Fluid fibers in true 3D ferroelectric liquids.

We demonstrate an exceptional ability of a high-polarization 3D ferroelectric liquid to form freely suspended fluid fibers at room temperature. Unlike fluid threads in modulated smectics and columnar phases, where translational order is a prerequisite for forming liquid fibers, recently discovered ferroelectric nematic forms fibers with solely orientational molecular order. Additional stabilization mechanisms based on the polar nature of the mesophase are required for this. We propose a model for such a mechanism and show that these fibers demonstrate an exceptional nonlinear optical response and exhibit electric field-driven instabilities.

Superparamagnetic nanoparticles with LC polymer brush shell as efficient dopants for ferronematic phases.

Liquid crystal (LC) based magnetic materials consisting of LC hosts doped with functional magnetic nanoparticles enable optical switching of the mesogens at moderate magnetic field strengths and thereby open the pathway for the design of novel smart devices. A promising route for the fabrication of stable ferronematic phases is the attachment of a covalently bound LC polymer shell onto the surface of nanoparticles. With this approach, ferronematic phases based on magnetically blocked particles and the commercial LC 4-cyano-4'-pentylbiphenyl (5CB) liquid crystal were shown to have a sufficient magnetic sensitivity, but the mechanism of the magneto-nematic coupling is unidentified. To get deeper insight into the coupling modes present in these systems, we prepared ferronematic materials based on superparamagnetic particles, which respond to external fields with internal magnetic realignment instead of mechanical rotation. This aims at clarifying whether the hard coupling of the magnetization to the particle's orientation (magnetic blocking) is a necessary component of the magnetization-nematic director coupling mechanism. We herein report the fabrication of a ferronematic phase consisting of surface-functionalized superparamagnetic Fe3O4 particles and 5CB. We characterize the phase behavior and investigate the magneto-optical properties of the new ferronematic phase and compare it to the ferronematic system containing magnetically blocked CoFe2O4 particles to get information about the origin of the magneto-nematic coupling.

Efficient ferronematic coupling with polymer-brush particles.

Switching of liquid crystal phases is of enormous technological importance and enables digital displays, thermometers and sensors. As an alternative to electric fields or temperature, magnetic fields are an interesting trigger, as they are on the one hand versatile to design, and on the other hand, they are compatible with a bouquet of applications. An interesting option to enable the magnetic switchability of nematic phases is by doping them with functional magnetic nanoparticles, but it remains a challenge to achieve well-compatibilized and stable ferronematic phases. Here, we report a new approach for the experimental realization of finely dispersed MNPs and nematic LC by creation of a surface-coupled mesogen-functionalized polymer brush, and the determination of their corresponding magneto-optical response. For this purpose, CoFe2O4 particles are equipped with a covalently attached polymeric shell carrying mesogenic groups and successfully dispersed in 4-pentyl-4'-cyanobiphenyl (5CB) to form a stable ferronematic phase at ambient concentration up to ∼1 vol%, as shown by DSC and Abbé refractometry. The magneto-optic response is detected in planar aligned LC cells. As compared to undoped 5CB, the hybrid system shows a significantly increased magnetic sensitivity, and the magneto-nematic surface anchoring is quantified by analysis of the magneto-nematic cross-correlation.

Frequency-dependent conversion of the torque of a rotating magnetic field on a ferrofluid confined in a spherical cavity.

The dynamics of magnetic nanoparticles in rotating magnetic fields is studied both experimentally and theoretically. The experimental investigation is focused on the conversion of the magnetic forces to a mechanical torque acting on a ferrofluid confined in a spherical cavity in a rotating magnetic field. Polydispersity usually present in diluted ferrofluids is shown to play a crucial role in the torque conversion. Important features observed experimentally are reproduced theoretically in studies on the dynamics of particles with uniaxial magnetic anisotropy in the presence of thermal noise. The phase lag between the rotating magnetic field and the induced rotating magnetization, as well as the corresponding torque which is transferred to the carrier fluid because of the mutual coupling between both, is analyzed as a function of the particle size. It is shown that for large particles the magnetic moment is locked to the anisotropy axis. On lowering the particle radius, Néel relaxation becomes increasingly important. Illustrative numerical calculations demonstrating this behavior are performed for magnetic parameters typical for iron oxide.

