Biomedical subjects

A V Zakharov

Publications and source records attributed to A V Zakharov.

At least 19 recordsLinked to original sources

PubMed2006

Nematiclike viscosity coefficients of ferroelectric liquid crystals in their smectic-C* phase.

The five nematiclike viscosity coefficients corresponding to the Smectic-C* (SmC*) phase are calculated by a combination of the existing statistical-mechanical approach and NMR theory, based on a rotational diffusion model for two ferroelectric smectogens. The order parameter S2, the smectic tilt angle Theta, and the rotational diffusion coefficient D(perpendicular), corresponding to molecular tumbling in the SmC* phase, for the ferroelectric smectogens 4-[4'-(1-methylheptyloxy)]biphenyl'-(10-undecenyloxy)benzoate and (S)-[4-(2-methylbutyl)phenyl]-4'-n-octylbiphenylcarboxylate have been obtained by means of 2H NMR spectroscopy and these data have been used to calculate viscosity quantities. For practical purposes a specific form of the functional dependence of S4 on S2 is used. As the main result, our calculations also predict a laminar flow regime in a high shear flow for these liquid crystal compounds.

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PubMed2006

Monolayer alignment on azobenzene surfaces during UV light irradiation: analysis of optical polarized absorption measurement results and theoretical treatment.

The influence of the charge separation during the trans-cis conformational change on the surface of azobenzene 6Az10PVA monolayer on the polar liquid-crystal monolayer film, such as 4-n-pentyl-4'-cyanobiphenyl(5CB), is investigated. The effective anchoring energy (in the Rapini-Papolar form) is phenomenologically described in the framework of the molecular model, which takes into account the interaction between the surface polarization and surface electric field, for number of conformational states of the boundary surface. It is shown, using the experimental data for the voltage across the 6Az10PVA+5CB film, provided by the surface-potential technique, that the charge separation during the conformational changing, caused by the UV irradiation, may lead to changing of the surface alignment of liquid-crystalline molecules. The influence of the photoisomerization process on the orientational order parameter S2(t) using the optical polarized absorption measurement is also investigated.

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PubMed2005

Intrinsic torsional reorientations in a twisted nematic liquid crystal cell.

The nature of the orientational relaxation process of the director n to its equilibrium orientation neq, in the twisted nematic cell, under the influence of an external electric field, is investigated. The influence of the electric, elastic, and viscous torques on the dynamics of the director is reflected in the relaxation of the director n to neq, with different relaxation times. It is shown that the relaxation time, both for the cases of a strong and weak anchoring, exhibits an anomalous increase with decreasing of an external electric field, whereas the influence of the azimuthal anchoring energy, in the case of the twisted nematic cell is characterized by a weak effect. It is also shown that these torques exerted on the director may excite the traveling wave spreading from one edge of the cell to their second edge. Calculations of the relaxation processes in the vicinity of a nematic-smectic- A (NA) phase transition temperature T(NA) , e.g., at a few tens of mK from T(NA) in the nematic phase, shows that the director distortion in the gap between two plates is maintained to be constant across the sample both in the case of a strong and weak anchoring.

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PubMed2005

Effect of aerosil dispersions on the nematic-to-isotropic interface.

The effect of silica aerosils on the kinetics of the first-order nematic-isotropic (NI) phase transition is phenomenologically described in the framework of the time-dependent Landau-Ginzburg equation. A steady-state solution to the equation is presented such that the NI interface may propagate with a solitary-like wave profile under constant quenching. The results provide a plausible basis for the interpretation of the dynamical effects of quenched disorder in the liquid-crystal systems, caused by randomly interconnected porous media, such as aerosils. In the low silica aerosil rho(s) ( < or =0.1 g/cm;3) regime, the calculated values of the interface velocity v(T,rho(s)), the interface thickness kappa(T,rho(s)), and the critical radius of a spherical nucleus of new nematic phase in a bulk isotropic environment, composed of polar molecules, such as 4-n-octyl- 4(')- cyanobiphenyl and 4-n-heptyl- 4(')- cyanobiphenyl shows that the effect of silica aerosils on the kinetics is reflected in a shifting of the set of temperature-dependent curves to lower temperature values.-1.

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PubMed2005

[Glial tumors of the brain: classification, present-day aspects of immunopathogenesis and immunogenetic diagnostics].

Glial tumors of the brain present a complicated problem in neurosurgery due to the peculiarities of their growth biology, their prevalence, infiltrative growth and diagnostic difficulties. The complexity of treatment, which includes surgical removal of a tumor, radiotherapy, chemotherapy, and immune correction, makes the problem even more topical. Modem classification of glial neoplasms is of no less importance, because treatment depends on the histologic structure of a tumor. Thereupon, the authors of the article pay special attention to the modem WHO classification of gliomas. As immune diagnostics and treatment are indispensable in modern neurooncology, application of neurospecific oncologic markers is being widely introduced into clinical and scientific practice in many scientific centers. However, there are no objective tests to verify the diagnosis of astrocytic glioma with reliable accuracy. Hereditary syndromes associated with glial tumors have been distinguished and are being studied. The most interesting and promising direction is research into genetic anomalies in patients with glial processes.

