Biomedical subjects

A L Alexe-Ionescu

Publications and source records attributed to A L Alexe-Ionescu.

6 recordsLinked to original sources

PubMed2005

Comment on "local elastic stability for nematic liquid crystals".

We discuss a theory recently proposed by Virga and co-workers for the analysis of the local elastic stability of nematic liquid crystals [R. Rosso, E. G. Virga, and S. Kralj, Phys. Rev. E. 70, 011710 (2004)]. The periodic instability in a nematic sample induced by the saddle-splay elastic constant or by the coupling of an external field with the flexoelectric polarization is reconsidered. We show that in the case in which the two surfaces limiting the sample are characterized by the same easy angle the analysis previously proposed by us coincides with the one proposed by Virga.

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PubMed2002

Periodic deformations in nematic liquid crystals.

We reconsider the possibility of periodic deformations in nematic liquid crystal samples, and present a simple method to analyze their stability near the threshold. Our method consists in finding the matrix characterizing the total energy in terms of the integration constants of the linearized solutions of the variational problem. In the undeformed state all the integration constants are identically zero. Hence the analysis of the stability of the undeformed state reduces to the analysis of the sign of the determinants of the principal minors of the matrix of the quadratic form representing the total energy of the nematic sample. We discuss the role of the saddle-splay elastic constant and of the anchoring energy strength in the stability of the modulated structure. The role of the thickness of the sample, as well as of the polar and azimuthal anchoring energies, in the phenomenon is also considered.

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PubMed2001

Statistical approach to the orienting photopolymer-nematic-liquid-crystal anchoring energy.

A statistical approach for the nematic order on an orienting photopolymer, also taking into account the surface anisotropy, has been formulated in the framework of mean field theory. This approach gives a Boltzmann-type orientational distribution function depending on both nematic-nematic and nematic-polymer interaction energies. The azimuthal anchoring energy coefficient has been evaluated from the extra Helmholtz free energy within an interface of thickness xi that may be interpreted as the length over which the density changes from pure polymer to pure liquid crystal or, generally speaking, as the typical length over which the interaction between polymer and liquid crystal takes place. In the case of surface anisotropy given by linearly polarized UV photopolymerization, the anchoring energy coefficient depends on the exposure time and on xi.

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PubMed2001

Liquid-crystal-electrochromic-material interface: a p-n-like electro-optic junction.

A liquid crystal (LC) cell has been made by using a standard sandwich configuration with one of the indium tin oxide (ITO) electrodes covered by a thin layer of tungsten trioxide (WO3). In this kind of cell optical polarization switching (observed under a crossed polarizer microscope) occurs for only one of the two polarities of an ac external applied electric field, while in the usual liquid crystal cells the electro-optic response does not depend on the sign of the field. The inhibiting switching configuration corresponds to anodic polarization of the tungsten trioxide film in which the deintercalation of cations occurs. Here we present the time behavior of charge and discharge for both the anodic and cathodic currents. A model based on charge carrier exchange between the ITO-WO3 and WO3-LC interfaces and also electrochemical processes is reported. Our model is also capable of explaining the electric and electro-optic asymmetric responses of the cell. Numerical calculations confirm the model.

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PubMed1999

Nematic ordering in a cell with modulated surface anchoring: effects of flexoelectricity.

We have analyzed molecular ordering in a nematic sample sandwiched between two parallel substrates, characterized by a periodically varying anchoring easy axis. If the periodicity lambda is smaller than the Debye screening length l(D) and the nematic material possesses flexoelectric properties, it is necessary to take into account also the electrostatic and flexoelectric contributions in the thermodynamical potential when the actual director field is determined. In this framework, for small deviations from the homeotropic alignment we have derived analytical expressions for the tilt angle (theta) and the electrical potential. To establish a connection with experimentally observable quantities, we have related the theta profile to the average and investigated its behavior for different values of lambda, the flexoelectric coefficient, and the anchoring strength w. Our results indicate that in a nematic with pronounced flexoelectric properties for small enough lambda, a kind of subsurface deformation appears, which substantially decreases . Therefore, effects of flexoelectricity cannot be neglected in treating nematic cells with modulated anchoring which allows bistable ordering.

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PubMed1999

Surface breaking in lyotropic nematic liquid crystals induced by a magnetic field.

The twist deformation induced by a magnetic field H in a lyotropic liquid crystal is analyzed. Our results show the existence of three regimes for the surface deformation related to two critical values for the external magnetic field. The first, for low magnetic field, is an elastic one in which the surface deformation disappears when the field is removed. In a second regime, a permanent deformation of the lyotropic liquid crystal is induced by the distorting field. This regime is characterized by a threshold for H, called H(*)(c). In the third regime, the sample is uniformly oriented along H. Also this regime has a well defined threshold value, H(**)(c), and the alignment remains even when the distorting field is removed. To interpret the experimental data recently published, we proposed a phenomenological model according to which there is an elastic coupling between the surface layer of thickness l and the bulk. This elastic coupling has a saturation for a given value of the deformation of the bulk with respect to the surface layer.

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