The role of structural anisotropy in the magnetooptical response of an organoferrogel with mobile magnetic nanoparticles.

We investigate the structure and the magnetooptical response of isotropic and anisotropic fibrillous organoferrogels with mobile magnetic nanoparticles (MNPs). We demonstrate that the presence of the gel network restricts the magnetooptical response of the ferrogel. Even though the ferrogel exhibits no magnetic hysteresis, an optical hysteresis has been found. This suggests that the magnetooptical response is primarily determined by the dynamics of self-assembly of the MNPs into shape-anisotropic agglomerates. Furthermore, we show that the optical anisotropy of the system can be fine-tuned by varying the concentration of the gelator and the MNPs, respectively. The optical response in structurally anisotropic gels becomes orientation-dependent, revealing an intricate interplay between the gel mesh and the MNPs.

Light-Responsive Microstructures in Droplets of the Twist-Bend Nematic Phase.

We report on the structure and optical manipulation of the director configurations in emulsions of liquid-crystalline droplets of a compound exhibiting the nematic (N) and the twist-bend nematic (NTB) phases. We demonstrate a decrease in the ratio of the bent elastic constant K33 to the splay constant K11 by nearly 2 orders of magnitude with decreasing temperature in the N phase. The director structures in liquid-crystal droplets doped with a photoswitchable surfactant without and under ultraviolet (UV) light are discussed in light of the strong elastic anisotropy of the investigated compound. We also compare our findings with the results obtained in doped nematic droplets of a conventional 4-cyano-4'-pentylbiphenyl (5CB) liquid crystal. The dynamics of droplets in the NTB phase by UV light irradiation are also studied using polarizing and confocal microscopies.

Optically driven translational and rotational motions of microrod particles in a nematic liquid crystal.

A small amount of azo-dendrimer molecules dissolved in a liquid crystal enables translational and rotational motions of microrods in a liquid crystal matrix under unpolarized UV light irradiation. This motion is initiated by a light-induced trans-to-cis conformational change of the dendrimer adsorbed at the rod surface and the associated director reorientation. The bending direction of the cis conformers is not random but is selectively chosen due to the curved local director field in the vicinity of the dendrimer-coated surface. Different types of director distortions occur around the rods, depending on their orientations with respect to the nematic director field. This leads to different types of motions driven by the torques exerted on the particles by the director reorientations.

Photomanipulation of the anchoring strength using a spontaneously adsorbed layer of azo dendrimers.

We systematically studied the photoinduced anchoring transition in a nematic liquid crystal containing azo dendrimers. Because the azo dendrimers in the trans-isomer state were spontaneously adsorbed at substrate surfaces, which was confirmed by optical second-harmonic generation (SHG), a homeotropic orientation was established at the first stage. Ultraviolet (UV) light irradiation triggered a transition into a planar state which was accompanied by a suppression of the SH generation. The monotonic decrease of the effective scalar order parameter with increasing UV light intensity was determined by polarized attenuated total reflection infrared (ATR-IR) spectroscopy. The variation of anchoring strength and extrapolation length was evaluated by observing the Fréedericksz transition as a function of UV light intensity at a certain visible (VIS) light intensity. Such a photoinduced variation can be interpreted as a variation of the anchoring strength depending on the trans/cis ratio at the surfaces based on a modified Rapini-Papoular model. Thus, this system provides the opportunity for a controlled change in the anchoring strength.

Doping of nematic cyanobiphenyl liquid crystals with mesogen-hybridized magnetic nanoparticles.