Antigens, CD↗

PubMed2004

Relaxation processes in Langmuir films under lateral compression.

The orientational relaxation process of the director n to its equilibrium orientation n(eq), in the Langmuir film, during the lateral compression in absence of flow, is investigated. The relaxation time, during compression of 4-n-pentyl- 4(') -cyanobiphenyl monolayer (multilayer) films on the water surface, using the Ericksen-Leslie theory, has been calculated for a number of dynamic regimes. It is also shown that the viscous and electric forces exerted per unit volume of the monolayer Langmuir film may excite the solitary wave propagating along the air-water interface.

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PubMed2004

Pretransitional anomalies in the shear flow near a second-order nematic-smectic- a phase change.

Pretransitional anomalies in a shear flow near a second order nematic-smectic- A ( NA ) phase transition temperature T(NA), for liquid crystals (LC's), taking into account the fluctuations of the local smectic order parameter above the T(NA), are investigated. It is shown that the tumbling instability of the Couette shear flow for polar LC compounds, such as 4-n-octyl- 4(') -cyanobiphenyl (8CB), in the vicinity of T(NA), e.g., at a few tens of mK from T(NA) in the nematic phase, occurs at any shear rate gamma;.

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PubMed2004

Effect of silica aerosils on the nematic to isotropic transition: A theoretical treatment.

The effect of silica aerosils on a first-order nematic-isotropic (NI) transition temperature shift DeltaT(NI)(rho(s)) and on the enthalpy jump DeltaH(rho(s)) is phenomenologically described in the framework of the Landau-de Gennes theory. It is shown that the aerosil network dispersed in the liquid crystal phase creates an additional curvature perturbation, resulting in linear and nonlinear slopes both in the transition temperature shift DeltaT(NI)(rho(s))=-T(NI)(0)alpharho(s) and in the enthalpy jump DeltaH(rho(s))=1 / 4DeltaH(0)(1+sqrt[1-kapparho(s)])(2)(1-alpharho(s)), where rho(s) is the aerosil density, and both alpha and kappa are constants of the theory. In the low rho(s)(</=0.2 g/cm(3)) regime, both the calculated and the measured values of DeltaH(rho(s)) for polar liquid crystals, such as 4-n-octyl-4(')-cyanobiphenyl (8CB) and 4-n-heptyl-4(')-cyanobiphenyl (7CB) show a good agreement between theory and experiment. The comparisons between the calculated and measured values of DeltaT(NI)(rho(s)), in the low-rho(s) regime, show a better agreement in the 7CB+aerosil than in the 8CB+aerosil systems.

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PubMed2003

Orientational relaxation phenomena in Langmuir-Blodgett films at the air-water interface.

The nature of the orientational relaxation process of the director n to its equilibrium orientation n(eq), in the mono-(multi)layer(s) Langmuir-Blodgett film, during the lateral compression in absence of flow, is investigated. The relaxation time, during compression of 4-n-pentyl-4(')-cyanobiphenyl mono-(multi)layer(s) film on the water surface, using the Ericksen-Leslie theory, has been calculated for the number of dynamic regimes.

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PubMed2002

Monolayers at the air-water interface: Maxwell displacement current and optical second-harmonic generation studies and theoretical treatment.

The structural properties and the surface pressure pi-A isothermal diagram have been investigated during compression of the 4-n-pentyl-4'-cyanobiphenyl (5CB) monolayer film on the water surface, both experimentally, using both the Maxwell displacement current (MDC) and optical second-harmonic generation (SHG) techniques, and theoretically, in the framework of the molecular model, assuming that the randomly tilted polar 5CB molecules on the water surface are replaced by the collective tilted array of unit vectors. The average angle is evaluated based on the both MDC and SHG techniques. The reasonable agreement between the calculated and experimental values of pi(A) was obtained.

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PubMed2002

Flow alignment in a shearing nematic liquid crystal near a charged surface.

The flow alignment angle theta(eff)(y) and effective rotational viscosity coefficient gamma(eff)1 are investigated for polar liquid crystals (LCs), such as 4-n-octyloxy-4'-cyanobiphenyl (8OCB), in the vicinity of a charged bounding surface. gamma(eff)1 and theta(eff)(y) are calculated, for the stationary regime, in the framework of the conventional Ericksen-Leslie theory. Calculations of gamma(eff)1, for the homeotropic alignment of 8OCB molecules, at a charged indium tin oxide-coated glass plate show an increase in gamma(eff)1 up to 7.8%.

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PubMed2002

Surface-induced smectic-ordering effect in thin nematic liquid-crystal cell.

The influence of the surface-induced smectic A (SmA) phase on the threshold field, for homeotropic alignment of polar liquid crystals is discussed from the energy point of view. It is shown that the Freedericksz critical field, for 4-n-octyl-4'-cyanobiphenyl (8CB), in a very thin cell near the second-order nematic-SmA phase transition, exhibits an anomalous increase with decreasing of temperature.

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PubMed2001

Surface polarization and effective anchoring energy in liquid crystals.

