Emil Petrescu

Behaviour of ferrocholesterics under external magnetic fields

Abstract The influence of an external magnetic field on the orientational behaviour of a ferrocholesteric with a positive magnetic anisotropy is investigated. Both the phenomena arising when the field was switched on or switched off are considered. It is found that the field needed for a ferrocholesteric–ferronematic transition B FC ↑ is higher when compared to that obtained for the pure cholesteric ( B C ↑). A similar result was obtained when estimating the critical field for the homeotropic ferronematic–ferrocholesteric (focal conic) transition, occurring when the magnetic field was decreased or switched off. We found that B FC ↓> B C ↓. These results are explained when considering that the magnetic moments of the magnetic powder are not oriented parallel to the liquid crystal molecular directors, therefore hindering their orientation under a magnetic field.

Surface effects on magnetic Freedericksz transition in ferronematics with soft particle anchoring

Abstract The magnetic field-induced Freedericksz transition in a homeotropically aligned ferronematic (FN) in the case of weak anchoring molecules to the walls is studied. A correction to the Burylov–Raikher expression for the second-order transition threshold field is found. The possibility of a first-order transition is discussed in connection with the similar results for nematic liquid crystals. The saturation magnetic field is also obtained in terms of FN, surface and device parameters.

Ferronematics with soft particle anchoring in magnetic and laser fields

The influence of a magnetic field and an optical laser field on the orientational behavior of a ferronematic with positive magnetic and dielectric anisotropy is investigated. A slow transition from the homeotropic to the distorted alignment of the nematic director is found when the field strengths exceed certain threshold values. A correlation between the threshold values of the two fields is obtained and a phase diagram for this transition is determined. It is also found that in the presence of the two fields the distorted arrangement is more stable than the homeotropic one (its free energy per unit area is smaller).

Thermally Stimulated Depolarization Currents and Optical Transmission Studies on UV Cured Polymer Dispersed Liquid Crystal Films

Polymer-dispersed-liquid-crystal films were obtained using the photopolymerization-induced-phase-separation method. The thermally stimulated depolarization current measurements indicate a space-charge-limited current. Conduction of electronic type was proven and the activation energy, specific to the conduction process, was calculated. Optical measurements have been performed simultaneously with the measurements of thermally stimulated currents and the nematic-isotropic phase transition was detected by the switch of the optical transmission. The change of the optical transmission versus applied a.c. voltage was the indication for the on-off switch behavior, useful in electro-optic applications.

Static and Dynamic Behavior of Ferronematics Under Magnetic Fields

The static and dynamic behavior under magnetic elds of ferronematics obtained by adding an organomagnetic material to the nematic E3 is investigated. First, by using both optical and electrical measurements, an increase of the threshold field for magnetic Freedericksz transition was noticed. Second, we examined both theoretically and experimentally the relaxation phenomena occurring in these ferronematics when the magnetic field was suddenly varied. We found that for ferronematics the relaxation processes are delayed when compared to those occurring in pure nematics. The threshold fields for magnetic Freedericksz transition were determined by means of the relaxation time and the results are in good agreement with the ones obtained when using the optical and electrical methods reported before.

Dynamic behavior of a nematic liquid crystal mixed with CoFe2O4 ferromagnetic nanoparticles in a magnetic field

The dynamic behavior of a mixture of 4-cyano-4′-pentylbiphenyl (5CB) with 1% CoFe2O4 nanoparticles was analyzed. Experimental data indicate a high stability of the nematic director in the mixture compared to a reference 5CB sample in the magnetic field. The ferrite nanoparticles agglomerate forming long chains as observed in polarized microscopy images. These chains have a very high influence on the magneto-optic effect of the cell. When the magnetic field is applied on the mixture, the chains tend to align with the field direction but, due to their large size, they remain oriented obliquely between the support plates. Thus, the nematic molecules anchored on their surface can not reorient with the field and only a small distortion angle of the liquid crystal molecular director is observed. A comparison with a previously developed theoretical model confirms this small deviation.

Laser-Induced Optical Nonlinearities In Ferronematics

Nematic and ferronematic samples were subjected to high power laser irradiation. Optical nonlinearities such as diffraction rings and self focussing effects were examined as function of light intensity. It is shown that the self-focusing effect appearing in a ferronematic(obtained by dispersing 0.1%yttrium iron garnet into nematic 4799 Merck)is smaller when compared to that encountered in pure nematic. A theoretical approach was elaborated to explain these optical nonlinearities in ferronematics and good agreement with the experimental findings was found.

Dynamic behavior of a nematic liquid crystal with added carbon nanotubes in an electric field

The dynamic behavior of a nematic liquid crystal with added carbon nanotubes (CNTs) in an electric field was analyzed. A theoretical model based on elastic continuum theory was developed and the relaxation times of nematic liquid crystals with CNTs were evaluated. Experiments made with single-walled carbon nanotubes dispersed in nematic 4-cyano-4’-pentylbiphenyl (5CB) indicated a significant difference of the relaxation time when compared to pure liquid crystal. We also noticed that the relaxation time when the field is switched off depends on how long the field was applied. It is shorter when the field is switched off immediately after application and longer when the field was applied for at least one hour.

Relaxation phenomena in nematic liquid crystals with multiwall carbon nanotubes adding

Theoretical and experimental studies about the influence of an external magnetic field on a nematic liquid crystal with carbon nanotubes adding are presented. Planar oriented cells filled with a mixture of MCL-6601 (Merck) nematic and multi-walled carbon nanotubes were subjected to a magnetic field higher than the critical one for the magnetic Freedericksz transition. A laser beam was used to observe the molecular director distortion. Dynamical measurements of the transmitted light intensity were performed when the magnetic field was switched on and off. The results were used to evaluate the relaxation times of the mixture and to show the influence of the carbon nanotubes on these parameters. We also present a theoretical model to explain the experimental results.

Dynamic behavior of nematic liquid crystal mixtures with quantum dots in electric fields

The dynamic behavior of a mixture consisting of liquid crystalline 4-cyano-4’-pentylbiphenyl (5CB) and CdSe/ZnS quantum dots in electric fields was theoretically studied. The model was based on elastic continuum theory considering the interaction of the nematic molecules with the surrounding molecules, with the quantum dots and with the electric field. Experimental data obtained by dynamic measurements on a sample containing 0.89% (mass fraction) of CdSe/ZnS quantum dots revealed a decrease of the relaxation time compared to pure 5CB.