O. Buluy

Magnetic sensitivity of a dispersion of aggregated ferromagnetic carbon nanotubes in liquid crystals

Using carbon nanotubes filled with α-Fe, we have shown that aggregated ferronematic colloids demonstrate reliable and very effective response to a weak (<5 mT) magnetic field. The magnetic field realigns the aggregates of the particles which results in a non-threshold reorientation of the LC nearby, leading to the optically observed director distortions. The distortion regions expand with the increase of the magnetic field and achieve maximum size of several micrometres, comparable with the size of the agglomerates. In the non-distorted regions the reorientation of the director begins at the magnetic field reaching the Freedericksz transition value. Taking into account the extreme sensitivity of aggregated ferronematics to magnetic field, the following experimental and theoretical studies of the individual response of the aggregated nanoparticles to magnetic field may became the topical task of the physics and applications of ferronematics.

Macroscopic optical effects in low concentration ferronematics

We present a detailed experimental and theoretical study of the optical response of suspensions of ferromagnetic nanoparticles (“ferroparticles”) in nematic liquid crystals (“ferronematics”), concentrating on the magnetic field-induced Frederiks transition. Even extremely low ferroparticle concentrations (at a volume fraction between 2 × 10−5 and 2 × 10−4), induce a significant additional ferronematic linear response at low magnetic field (<100 G) and a decrease in the effective magnetic Frederiks threshold. The experimental results demonstrate that our system has weak ferronematic behavior. The proposed theory takes into account the nematic diamagnetism and assumes that the effective magnetic susceptibility, induced by the nanoparticles, no longer dominates the response. The theory is in good agreement with the experimental data for the lowest concentration suspensions and predicts the main features of the more concentrated ones. The deviations observed in these cases hint at extra effects due to particle aggregation, which we have also observed directly in photographs.

Enhanced Magneto-Optical Properties of Suspensions of Spindle Type Mono-Dispersed Hematite Nano-Particles in Liquid Crystal

We explored how to increase liquid crystal sensitivity to external magnetic fields. Suspensions consisting of a mixture of liquid crystal and spindle type mono-dispersed hematite nano-particles at concentrations lower than 1 wt% were prepared. The Fréedericksz transition threshold for the suspensions appeared to be lower than for the pure liquid crystal. It was proved that adding canted antiferromagnetic nano-particles in liquid crystals increased their sensitivity to magnetic field while no change of the basic mesogenic properties of the matrix occurred.

Influence of Surface Treatment of Ferromagnetic Nanoparticles on Properties of Thermotropic Nematic Liquid Crystals

We observe a strong effect of surfactant structure on the aggregation of ferromagnetic nanoparticles in nematic liquid crystal (LC) hosts, and the colloidal stability and ordering of the resulting nematic matrix. We find that ferromagnetic nanoparticles coated with surfactants that possess terminal mesogenic groups induce minimal aggregation of the nanoparticles, resulting in a 1.5 times reduction in the value of the Frederikscz transition field in comparison with the parent LC host.

Strong orientational coupling in two-component suspensions of rod-like nanoparticles

We consider two-component suspensions of non-magnetic and magnetic rods and show that alignment of the magnetic component by a magnetic field orients the non-magnetic one. We studied suspensions of non-magnetic V2O5 rods doped with magnetic Fe3O4 nanoparticles. In the isotropic phase, the field-induced orientation of Fe3O4 particle chains aligns the V2O5 rods. Moreover, the nematic phase reorients towards the field at ≈50 Oe, much less than the ≈5 kOe needed for pure V2O5 suspensions. Such effects should exist in any suspension of two kinds of rods, one of which is sensitive to an external field.

Formation and dynamics of easy orientation axis in magnetic field on PVCN-F surface

We describe the experiments on a magnetically-induced drift of the easy axis on a soft surface of photoaligning material fluoro-polyvinyl-cinnamate. We found unexpected partial relaxation of the drift of the easy axis after switching the magnetic field off. This relaxation cannot be explained in a framework of the existing models and requires additional assumptions about the drift process. We propose a model that explains the experimental data suggesting elastic-like behaviour of the polymer fragments during the drift of the easy axis.

Magnetically induced alignment of FNS

In this work we report the observation of magnetically controlled anchoring of ferro-nematic suspensions. We found that application of a weak magnetic field to a cell with the ferro-suspension induces an easy orientation axis with weak anchoring energy on a glass surface. Varying the direction of the magnetic field can change the easy axis orientation. We believe that the magnetically induced alignment of the ferro-liquid crystal (LC) suspension is caused by adsorption of the ferro-particles on the glass surface and this alignment can be manipulated with a magnetic field. The developed system is very promising for ultra-sensitive magnetically controlled LC devices for information processing and storage.

Anchoring and gliding of easy axis of 5CB on photoaligning PVCN-F surface

Abstract— The photoaligning properties of the popular photoaligning material polyvinyl-4(fluorocinnamate) (PVCN-F) are presented. The aligning quality and azimuthal and zenithal anchoring energy were measured and the drift of the easy orientation axis (gliding effect) on the PVCN-F surface, depending on UV exposure, was studied. Special attention is paid to unraveling the contribution of the adsorption liquid-crystal molecules onto the aligning surface to the anchoring properties of PVCNF and measuring the drift of the easy orientation axis over the PVCN-F surface. It is shown that a relatively weak azimuthal anchoring energy (Waz ∼ 10−7 − 10−5 J/m2) leads to strong drift of the easy axis in the azimuthal plane that was observed in a moderate (∼0.1–0.3 T) magnetic field. A much stronger polar anchoring (Wzen ∼ 10−4 J/m2) allowed us to observe the essential gliding of the easy axis in the zenithal plane in a rather strong electric field (∼5 V/μm).

Magnetically induced alignment of ferro-nematic suspension on PVCN-F layer

We have observed magnetically controlled anchoring of ferro-nematic suspensions (FNS) on a fluoro-poly(vinyl)-cinnamate (PVCN-F) layer. We found that with the application of a weak magnetic field to a cell filled with the ferro-suspension, we could induce an easy (orientational) axis of weak anchoring energy on the polymer surface. By varying the direction of the magnetic field, one can change the easy axis orientation of liquid crystal (LC). We believe that the magnetically induced alignment of the ferro-nematic suspension is caused by the precipitation of ferro-particles on the polymer substrate. As a result, this alignment can be manipulated by a small magnetic field. The developed system is very promising for ultra-sensitive magnetically controlled LC devices for information processing and storage.

P-129: Rejuvenated Photoalignment of Liquid Crystal on Cinnamoil-Containing Polymers

We report on easy orientation axis alternation in new photoaligning polymers (PG materials) based on cellulose backbone containing photosensitive cinnamoil groups. Materials have strong application potential due to high aligning quality and photosensitivity, strong anchoring energy, small sticking effect. Rejuvenation of polymer after UV-irradiation makes PG attractive for various applications.