Magnetic nanoparticles (MNPs) functionalized with (pro-)mesogenic ligands are implemented into a nematic liquid crystal (LC) and studied regarding both colloidal stability and magneto-optical behavior. In this study, the particle surface is specifically engineered to tune the MNP interactions with the LC host. For this purpose, four types of (pro-)mesogenic ligands (ML) are synthesized, which are composed of three structural parts, i.e., a rigid, LC motif (i.e., cyanobiphenyl) and a functional group for nanoparticle binding, both linked via a flexible spacer of different alkyl chain lengths. Electrostatically stabilized CoFe2O4 and γ-Fe2O3 nanoparticles with narrow size distribution and sizes below 3 nm are obtained via co-precipitation and subsequently functionalized to yield MNP@ML nanoparticles. Studies on the behaviour of the MNP@ML nanoparticles in the commercial LC host (i.e., 4-pentyl-4'-cyanobiphenyl (5CB)) in the bulk and in thin films in LC test cells, reveal the initial formation of some heterogeneities after transition from the isotropic to the nematic phase. Homogenous MNP@ML-5CB hybrids with long-term, colloidal stability, however, are obtained after magnetic separation of initially formed particle aggregates. In particular, MLs with carboxy groups and high structural flexibility (i.e., long linker lengths) are shown to be well suited to form stable MNP colloids, allowing for high MNP doping levels. As compared to undoped 5CB, the CoFe2O4@MLx-5CB hybrids show an increased sensitivity to the magnetic field, affecting the Fréedericksz transition. The strongest effect, however, is observed in magnetic and electric fields. The coupling of the ultrasmall, spherical MNPs with the LC director in the magnetic field suggests the formation of LC-induced, anisometric MNP clusters.

Pattern-stabilized decorated polar liquid-crystal fibers.

Geometric frustration gives rise to new fundamental phenomena and is known to yield the formation of exotic states of matter, such as incommensurate crystals, modulated liquid-crystalline phases, and phases stabilized by defects. In this Letter, we present a detailed study of polar structure of freely suspended fluid filaments in a polarization modulated liquid-crystal phase. We show that a periodic pattern of polarization-splay stripes separated by defect boundaries and decorating smectic layers can stabilize the structure of fluid fibers against the Rayleigh-Plateau instability. The instability is suppressed by the resistance of the defect structure to a radial compression of the cylindrical fibers. Our results provide direct experimental observation of a link between the stability of the liquid fibers, internal polar order, and geometrical constraints. They open a new perspective on a wide range of fluid polar fiber materials.

Structure and mechanical properties of liquid crystalline filaments.

The formation of stable freely suspended filaments is an interesting peculiarity of some liquid crystal phases. So far, little is known about their structure and stability. Similarly to free-standing smectic films, an internal molecular structure of the mesophase stabilizes these macroscopically well-ordered objects with length to diameter ratios of 10(3) and above. In this paper, we report observations of smectic liquid crystal fibers formed by bent-shaped molecules in different mesophases. Our study, employing several experimental techniques, focuses on mechanical and structural aspects of fiber formation such as internal structure, stability, and mechanical and optical properties.

Labyrinthine instability in freely suspended films of a polarization-modulated smectic phase.

We report on fingering and labyrinthine instabilities of the layer dislocation lines in freely suspended polar liquid-crystalline films. These polar fingerlike and labyrinth structures reversibly form upon a transition into a modulated phase. External electric fields of several kV/m applied in the film plane can reversibly influence the formation of the finger textures. We show that the labyrinthine pattern is intrinsically related to regular splay deformations of the polarization.

Transitions between paraelectric and ferroelectric phases of bent-core smectic liquid crystals in the bulk and in thin freely suspended films.

We report on the contrasting phase behavior of a bent-core liquid crystal with a large opening angle between the mesogenic units in the bulk and in freely suspended films. Second-harmonic generation experiments and direct observation of director inversion walls in films in an applied electric field reveal that the nonpolar smectic C phase observed in bulk samples becomes a ferroelectric "banana" phase in films, showing that a mesogen with a small steric moment can give a phase with polar order in freely suspended films even when the corresponding bulk phase is paraelectric.

Structure characterization of free-standing filaments drawn in the liquid crystal state.

Stable free-standing liquid filaments formed by some layered mesophases of bent-core mesogens are unique structures. Some of their physical properties have been analyzed in recent studies, but their microscopic structure and conditions for stability have still been unclear. We explore details of filament shapes and surface profiles of filaments drawn in liquid crystal phases of bent-core mesogens by AFM and SEM measurements, and we present a microscopic structure model. Conclusions on the stabilizing mechanisms are drawn. Qualitative differences in mechanical properties are found for different mesophases, even though the macroscopic appearance of the filaments is very similar.