Resumo em Inglês:

Liquid crystals have been very fruitful systems to study equilibrium phase transitions. Recently, they have become an important system to study dynamics of first-order phase transitions. The moving nonequilibrium nematic-isotropic interface is a model system to study growth of stable states into metastable states and displays a myriad of dynamical instabilities that, far from equilibrium, drive the system to a scenario of spatio-temporal chaos. We present a mean-field theory for the time evolution of a planar nonequilibrium nematic-isotropic interface for pure liquid crystals using a time dependent Ginzburg-Landau equation, which is one of the simplest approaches to dissipative dynamics. We obtain a theoretical expression for the growth kinetics of the nematic phase into a metastable isotropic phase and compare it with our experimental results. In a directional solidification arrangement we study instabilities of the nematic-isotropic interface of the liquid crystal 8CB doped with water and hexachloroethane. The observed instabilities are similar to cellular instabilities that appear during growth of crystal-melt interfaces of binary mixtures. We then compare our results with known theories of morphological instabilities during crystal growth.

Resumo em Inglês:

In a brief review of the theory of pseudo-Casimir interaction in nematic liquid crystals, we describe long- and short-range fluctuation-induced forces in these materials and discuss the role of the anchoring strength in this context. We also analyze the pretransitional behavior of the interaction in the vicinity of the clearing point.

Resumo em Inglês:

Nuclear Quadrupole Resonance studies in Nematic and Smectic C thermotropic liquid crystalline mesophases of p-heptyl azoxyanizole are reported. The local molecular microstructure is modeled from NQR spectra. In the Smectic C mesophase, the NQR data is explained by assuming two features: i) biaxiality in the electric field gradients at the sites occupied by 14N nuclei, and ii) the elemental unit cell of the mesophase is composed by two HpAb molecules. These bimolecular units coexist together with single molecules and the relative proportions change with temperature, the quantity of individual molecules and the relative proportions change with temperature, the quantity of individual molecules being predominant as the temperature approaches the Nematic phase transition. At the lowest temperatures in the Nematic mesophase the NQR spectrum behaves similarly to the corresponding at low temperatures in the SmC mesophase; this means that we can propose again the existence of a huge number of bimolecular and biaxial unit cells. At higher temperatures the transition to single molecular units is deduced from NQR spectra. That transition is characterized by the one describing the passage from a partially disordered molecular system -the system is conserving some local anisotropic properties of symmetry from the crystalline solid state- to a partially ordered system that resembles an oriented liquid.

Resumo em Inglês:

Matter diffusion experiments are carried out in hexagonal columnar phases of mesogenic disk-like and polycatenar compounds. The diffusion characteristics of two dyes of different sizes diffused into these mesophases are determined by measuring the diffusion coefficients along the columns and perpendicularly to them. The nature of diffusion versus the structure of these mesophases in discussed in comparison with previous results obtained in nematic and smectic systems.

Resumo em Inglês:

Liquid crystals are states of matter with symmetries intermediate between those of isotropic liquids and crystalline solids. The role of permanent dipoles in determining the stability of the nematic phase was discussed soon after the discovery of liquid crystals. Thought it is now well known that polar interactions are not essential for the stability of the mesophases, strong longitudinal dipoles give rise to many unusual phenomena like reentrant phases, polymorphism of smectic A and nematic liquid crystals, etc. The article gives a summary of these phenomena and phenomenological as well as molecular theories which have been developed to describe them.

Resumo em Inglês:

Nuclear magnetic resonance relaxation experiments with field cycling techniques proved to be a valuable tool for studying molecular motions in liquid crystals, allowing a very broad Larmor frequency variation, sufficient to separate the cooperative motions from the liquidlike molecular diffusion. In new experiments combining NMR field cycling with the Jeener Broekaert order-transfer pulse sequence, it is possible to measure the dipolar order relaxation time (T1D), in addition to the conventional Zeeman relaxation time (T1Z) in a frequency range of several decades. When applying this technique to nematic thermotropic liquid crystals, T1D showed to depend almost exclusively on the order fluctuation of the director mechanism in the whole frequency range. This unique characteristic of T1D makes dipolar order relaxation experiments specially useful for studying the frequency and temperature dependence of the spectral properties of the collective motions.

