| | 1 | Author
| Heidrun Amm, Maren Grigutsch, Ralf Stannarius | Requires cookie* | | | Title
| Spatio-temporal Analysis of Electroconvection in Nematics  | | | | Abstract
| We investigate electroconvection in nematic liquid crystals by means of optical microscopy. Time resolved optical images are used to study the director dynamics. For the first time we present instant images in the dielectric regime. A numerical simulation of the optical transmission patterns is performed on the basis of Fermat's principle. In the instant images of dielectric rolls, the periodicity of the observed optical pattern is equal to the wavelength Ao of the convection rolls. The well known low contrast 'stationary' optical texture observed in conventional experiments results from time averaging of these instant images; its wavelength is Ao/2. | | | |
Reference
| Z. Naturforsch. 53a, 117—126 (1998); received December 10 1997 | | | |
Published
| 1998 | | | |
Keywords
| Liquid Crystals, Pattern Formation, Electrohydrodynamic Instability, Optical Microscopy | | | |
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| default:Reihe_A/53/ZNA-1998-53a-0117.pdf | | | | Identifier
| ZNA-1998-53a-0117 | | | | Volume
| 53 | |
| 2 | Author
| S. Urban, B. Gestblom, R. Dąbrowski, H. Kresse | Requires cookie* | | | Title
| Dielectric Studies of a Substance with Negative Dielectric Anisotropy | | | | Abstract
| The results of dielectric studies of 6BAP(F) (l-[4-(hexylbicyclo[2,2,2]octyl]-2-(3-fluoro-4-methoxy-phenyl)ethane) in the nematic and isotropic phases are presented. The substance has a negative dielec-tric anisotropy. By applying two experimental techniques, using a network analyzer and time domain spectrometer (TDS), the two main relaxation processes, connected with the molecular reorientation around the short and long axes, respectively, were resolved in the phases studied. The activation bar-riers hindering the motions were obtained. By extrapolation of the longitudinal and transverse relaxa-tion times from the isotropic to the nematic phase the retardation factors, and g x , and the nematic po-tential versus temperature could be calculated. These are discussed together with the order parameter S obtained from the refractive index, and are compared with the predictions of the mean-field theories. | | | |
Reference
| Z. Naturforsch. 53a, 134—140 (1998); received February 20 1998 | | | |
Published
| 1998 | | | |
Keywords
| Liquid Crystals, Dielectric Anisotropy, Dielectric Relaxation, Retardation Factors | | | |
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| default:Reihe_A/53/ZNA-1998-53a-0134.pdf | | | | Identifier
| ZNA-1998-53a-0134 | | | | Volume
| 53 | |
| 3 | Author
| A. Wiirflinger | Requires cookie* | | | Title
| Nematic Potential and Order Parameter Determined from Dielectric Measurements | | | | Abstract
| The Maier-Saupe theory is employed in order to calculate order parameters S from the nematic po-tential q. It is found that one of the basic assumptions of the Maier-Saupe theory, q ~S, is approximate-ly fulfilled. The relation between q and S is analysed for various state changes. Previously reported find-ings for 7 PCH that q ~S, not fulfilled along isochoric changes, can be explained by taking into account the pressure and temperature dependences of q. The procedure described in this paper allows to treat experimental data for the nematic potential in a unique way, without being affected by inadequacies of experimentally determined order parameters. | | | |
Reference
| Z. Naturforsch. 53a, 141—144 (1998); received February 11 1998 | | | |
Published
| 1998 | | | |
Keywords
| Liquid Crystals, High Pressure, Nematic Potential, Dielectric Relaxation | | | |
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| default:Reihe_A/53/ZNA-1998-53a-0141.pdf | | | | Identifier
| ZNA-1998-53a-0141 | | | | Volume
| 53 | |
| 4 | Author
