present a unique example of a dynamic magneto-optic coupling in a ferromagnetic nematic liquid, in which long-range orientational order of liquid crystalline molecules is accompanied by long-range magnetic order of magnetic nanoplatelets. We investigate the dynamics of the magneto-optic response experimentally and theoretically and find out that it is significantly affected by the dissipative dynamic cross-coupling between the nematic and magnetic order parameters. The cross-coupling coefficient determined by fitting the experimental results with a macroscopic theory is of the same order of magnitude as the dissipative coefficient (rotational viscosity) that governs the reorientation of pure liquid crystals.

cond-mat.soft↗

arXiv2018

Magneto-optic dynamics in a ferromagnetic nematic liquid crystal

We investigate dynamic magneto-optic effects in a ferromagnetic nematic liquid crystal experimentally and theoretically. Experimentally we measure the magnetization and the phase difference of the transmitted light when an external magnetic field is applied. As a model we study the coupled dynamics of the magnetization, M, and the director field, n, associated with the liquid crystalline orientational order. We demonstrate that the experimentally studied macroscopic dynamic behavior reveals the importance of a dynamic cross-coupling between M and n. The experimental data are used to extract the value of the dissipative cross-coupling coefficient. We also make concrete predictions about how reversible cross-coupling terms between the magnetization and the director could be detected experimentally by measurements of the transmitted light intensity as well as by analyzing the azimuthal angle of the magnetization and the director out of the plane spanned by the anchoring axis and the external magnetic field. We derive the eigenmodes of the coupled system and study their relaxation rates. We show that in the usual experimental set-up used for measuring the relaxation rates of the splay-bend or twist-bend eigenmodes of a nematic liquid crystal one expects for a ferromagnetic nematic liquid crystal a mixture of at least two eigenmodes.

cond-mat.mtrl-sci↗

arXiv2011

Finding the ciliary beating pattern with optimal efficiency

We introduce a measure for energetic efficiency of biological cilia acting individually or collectively and numerically determine the optimal beating patterns according to this criterion. Maximizing the efficiency of a single cilium leads to curly, often symmetric and somewhat counterintuitive patterns. But when looking at a densely ciliated surface, the optimal patterns become remarkably similar to what is observed in microorganisms like Paramecium. The optimal beating pattern then consists of a fast effective stroke and a slow sweeping recovery stroke. Metachronal coordination is essential for efficient pumping and the highest efficiency is achieved with antiplectic waves. Efficiency also increases with an increasing density of cilia up to the point where crowding becomes a problem. We finally relate the pumping efficiency of cilia to the swimming efficiency of a spherical microorganism and show that the experimentally estimated efficiency of Paramecium is surprisingly close to the theoretically possible optimum.

physics.bio-ph↗