I. N. de Oliveira

Effects of UV irradiation on the wettability of chitosan films containing dansyl derivatives

The morphological and wetting properties of chitosan films containing dansyl derivatives have been investigated. By means of dynamic contact angle measurements, we study the modification of surface properties of chitosan-based films due to UV irradiation. The results were analyzed in the light of the molecular-kinetic theory which describes the wetting phenomena in terms of the statistical dynamics for the displacement of liquid molecules in a solid substrate. Our results show that the immobilization of dansyl groups in the chitosan backbone leads to a pronounced enhancement of the UV sensitivity of polymeric films.

Reflection chromaticity of multilayered structures incorporating cholesteric liquid crystals

We investigate the chromaticity of the light reflected by multilayered structures containing cholesteric liquid crystals (ChLCs). We considered a single-pitched ChLC multilayered system with quasiperiodic Fibonaccian phase defects as well as an alternate sequence of single-pitched ChLC and isotropic dielectric layers. Using the Berreman 4×4 matrix formalism, we numerically obtain the reflection spectrum and the chromaticity diagram of these structures. The associated chromaticity is shown to depend on the incidence angle. However, for the ChLC Fibonaccian phase defect system, the color shift can be controlled by the defect angle α. For the F4 generation and α=π/3, the chromaticity of the reflected light remains close to the white color region for a wide range of incidence angles, followed by a blueshift at intermediate incidence angles. This is the setup that produces the nearest omnidirectional red-green-blue condition for the reflection spectrum. On the other hand, the structure with alternate ChLC and ...

A molecular dynamics study of ferroelectric nanoparticles immersed in a nematic liquid crystal

A large number of interesting phenomena related to the insertion of colloidal particles in liquid crystals (LC) have recently been reported. Here, we investigate effects caused by the addition of spherically shaped ferroelectric nanoparticles to a nematic liquid crystal. Using molecular dynamics (MD) simulations, the density of LC molecules, the orientational order parameter, and the polar and azimuthal angle profiles are calculated as functions of the distance to the center of the immersed nanoparticle for different temperatures of the system. We observe that the assembly of ferroelectric nanoparticles enhances the nematic order in the LC medium changing many properties of its host above the nematic-isotropic transition temperature T*NI .

Bosons with multifractal energy spectrum: specific heat log periodicity and Bose–Einstein condensation

The thermodynamics for systems of non-interacting bosons with multifractal energy spectrum is considered. The critical attractors of one-dimensional generalized logistic and circular maps are used to generate multifractal bounded spectra with well defined scaling exponents. The specific heat is then calculated for both cases of conserved and non-conserved particle number, showing a power-law behaviour which is modulated by log-periodic oscillations when the energy spectrum is not dense. The occurrence of Bose?Einstein condensation for systems with conserved particle number, at which the specific heat is discontinuous, is also analyzed.

Synthesis and spectroscopic characterization of a fluorescent pyrrole derivative containing electron acceptor and donor groups.

The synthesis and fluorescence characterization of a new pyrrole derivative (PyPDG) containing the electron donor-acceptor dansyl substituent is reported. The effects of temperature and solvent polarity on the steady-state fluorescence of this compound are investigated. Our results show that PyPDG exhibits desirable fluorescent properties which makes it a promising candidate to be used as the photoactive material in optical thermometry and thermography applications. Further, the electrochemical and emission properties of polymeric films obtained from the oxidation polymerization of PyPDG are also analyzed.

Defect structures in nematic liquid crystal shells of different shapes

ABSTRACT The present review is devoted to recent studies of structures with topological defects on nematic liquid crystal (NLC) shells of different shapes and to the possibility of using external electric fields to manipulate these structures. It is shown that principal curvatures of bounding surfaces have a strong effect on the number and arrangement of these defects on NLC shells. It is also shown that external electric fields can be used to control the number of valence centers occurring on these shells, thus unveiling their capability of acting as multivalent building blocks for nanophotonic devices.