Resumo em Inglês:

In this paper we review and discuss the possibility of having a 2D-nematic ordering of the surfactant polar heads in bilayer membranes. The coupling between the nematic order parameters and the curvature can yield instabilities in the shape of the membrane, generally leading to tubules with mesoscopic radiuses. The nematic directors in the two monolayers should present coupled disclinations with peculiar characteristics. Para-nematic membranes, i.e., nematic membranes that are in the isotropic phase when flat, should behave as the nematic membranes in the curved state.

Resumo em Inglês:

Liquid crystals are very sensitive to surface effects. In fact, these effects are very useful in designing eletro-optical devices. We present a review of the theoretical models that describe the surface interactions in liquid crystals, focusing on lyotropic systems. Experimental results will be presented and compared to theoretical predictions.

Resumo em Inglês:

We present a review of our recent progress in the description of the elastic and dynamical properties of the unstable periodic (walls) arising in a nematic liquid crystal sample. The Lonberg criterion for the selection of the period of these walls is analyzed. We present a detailed study of the universal character of these walls, which is the basis for a law of corresponding states of these periodic structures. We show the connection of corresponding states with the constant ratio between the Miesowicz viscosity coefficients. Finally, the collapse of these structures is investigated. We show that it is due to the exponential growth of the fluctuations localized along the lines where the director does not bend at all.

Resumo em Inglês:

We describe applications of Thermal Lens Spectrometry to study optical and thermal properties of lyotropic liquid crystal. In particular, we refer to methods for measurements of thermal diffusivity anisotropy at different temperatures and as a function of the ferrofluids doping. The theoretical basis for quantitative measurements is discussed together with the advantages and limitations as compared with conventional methods. Finally, future developments of photothermal techniques for these applications are assessed.

Resumo em Inglês:

Self-avoiding chains on regular lattices are frequently used as models to study thermodynamic properties of linear polymers, particularly regarding critical phenomena which may occur in these systems. We review these models, concentrating on their properties under geometric constraints, when the walks are placed inside strips or pores. In these cases the models may be solved using a transfer matrix formalism for the generating functions. A short range interaction between the sites visited by the walks and the walls is included in the model and the distribution of the sites and the forces on the walls are studied as functions of the strength of this interaction.

Resumo em Inglês:

We present results of numerical simulations of the infrared spectra of thin films of behenic acid (b.a.) in different spectroscopic configurations. A model of independent oscillators was used to derive the optical constants of the material in the infrared region and, from these, the reflectance expressions corresponding to several experimental situations. In this manner, the theoretical attenuated total reflection spectra could be fitted to the experimental result through a steepest-descent routine, and the spectroscopic parameters so optmized were used in the numerical computation of the behavior of the optical constants and of the different spectra of interest. We present results for both Langmuir-Blodgett thin films and thicker evaporated samples of b.a. deposited on ZnSe crystals. In the former case, we also analyze the best suited experimental configuration for the thickness estimate of the films. We have shown that the preliminary use of numerical simulation techniques can simplify the investigation of the spectroscopic properties of thin organic films by allowing the operating limits and capabilities of each experiment to be estimated. Since fatty acid multilayers are the prototypal organized molecular structures, we expect that these techniques become specially important in the spectroscopic investigation of molecular order in thin organic films.