The influence of the surface polarization (SP) on the effective anchoring energy for both homeotropic and planar alignments of polar liquid crystals at a solid substrate is discussed from the energy point of view. It is shown that in a certain range of surface charge density the destabilizing SP mechanism may lead to destruction of the homeotropic or planar alignment of liquid crystalline molecules, such as 4-n-pentyl-4(')-cyanobiphenyl.

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PubMed2001

Dielectric and elastic properties of liquid crystals.

The structural properties, the static and relaxation dielectric coefficients [epsilon(j) and epsilon(j)(omega) (j= ||, perpendicular)], the rotational diffusion constants D( perpendicular) and D( ||), the orientational correlation times tau(1)(i0) (i=0,1), and the bulk elastic constants K(i) (i=1,2,3) are investigated for polar liquid crystals, such as 4-n-pentyl-4(')-cyanobiphenyl (5CB). epsilon(j) are calculated by a combination of the existing molecular theory and statistical-mechanical approach (SMA) that takes into account translational and orientational correlations as well as their coupling, whereas epsilon(j)(omega) are calculated by combining SMA and nuclear magnetic resonance relaxation theory, both based on a rotational diffusion model in which the reorientation of an individual molecule is assumed as stochastic Brownian motion in a potential of mean torque. Reasonable agreement between the calculated and experimental values of epsilon(j) and epsilon(j)(omega) for 5CB is obtained. The bulk Frank elastic constants K(i) (i=1,2,3), for splay, twist, and bend distortion modes, as well as their ratios K(3)/K(1) and K(2)/K(1) are also obtained.

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PubMed2000

Rotational viscosity, dynamic phenomena, and dielectric properties in a long-chain liquid crystal: NMR study and theoretical treatment.

The rotational diffusion constants D(perpendicular) and D(parallel), rotational viscosity coefficients gamma(i) (i=1,2), the orientational correlation times tau(L)mn, and the dielectric permittivities for nematic liquid crystals (NLCs) are investigated. gamma(i) are calculated by a combination of existing statistical-mechanical approach (SMA) and NMR relaxation theory, both based on a rotational diffusion model. In the rotational diffusion model, it is assumed that the reorientation of an individual molecule is a stochastic Brownian motion in a certain potential of mean torque. According to the SMA, gamma(i) are found to be a function of temperature, density, rotational diffusion constant for tumbling motions, and the orientational order parameters. The order parameters and rotational diffusion constant are obtained from an analysis of NMR measurements. Reasonable agreement between the calculated and experimental values of gamma(i) for 4-n-octyloxy-4'-cyanobiphenyl (8OCB) is obtained. The orientational correlation times, and the longitudinal and transverse components of the real chi'(j)(omega) and imaginary chi"(j)(omega) (j= parallel, perpendicular) parts of the complex susceptibility tensor for 8OCB molecules in the nematic phase are also obtained.

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PubMed1999

Rotational viscosity in a nematic liquid crystal: a theoretical treatment and molecular dynamics simulation.

The rotational viscosity coefficient gamma(1) of 4-n-pentyl-4(')-cyanobiphenyl in the nematic phase is investigated by combination of existing statistical-mechanical approaches (SMAs), based on a rotational diffusion model and computer simulation technique. The SMAs rest on a model in which it is assumed that the reorientation of an individual molecule is a stochastic Brownian motion in a certain potential of mean torque. According to the SMAs, gamma(1) is found to be a function of temperature, density, rotational diffusion coefficient, and a number of order parameters (OPs). The diffusion coefficient and the OPs were obtained from an analysis of a trajectory generated in a molecular dynamics simulation using realistic atom-atom interactions. In addition, a set of experimentally determined diffusion coefficients and OPs was used for evaluation of gamma(1). Reasonable agreement between calculated and experimental values of gamma(1) is obtained. It is shown that near the clearing point gamma(1) is proportional to (-)P22, where (-)P2 is the second-rank OP. This limiting value of gamma(1) is in agreement with mean-field theory.

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PubMed1999

Statistical-mechanical study of the pair correlations for the dipolar Gay-Berne model.

A statistical-mechanical theory based upon the method of conditional distribution function has been applied to calculations of the nearest-neighbor (NN) and next NN correlators as well as order parameters. The method takes into account translational and orientational correlations as well as their coupling. Using the dipolar Gay-Berne interaction potential, calculations have been carried out for temperatures and densities corresponding to a nematic phase. The results in orientational distribution functions indicate a significant effect of the dipole-dipole interactions on the orientational order of the liquid crystal. It was found that NN particles tend to be mutually antiparallel, whereas a reversed tendency is observed for next NN particles. These results are in agreement with computer simulations and with interpretations of experimental data.

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PubMed1992

[The assessment of body functional systems by the indices of their nonspecific reactions].

The analysis of long standing researches in the military operators' procedure has shown some regularities in the changes of indices which were examined. Varying in time and magnitude the non-specific reactions at first manifest itself in unstable vegetative regulations, then proliferate its influence upon visual and motor coordination, and finally cause negative shifts in professional activities (worsening of accuracy and precision, appearance of errors, etc). It is obvious, that specific reactions impose their own features on the development of this program without changing its basic trend.

Electrocardiography↗