Polarization rotation of localized modes in magneto-photonic Fibonacci structures containing nematic layers

ABSTRACT In this study, we investigate the spectral characteristics of the oblique incident light transmitted by a multilayered structure composed of a quasi-periodic Fibonaccian sequence of nematic and magnetic layers. Using the Berreman 4 × 4 matrix formalism, we numerically obtain the transmission spectrum and the polarization rotation angle of the system as a function of the incidence angle. Our results reveal that the mismatch between the wavelengths of the localized transmitted mode within the band gap and the mode with pronounced polarization rotation angle is reduced when one departs from the normal incidence condition. Further, we report the dependence of the band gap width and the spectral characteristics of the transmitted mode as a function of the incidence angle. The relevance of the present finding to the development of new magneto-optical devices is discussed.

Tunable topological valence in nematic shells on spherocylindrical colloidal particles

We perform molecular dynamics simulations of the orientational ordering on nematic shells delimited by spherocylindrical nanoscopic colloidal particles. We show that under conditions of degenerate planar anchoring, the equilibrium director field structure in these shells exhibits pairs of +1/2 topological defects at the poles of spherical cups in the absence of an external electric field. In addition, a certain number of pairs of ±1/2 defects occurs on the spherical cups far from the poles, thus resulting in a total of eight valence spots. A strong field applied along the main spherocylindrical axis removes the ±1/2 defect pairs while it coalesces the polar ones into a single +1 topological defect. A strong transverse field destroys all defects on the spherical cups but generates four +1/2 defects in the cylindrical part. Therefore, an external field can be used to control the number of valence centers in spherocylindrical nematic shells, thus unveiling their capability of acting as multivalent building blocks for nanophotonic devices.

Suppression of Bose-Einstein condensation in one-dimensional scale-free random potentials

A perfect Bose gas can condensate in one dimension in the presence of a random potential due to the presence of Lifshitz tails in the one-particle density of states. Here, we show that scale-free correlations in the random potential suppress the disorder induced Bose-Einstein condensation BEC. Within a tight-binding approach, we consider free Bosons moving in a scale-free correlated random potential with spectral density decaying as 1 /k. The critical temperature for BEC is shown to vanish in chains with a binary nonstationary potential 1. On the other hand, a weaker suppression of BEC takes place in nonbinarized scale-free potentials. After a slightly increase in the stationary regime, the BEC transition temperature continuously decays as the spectral exponent →.

Electrically-controlled Faraday rotation and wavelength mismatch phenomena in a magnetophotonic structure containing nematic layers

In this study, we investigate the effects of the director reorientation on the spectral properties of a one-dimensional magnetophotonic crystal constituted by a sequence of magnetic and nematic layers, which exhibits a magneto-optical defect as the central layer. Using the Eidner-Oldano treatment to solve the Berreman equation, we numerically determine how the Faraday rotation and transmission spectrum of the photonic systems are affected by a field-induced reorientation in the nematic layers. Our results reveal that the transmittance spectrum of the magnetophotonic system is strongly sensitive to the director profile when the applied voltage is raised above the Freedericksz threshold. The director reorientation leads to the suppression of the typical wavelength mismatch between the Faraday rotation and light localization phenomena in one-dimensional photonic structures based on materials with different types of birefringence. Considering the polarization configurations of the propagation eigenmodes for a birefringent medium, the main mechanism behind mismatch suppression is analyzed, emphasizing the effects associated with the field-induced nematic reorientation. Our results show that the electric control of wavelength mismatch in magnetophotonic systems may be used as an efficient mechanism to develop a new class of optical switches and logic gate devices.In this study, we investigate the effects of the director reorientation on the spectral properties of a one-dimensional magnetophotonic crystal constituted by a sequence of magnetic and nematic layers, which exhibits a magneto-optical defect as the central layer. Using the Eidner-Oldano treatment to solve the Berreman equation, we numerically determine how the Faraday rotation and transmission spectrum of the photonic systems are affected by a field-induced reorientation in the nematic layers. Our results reveal that the transmittance spectrum of the magnetophotonic system is strongly sensitive to the director profile when the applied voltage is raised above the Freedericksz threshold. The director reorientation leads to the suppression of the typical wavelength mismatch between the Faraday rotation and light localization phenomena in one-dimensional photonic structures based on materials with different types of birefringence. Considering the polarization configurations of the propagation eigenmodes for a...