Resumo em Inglês:

Adsorption processes in layer-by-layer films are discussed using poly(o-methoxyaniline) (POMA) as a case study and also comparing with results for other polymers from the literature. This follows a brief overview of the materials and characterization techniques employed for self-assembled films, including their possible applications. The original paradigm of the self-assembly method is associated with spontaneous adsorption of oppositely charged polymer layers. While this rationale has been successful in explaining adsorption mechanisms for some polyelectrolytes, for polyanilines other interactions must be included. For POMA, in particular, at least three types of interactions are identified, namely van der Waals forces, ionic interactions and H-bonding. Furthermore, H-bonding is responsible for a number of effects even for charged POMA where electrostatic attraction was expected to predominate. Such effects include POMA dedoping upon contact with a glass substrate at early stages of adsorption, and the non-linear increase in the adsorbed amount with the number of POMA/poly(ethenesulfonic acid) (PVS) bilayers deposited in a multilayer structure. Adsorption of a POMA layer on a glass substrate or on an already formed POMA/PVS film occurs in two steps: a fast, first-order kinetics process with a characteristic time of a few seconds and a slower process represented by a Johnson-Mehl-Avrami function with a characteristic time of hundreds of seconds. These correspond basically to nucleation and growth mechanisms which is corroborated by atomic force microscopy measurements. The amount of material adsorbed in any given layer depends on experimental parameters, especially polymer concentration and pH, owing to the different extents of H-bonding that may allow POMA to adsorb on itself or on PVS molecules.

Resumo em Inglês:

Techniques based upon the electrical response of the quartz crystal microbalance (QCM) have been widely used in laboratories as a routine tools. In this article we present and discuss applications of the QCM (or its variant, the electrochemical quartz crystal microbalance, EQCM) to the viscoelastic characterization of films. It is pointed out that correlations between the motion of quartz crystal and contacting films and overlayers as well as the influence of the electronic circuit on the electric state of the whole system are of fundamental importance in interpreting the results.

Resumo em Inglês:

This paper presents a review of Electric Double Layered Magnetic Fluids (EDL-MF) based on spinel ferrite type [(M1-x+2Fex+3)]A [(Fe2-x+3 Mx+2)]BO4-2, with M(II) = Mn, Co, Ni, Cu and Zn. The chemical synthesis of the nanoparticles using hydrothermal techniques and their complete peptization in an aqueous medium results in a ultra stable magnetic colloid, EDL-MF. The characterization of the particles was performed using X-ray diffraction and electronic microscopy techniques. The resulted sols were investigated by magnetic, magneto-optical measurements and magnetic resonance techniques.

Resumo em Inglês:

Thermal diffusivity results obtained for different concentrations of the same ferrofluid are reported. The collinear mirage technique was used for its adequacy in obtaining the thermal properties of semitransparent materials with low thermal diffusivities. The results show that, as the ferrofluid concentration increases, the thermal diffusivity also increases, leading to a different trend compared to those reported for ferronematics. The different behaviors observed can be related to the different geometries of thermal waves propagating throught the material.

Resumo em Inglês:

Soil samples from the King George Island, Antarctica, have been studied by 57Fe Mossbauer spectroscopy, X-ray diffractometry, radiometry, neutron activation analysis and chemical analytical methods. X-ray diffractometry measurements have identified soils containing different volume ratios of quartz, feldspar, chlorite as well as hematite. The difference in the phase composition and in the iron distribution among the crystallographic sites of iron-bearing minerals (chlorite, magnetite and hematite) of samples from two different depths was derived from the complex Mössbauer spectra. The differences in the mineral composition, iron distribution, concentration of water soluble salts, pH and radioactivity of certain radionucliedes indicate the occurence of chemical weathering of minerals.

Resumo em Inglês:

An extended treatment of the one-dimensional q-harmonic oscillator, based on two examples of inequivalent representations of the Heisenberg quantum algebra which appeared recently in literature is presented. The dependence of several oscillator properties such as the energy spectrum, uncertainty relations and selection rules on the new parameters characterizing those generalized representation is also discussed.