| Lucia Calucci, Claudia Forte, Marco Geppi, CarloAlberto Veracini | Requires cookie* | | | Title
| Dynamics of Liquid Crystals by Means of 2 H-NMR: a Comparison between 4,4'-bis(hexyloxy)azoxybenzene and the Derivative Pd(II) Complex AZPAC | | | | Abstract
| In the present work we report a molecular dynamics investigation of the two mesogens 4,4'-bis(hexyloxy)azoxybenzene (HL) and its Pd(II) acetylacetonate derivative Azpac in their nematic phases. Deuterium Zeeman and quadrupolar spin-lattice relaxation times have been measured at 46.04 MHz on two isotopomers of Azpac (Azpac-d 4 and Azpac-c/ 26 > deuteriated on the aromatic core and on the alkoxy chains, respectively) and on HL-d 4 , an isotopomer of HL partially deuteri-ated on the aromatic core, by means of the Wimperis pulse sequence. The spectral densities obtained from the measured relaxation times are discussed in terms of internal and overall molecular motions. A small step rotational diffusion model for the overall molecular motions, superimposed on a free rotational model for internal motions, allowed diffusion coefficients for molecular spinning and tumbling and for phenyl ring rotations to be derived for HL; the same models were used in describing the dynamics of Azpac. | | | |
Reference
| Z. Naturforsch. 53a, 427—435 (1998); received October 31 1997 | | | |
Published
| 1998 | | | |
Keywords
| Liquid Crystals, Deuterium Relaxation, Molecular Dynamics, Metallomesogens, NMR | | | |
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| default:Reihe_A/53/ZNA-1998-53a-0427.pdf | | | | Identifier
| ZNA-1998-53a-0427 | | | | Volume
| 53 | |
| 5 | Author
| A. Würflinger, S. Urban | Requires cookie* | | | Title
| Relations between Nematic Potential and Order Parameters (P 2 ) and (P 4 > | | | | Abstract
| The Maier-Saupe theory is employed in order to calculate order parameters (P 2), (P 4) from the nema-tic potential q. The relation between <r=q/(RT) and S = (P 2) corresponds well with a recently establis-hed formula by Kalmykov. The relation between the order parameters is in accordance with the analy-tic expression (P 4)=5/l (P 2) 2 proposed by Zanonni, but deviates significantly from the Faber model, according to which log (P 4)=(10/3) log (P 2). Experimental results for 5 CB obtained from Raman mea-surements, however, are in better agreement with the Faber model. | | | |
Reference
| Z. Naturforsch. 53a, 883—886 (1998); received September 12 1998 | | | |
Published
| 1998 | | | |
Keywords
| Liquid Crystals, Nematic Potential, Order Parameters | | | |
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| default:Reihe_A/53/ZNA-1998-53a-0883.pdf | | | | Identifier
| ZNA-1998-53a-0883 | | | | Volume
| 53 | |
| 6 | Author
| A. Kapanowski, K. Sokalski | Requires cookie* | | | Title
| Elastic Constants of Binary Liquid Crystalline Mixtures | | | | Abstract
| Microscopic expressions for the elastic constants of binary liquid crystalline mixtures composed of short rigid uniaxial molecules are derived in the thermodynamic limit at small distorsions and a small density. Uniaxial and biaxial nematic phases are considered. The expressions involve the one-particle distribution functions and the potential energy of two-body short-range interactions. The theory is used to calculate the phase diagram of a mixture of rigid prolate and oblate molecules. The concentration dependence of the order parameters and the elastic constants are obtained. The possibility of phase separation is not investigated. | | | |
Reference
| Z. Naturforsch. 53a, 963—976 (1998); received September 24 1998 | | | |
Published
| 1998 | | | |
Keywords
| Liquid Crystals, Nematics, Elastic Constants, Mixtures | | | |
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| default:Reihe_A/53/ZNA-1998-53a-0963.pdf | | | | Identifier
| ZNA-1998-53a-0963 | | | | Volume
| 53 | |
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