RESUMO
We theoretically study how the measurements of the flexoelectric coefficients in nematic liquid crystals are affected by the inclusion of the director-density coupling energy. It is shown that this investigation is quite relevant for interpreting the data of experiments.
RESUMO
The director-density coupling theory was formulated with two parameters (u(1) and u(2)) to explain the acousto-optic effect in nematic liquid crystals. The assumption that the director is not able to accompany rapid oscillations of the sound wave, so that it actually couples to the time-averaged interaction, renders it effectively a u(1)-independent theory. In this paper, we investigate a route in which the time average is postponed to the end of the calculation. This approach allows us to derive measurable quantities that depend on both u(1) and u(2).
Assuntos
Algoritmos , Ondas de Choque de Alta Energia , Cristais Líquidos/química , Cristais Líquidos/efeitos da radiação , Modelos Químicos , Modelos Moleculares , Simulação por Computador , Espalhamento de RadiaçãoRESUMO
The director-density coupling theory has been proposed to describe the acousto-optic effect in nematic liquid crystals. On the basis of this theory, we make predictions for an experimental test using two superimposed ultrasonic waves. As a result of the analysis, low-frequency oscillations of the director and optical transparency are predicted to occur around their mean values. The possibility of either verifying or invalidating this prediction is important in order to distinguish the theory from other competitive ones.
RESUMO
Experiments with nematic liquid crystals have proved that an ultrasonic wave exerts a torque on the liquid-crystal molecules, causing a change in its optical properties (acousto-optic effect). In this work we report a theoretical study on the theory proposed by Selinger et al. [Phys. Rev. E 66, 051708 (2002).] and, independently, by Boneto et al. [Chem. Phys. Lett. 361, 237 (2002).] for this effect. We solved exactly the Euler-Lagrange equation, which determines the equilibrium configuration of the director profile. The liquid-crystal director is also calculated in powers of the acoustic intensity and a comparison of this expansion with the solution in a closed form is given. We show the existence of minimizers that does not satisfy the Euler-Lagrange equation and report the possibility of observing a Fréedericksz-type transition. Finally, a possibility of controlling light by ultrasonic wave is also discussed in the limit of low acoustic intensity.
RESUMO
The control of the light trajectories in a liquid crystal is studied through a theoretical model based in the variation of the molecular orientation of a ferronematic material. In this model, the director field is mapped into a Riemannian space where the light paths are obtained numerically through the calculation of the geodesics for the effective geometry perceived by light.
RESUMO
We have investigated the rate of substitution of advantageous mutations in populations of haploid organisms where the rate of recombination can be controlled. We have verified that in all the situations recombination speeds up adaptation through recombination of beneficial mutations from distinct lineages in a single individual, and so reducing the intensity of clonal interference. The advantage of sex for adaptation is even stronger when deleterious mutations occur since now recombination can also restore genetic background free of deleterious mutations. However, our simulation results demonstrate that evidence of clonal interference, as increased mean selective effect of fixed mutations and reduced likelihood of fixation of small-effect mutations, are also present in sexual populations. What we see is that this evidence is delayed when compared to asexual populations.