Resumo em Inglês:

We investigate the learning of a rule from examples of the case of boolean perceptron. Previous studies of this problem have been made using the full quenched theory. We consider here two alternative approaches that can be applied easily. the two-replicas interactions approach considerably improves upon the well-known first-order approach. The mean field approach proved some results that have been obtained previously using the complex full quenched theory. Both approximations have been applied to both continuous weights and discrete weights perceptron.

Resumo em Inglês:

Squeezed Displaced Number States of the light were introduced in the recent literature. They exhibit various nonclassical properties as sub-Poissonian statistics, squeezing and oscillations in the photon-numer distribution. Here we investigate other properties of these fields, as waiting-time and photoelectron-counting distributions. We considerably simplify previous calculations in the literature while showing that these states constitute a unified approach for number, coherent, and squeezed states.

Resumo em Inglês:

In this paper we use the Ziglin-Yoshida method to discuss the determination of non-integrability domains for some classes of homogeneous hamiltonian systems. In particular, we demonstrate the non-integrability of Störmer problem through the reduction of the system to a two-dimensional homogeneous potential. We have also found the non-integrability domains of potentials of the form <img src="http:/img/fbpe/bjp/v28n4/image197.gif" alt="Image197.gif (1160 bytes)" align="top"> and <img src="http:/img/fbpe/bjp/v28n4/image199.gif" alt="Image199.gif (1022 bytes)" align="top">.

Resumo em Inglês:

The spin dynamics of two dimensional XY ferromagnet are reexamined at low temperatures in the framework of the Mori continued fraction formalism using the Gaussian approximation. In this formalism, the terms on denominators for the Laplace transform of the relaxation function R(t) are related to the frequency moments<img src="http:/img/fbpe/bjp/v28n4/sd6.gif" alt="sd6.gif (174 bytes)" align="absmiddle"> of the relaxation shape function R(q,w). In the Gaussian approximation scheme, we truncate the continued fraction for R(t) on the second stage. Adopting this approximation, we calculated up to the sixth moment. The moments are writing in terms the static spin correlation functions. In the estimation of the fourth and sixth moment at finite temperature, the four and six-spin correlation functions may be approximated by a sum of products of appropriate pair correlation functions(mode-mode decoupling). In this work we calculated the static spin correlation functions for the expressions to the fourth and sixth moments, needed in the study of the dynamics.

Resumo em Inglês:

The Duffin-Kemmer-Petiau (DKP) relativistic equation has been recently uded to study the interactions of spinless mesons with nuclei. In view of this interest and also the interest to determine the phase space picture for hadronic quantum theory, we present a derivation of the DKP equation of the generalized phase space proposed by Bohm and Hiley. Our development is based on the algebraic calculus introduced by Schönberg and uses the idea of algebraic spinors due to Riesz and Cartan. The free DKP particle and the more general case of the DKP particle in a prescribed external electromagnetic field are considered, and we obtain the DKP Liouville type equations for these cases.

Resumo em Inglês:

We present a tree level approach to the issue of the deflection of photons by the gravitational field of the Sun, treated as an external field, on the basis of R + R2 gravity. We show that the deflection angle of a photon grazing the surface of the Sun is exactly the same as that given by general relativity. An explanation for this strange coincidence is provided.

Resumo em Inglês:

Recent DESY-HERA data on J/<FONT FACE="Symbol">Y</font> elasticity distribution show that it emerges mostly as a fast particle. Interpreting photoproduction as a collision between a pre-formed charmed hadron and the proton, the outcoming J/<FONT FACE="Symbol">Y</font> is a leading particle of the collision. We analyse these data using a model formulated to describe energy flow in hadron-hadron reactions. The measured J/<FONT FACE="Symbol">Y</font> spectrum can be successfully described in terms of this model. We conclude that the observed transparency of the charmed hadron-proton collisions arises because of the particularly small gluonic content of the initial c - <img src="http:/img/fbpe/bjp/v28n4/image200.gif" alt="Image200.gif (847 bytes)" align="absbottom"> state.