We report on the development of a dilute suspension of ferroelectric particles in a nematic liquid-crystal (LC) host. We found that the submicron particles do not disturb the LC alignment and the suspension macroscopically appears similar to a pure LC with no readily apparent evidence of dissolved particles. The suspension possesses enhanced dielectric anisotropy, and is sensitive to the sign of an applied electric field.

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We investigated the physical properties of low concentration ferroelectric nematic colloids, using calorimetry, optical methods, infrared spectroscopy and capacitance studies. The resulting colloids normally remain homogeneous, but the nematic orientational coupling is significantly amplified. In particular cases, the nematic orientation coupling increases by 10% for particle concentrations of 0.2%. A manifestation of the increased orientational order is that the clearing temperature of a nematic colloid increases up to 40 Celsius degrees compared to the pure LC host. A theoretical model is proposed in which the ferroelectric particles induce local dipoles whose effective interaction is proportional to the square of the orientational order parameter.PACS numbers: 64.70. Md, 82.70.Dd, Colloids in which the solute is liquid crystalline (LC) are known to possess an extremely rich set of behaviors [1,2,3,4,5]. The anchoring between the LC and microcolloidal particles ( 1µm) can produce long-range orientational distortions around the particles. This results in strong inter-particle interactions -sometimes repulsive and sometimes attractive -in the mesophase. The interactions can give rise to well-ordered structures of particles in the liquid crystal matrix (both lattices and chains) [2,3]. However, in most cases a prerequisite for interesting LC colloidal behavior has been a high concentration dispersion, typically with particle volume fraction c part 30%. In such systems aggregated particles produce director distortions extending over macroscopic scales. These suspensions scatter light strongly, and possess unique structural, mechanical, electro-and magneto-optical properties [5,6].Recently, we have shown that even at low concentrations (c part 1%), LC colloids differ strongly from the pure host material [7,8,9,10]. These colloids consist of submicron ferroelectric particles suspended in the LC host. In these systems, unlike in classic LC colloids, the suspensionmatrix interaction is insufficient to disturb the LC orientation. This small concentration dramatically increases the dielectric anisotropy, significantly decreases the Freedericksz transition voltage, and significantly accelerates electric field-induced director reorientation.In this letter, we report results which show that these phenomena are general properties of liquid crystal suspensions containing ferroelectric colloidal nanoparticles. The addition of impurities normally decreases the nematic clearing temperature T N I [11]. However, our measurements show massive increases in T N I , of the order of 40 • C, for mass impurity concentrations of the order of 0.2%. These results imply an increase in the effective nematic interaction parameter. Measurements of the birefringence, dielectric anisotropy and order parameter in the suspension are consistent with this picture.We have also constructed a theoretical model. The ferroelectric particles in the suspension produce large electric fields in their neighborhood. These electric fields produce induce...

To cite this article: E. Ouskova , O. Buchnev , V. Reshetnyak , Yu. Reznikov & H. Kresse (2003) Dielectric relaxation spectroscopy of a nematic liquid crystal doped with ferroelectric Sn 2 P 2 S 6 nanoparticles, Liquid Crystals, 30:10, It was found that doping a nematic liquid crystal (LC) with a small amount of ferroelectric nanoparticles strongly affects the dielectric properties of the system. In particular, adding the ferroelectric particles results in a shift of the absorption bands corresponding to the rotation of liquid crystal molecules around their short axes to lower frequencies and in an increase of the amplitude and with of the absorption bands. This suggests that strong interactions occur between the LC molecules and the particles, caused by the large dipole moment and high polarizability of the ferro-particles. The ferroelectric particles affect not only dielectric losses, but also dielectric permittivity of the system. Specifically, the static dielectric permittivity and the dielectric anisotropy of the suspension are more than twice that of the pure LC.

An appearance of an easy orientation axis of a nematic liquid crystal on the isotropic aligning surface under the light absorption by in-bulk azo dye molecules is observed. It is shown that orientation of LC molecules in the irradiated region is close to the direction of the exciting beam polarization. Accompanying to aligning, the surface driven reorientation effect has been studied. A threshold as well as a nonthreshold reorientation of the director toward the light-induced easy axis were observed. The value of the light-induced anchoring energy depends on the exposure time and achieves 2.5×10-3 erg/cm2. The experimental data are explained within the framework of adsorption of phototransformed molecules from the LC bulk onto the aligning surface.

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This paper examines ferronematic switching in a homeotropic cell in the presence of a magnetic field normal to the cell plane. At low fields we find thresholdless switching of the nematic director, consistent with experimental data. At higher fields, there are three regimes, depending on the strength of the director-ferroparticle coupling. For low coupling, there is an inverse Frederiks effect: the nematic reorientation increases and then reduces, disappearing at a critical field. At intermediate coupling, the reorientation reduces at high fields but remains finite. For high coupling, however, the director switching saturates. There is a dimensionless temperature scale t involving the temperature, the mean nematic elastic constant, the colloidal density and the cell dimension. For low t, high magnetic fields can cause the ferroparticles to segregate. The segregation is coupled to the director distortion, and this can drive the inverse Frederiks transition first order, causing bistability for intermediate fields. These features are perturbed but not changed structurally by the effect of a small bias magnetic field (< 10 Oe) normal to the unperturbed director.