Showing papers by "Martina Koneracka published in 2011"

Structuring from nanoparticles in oil-based ferrofluids

Abstract: The effect of magnetic field on the structure formation in an oil-based magnetic fluid with various concentrations of magnetite particles was studied. The evaluation of the experimental data obtained from small-angle X-ray scattering and ultrasonic attenuation indicates the formation of chain-like aggregates composed of magnetite particles. The experimental data obtained from ultrasonic spectroscopy fit well with the recent theoretical model by Shliomis, Mond and Morozov but only for a diluted magnetic fluid. In this model it is assumed that a dimer is the main building block of a B -field-induced chain-like structure, thus the estimation of the nematic order parameter does not depend on the actual length of the structure. The scattering method used reveals information about the aggregated structure size and relative changes in the degree of anisotropy in qualitative terms. The coupling constant [Formula: see text] , concentrations [Formula: see text] , average particle size d and its polydispersity [Formula: see text] were initially obtained using the vibrating sample magnetometry and these results were further confirmed by rheometry and scattering methods. Both the particles' orientational distribution and the nematic order parameter S were inferred from the ultrasonic measurements. The investigation of SAXS patterns reveals the orientation and sizes of aggregated structures under application of different magnetic-field strengths. In addition, the magnetic-field-dependent yield stress was measured, and a relationship between the yield stress and magnetic-field strength up to 0.5 T was established.

Phase Transitions in Liquid Crystal Doped with Magnetic Particles of Different Shapes

Abstract: In this study, observations of structural transitions in ferronematics based on the thermotropic nematic 4-trans-4′-n-hexyl-cyclohexyl-isothiocyanato-benzene (6CHBT) are described. Droplets of the nematic phase in the isotropic phase were observed in solutions of nematogenic 6CHBT dissolved in phenyl isocyanate and 6CHBT dissolved in phenyl isocyanate and doped with magnetic particles of different shapes (nanorods and chain-like particles). Magneto-dielectric measurements of structural transitions in these new systems enable to estimate of the type of anchoring of the nematic molecules on the magnetic particles surface.

The sensitivity of liquid crystal doped with functionalized carbon nanotubes to external magnetic fields

Abstract: Stable colloidal suspensions of single-walled carbon nanotubes (SWCNT) and magnetite-labeled SWCNT (SWCNT/Fe3O4) in thermotropic liquid crystal 4-(trans-4′-n-hexylcyclohexyl)isothiocyanatobenzene (6CHBT) were investigated. The presence of the magnetite in SWCNT/Fe3O4 product has been checked with infrared spectroscopy. Transmission electron microscopy has been employed to obtain the structural and dimensional data for the used samples. Magnetic properties of the samples have been investigated by a SQUID magnetometer and a significant increase of the saturation magnetization has been shown for magnetite labeled SWCNT. The structural changes in the liquid crystal doped with SWCNT or SWCNT/Fe3O4 in electric and magnetic fields were investigated. The density of anchoring energy at the nematic–magnetic particle boundary was determined and it was shown that the density of the anchoring energy for liquid crystal doped with SWCNT/Fe3O4 is higher than that for the liquid crystal doped with net SWCNT.

Magnetic-Field Induced Isotropic to Nematic Phase Transition in Ferronematics

Abstract: In this work the thermotropic liquid crystal 4-(trans-4'-n-hexylcyclohexyl)-isothiocyanato-benzene (6CHBT) was doped with differently shaped magnetic nanoparticles with the aim to increase the sensitivity of the liquid crystal on the external magnetic field. The volume concentration of the magnetic particles was 2×10-4. The phase transition temperature from isotropic to nematic phase in external magnetic fields up to 12 T was monitored by precise capacitance measurements in capacitance cells filled with the prepared ferronematic samples. The shift in the temperature from isotropic to nematic phase was observed in the liquid crystal doped with rodlike particles. To our knowledge, this is the first observation of such a magnetic-field induced transition in ferronematics based on a calamitic liquid crystal.