Ferronematic Liquid Crystal Research Articles

Lyotropic ferronematic liquid crystals based on new Ni, Cu and Zn Ionic magnetic fluids

The properties of lyotropic ferronematic liquid crystals based on new Ni, Cu and Zn ionic magnetic fluids are discussed. The efficiency of these new ferrofluids in the lyotropic nematic liquid crystal doping is verified and compared with the conventional surfacted ferro fluid lyotropic doping. It was observed that the structural characteristics of the lyotropics and ferro fluids determine the good formation of the ferronematics.

Nematic kink states in a laser field

We have investigated the nonlinear optical interaction of uniform and kink states of a nematic and a ferrofluid-doped nematic (ferronematic) liquid crystal with an incident laser field. We find that the transition between the permitted uniform oreintational states of these systems is of first order in the case of nematics, and of second order in the case of ferronematics. In the latter case we also find the phenomenon of reentrance. We find new kink states in a magnetic field with topological winding different from pi in the case of nematics, and 2pi in the case of ferronematics. In ferronematics, due to grain segregation the phase diagrams for uniform and kink states are entirely different. In these systems we find a first or second order structural transformation from a single kink into a pair of kinks. Further, we obtain a rich variety of kink states as the intensity of the laser field is varied.

Time dependence of the magnetic grain concentration and secondary grain aggregation in ferronematic lyotropic liquid crystals subjected to magnetic field gradients.

The dynamical behavior of lyotropic nematic liquid crystals doped with ionic and surfacted magnetic fluids (ferronematics) is studied using a linear optical technique. The response of these mesophases to a combination of a static and a pulsed magnetic field is investigated by measuring the relaxation times as a function of the pulse width. A reversible modification of the magnetic grain concentration in the bulk of the samples and a secondary aggregation process due to the presence of a field gradient introduced by the pulsed field is discussed.

Measurement of the splay-bend elastic constant in lyotropic ferronematic liquid crystals: The influence of the bounding surfaces

We report the first experimental investigation devoted to analyze the influence of a ferrofluid on the surface properties of lyotropic liquid crystals doped with magnetic particles. By means of optical methods, the bulk orientation of the director vs the temperature is determined. The theoretical analysis shows that near the nematic–isotropic transition, the thermodynamical model for the temperature surface transition works reasonably well. On the contrary, in the low temperature region near the lamellar–nematic transition, important deviations from the theoretical behavior are observed. In order to interpret our experimental data, we propose a simple phenomenological extension of the thermodynamical model that takes into account the different contributions to the surface energy and of the residual lamellar order in the nematic phase. We show, furthermore, that the effective splay-bend elastic constant depends on the ferrofluid doping concentration, in agreement with a recently proposed theory.

Ferronematics: enhanced magneto-optical response of a liquid crystalline system

Ferronematic liquid crystals or ferronematics (FNs) is the name for suspensions of monodomain ferro- or ferrimagnetic particles in nematic liquid crystals. The most significant feature of these systems is a strong orientational coupling between the disperse phase (ferroparticles) and the liquid crystalline matrix. The applied magnetic field, changing the orientation of the particles, via them affects the texture of the nematic matrix. Under appropriate conditions, it enables us to acquire a full-scale control over the orientational state of FNs with the aid of rather weak, much less than 100 Oe, magnetic fields.

Dynamics of lyotropic ferronematic liquid crystals submitted to magnetic fields.

Dynamics of lyotropic ferronematic liquid crystals submitted to magnetic fields.

Magnetic-field-induced birefringence in a homeotropic ferronematic liquid-crystal wedge

The magnetic-field-induced birefringence in wedged homeotropic ferronematic liquid crystals is studied. Theoretical results for small distortions are obtained in terms of the elastic continuum theory of liquid crystals. The results of birefringence measurements are in good agreement with the predictions of the theory.

Electric-field-induced molecular reorientation of a magnetically biased ferronematic liquid-crystal film.

Electric-field-induced molecular reorientation is studied in the magnetically biased ferronematic liquid-crystal (FNLC) film. A theory derived from the free energy of the system is given. To investigate the field-induced distortion, we have measured the optical birefringence which depends on the integration of the director orientation. In our experiment, the electric-field-induced molecular reorientation of FNLC biased at different magnetic fields has been obtained. The electric-field-induced molecular reorientation of FNLC films biased at fixing magnetic field with different thicknesses has also been measured. Furthermore, hysteresis phenomena have been observed. At higher magnetic field intensities, it seems that the magnetic particles flocculate into large clumps.

Electro-optical effect of a magnetically biased ferronematic liquid crystal

The electro-optical effect of a magnetically biased ferronematic liquid-crystal film is investigated by using birefringence measurements. When a magnetic field is applied, the threshold voltage of the Freedericksz transition no longer exists. The dependence of the birefringence on the magnetic field strength in the low field regime is presented. A theory that accounts for the results is given.

The Magnetic-Field-Induced Birefringence of the Mixtures of the Chiral Molecules and the Ferronematic Liquid Crystals

Abstract The ferronemtaic state which was described theoretically by Brochard and de Gennes can be achieved by doping the liquid crystal with anisotropic ferromagnetic particles. The homeotropic liquid crystal film was successfully prepared for the ferronematic liquid crystal doped with low concentration chiral molecules. The magnetic-field-induced-birefringence due to molecular reorientation was treated both theoretically and experimentally. In the low field regime (<2.5G), it is theoretically predicted that the induced phase difference between the ordinary and extraordinary ray of a normally incident probe beam is proportional to the square of the external magnetic field and to the fifth power of the thickness of the sample film. This is verified by the experimental results. The effect of the chiral molecules appears in the effective elastic constant, ke , which will decrease and then increase with respect to the increasing of the concentration of the chiral molecules.

354. Beam probing of rf beam-plasma interactions: A B Canuara and F W Crawford, J Appl Phys, 36 (10), Oct 1965, 3132–3135

The effects of anchoring phenomenon on the electric Frederiks transition threshold field in a nematic liquid crystal doped with ferroelectric nanoparticles are discussed. The polarizability of these nanoparticles in combination with confinement effects cause the drastic effects on the ferronematic systems. This study is based on Frank free energy and Rapini–Papoular surface energy for ferronematic liquid crystal having finite anchoring condition. In the case of different anchoring boundary conditions, the Euler–Lagrange equation of the total free energy is numerically solved by using the finite difference method together with the relaxation method and Maxwell construction to select the physical solutions and therefore investigate the effects of different anchoring strengths on the Frederiks transition threshold field. Maxwell construction method is employed to select three periodic solutions for nematic liquid crystal director at the interfaces of a slab. In the interval from zero to half-π, there is only one solution for the director orientation. In this way, NLC director rotates toward the normal to the surface as the applied electric field increases at the walls. Our numerical results illustrate that above Frederiks transition and in the intermediate anchoring strength, nematic molecules illustrate the different orientation at slab boundaries. We also study the effects of different anchoring strengths, nanoparticle volume fractions and polarizations on the Frederiks transition threshold field. We report that decreasing in the nanoparticle polarization results in the saturation Frederiks threshold. However, this situation does not happen for the nanoparticles volume fraction.