Cristina Cîrtoaje

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.

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.