Convection in anisotropic inclined porous layers

Two-dimensional convective flow in a system of fluid-saturated porous layers is considered for small fluid velocities. The layers are inclined at an angle to the horizontal and there is a temperature gradient across them. A mathematical model of the system is constructed and the resulting series solution is numerically summed. The unique features of this model include its ability to represent non-symmetric multi-layered systems and materials whose permeabilities are anisotropic.The model is applied to various physical configurations. System aspect ratios, system symmetry and permeablity anisotropies are found to influence the effect that layers of different materials have on the convective flow.     

An anchoring transition at surfaces with grafted liquid-crystalline chain molecules

The anchoring of nematic liquid crystals on surfaces with grafted liquid-crystalline chain molecules is studied by computer simulations and within a mean-field approach. The computer simulations show that a swollen layer of collectively tilted chains may induce untilted homeotropic (perpendicular) alignment in the nematic fluid. The results can be understood within a simple theoretical model. The anchoring on a layer of mutually attractive chains is determined by the structure of the interface between the layer of chain molecules and the solvent. It is controlled by an interplay between the attractive chain interactions, the translational entropy of the solvent and its elasticity. A second-order anchoring transition driven by the grafting density from tilted-to-homeotropic alignment is predicted. Received 5 July 2001 and Received in final form 16 November 2001     

黄藤细胞壁微纤丝取向偏振光拉曼光谱研究

采用532 nm共聚焦显微拉曼光谱技术原位状态下研究了黄藤藤茎纤维及导管细胞壁中纤维素微纤丝空间取向差异。在高数值孔径（NA=1.25）物镜测试条件下,C-H伸缩振动（2 771~3 000 cm-1）特征峰峰面积拉曼成像成功的区分出细胞角隅、复合胞间层以及次生壁。进一步发现纤维细胞次生壁呈宽窄交替的同心层状结构,而导管次生壁无明显的分层结构。采用平行于细胞径向壁的拉曼偏振激光进行光谱成像发现纤维细胞次生壁窄层纤维素C-O-C（1 097 cm-1）拉曼信号强度明显高于宽层,即窄层中微纤丝取向更加平行于入射激光偏振方向,与细胞轴夹角更大,而导管次生壁中微纤丝取向较为均一。细胞壁不同形态区域拉曼光谱分析发现纤维素C-O-C特征峰以及CH和CH₂特征峰的拉曼信号强度与入射激光的偏振方向存在明显的相关性。当入射偏振激光的电矢量方向从平行变化到垂直于微纤丝方向时,其糖苷键C-O-C非对称伸缩振动信号减弱,而CH和CH₂的取向在与入射偏振激光的电矢量方向垂直时,其拉曼信号强度相较于平行状态略微降低,表明纤维素特征峰中的糖苷键C-O-C的非对称伸缩振动比CH和CH₂伸缩振动对拉曼偏振光的方向改变更为敏感。比较纤维细胞宽层与窄层的拉曼光谱发现径向次生壁窄层1 097 cm-1处拉曼信号强度明显高于弦向次生壁窄层,而径向次生壁宽层的2 897 cm-1处拉曼信号强度低于弦向次生壁宽层。拉曼特征峰比值（I_{1 095}/I_{2 897}）可用来定性研究细胞壁微纤丝角,结果发现这一比值在导管次生壁、纤维细胞窄层和纤维细胞宽层中分别为1.32~1.10,0.92~0.55和0.42~0.33,表明导管次生壁具有最大的微纤丝角,纤维细胞窄层次之,宽层最小。该研究为解析藤材细胞壁骨架空间结构、化学成分分布以及微力学特性提供了新型的分析手段和重要的理论指导。     

Complete wetting in the three-dimensional transverse Ising model

We consider a three-dimensional Ising model in a transverse magnetic fieldh and a bulk fieldH. An interface is introduced by an appropriate choice of boundary conditions. At the point (H=0,h=0) spin configurations corresponding to different positions of the interface are degenerate. By studying the phase diagram near this multiphase point using quantum mechanical perturbation theory, we show that the quantum fluctuations, controlled byh, split the multiphase degeneracy giving rise to an infinite sequence of layering transitions.     

Azimuthal anchoring energy at the interface between a nematic liquid crystal and a PTFE substrate

The azimuthal anchoring energy of the nematic liquid crystal 4-pentyl-4'-cyanobiphenil (5CB) on a poly(tetrafluoroethylene) (teflon, PTFE) film is measured for the first time. The PTFE film is deposed using the Wittmann and Smith technique which consists on rubbing a bar of this polymer against a glass substrate at a controlled temperature and pressure. Measurements of the azimuthal anchoring energy are made with a reflectometric technique which provides high accuracy and sensitivity. The dependence of the azimuthal anchoring energy on temperature and on the rubbing pressure is investigated. The extrapolation length remains virtually constant in the whole temperature range of the nematic phase except for an increase of 25% close to the clearing temperature. The azimuthal anchoring energy is somewhat strong and increases with increasing the deposition pressure of PTFE. The observation of a relevant pre-transitional anisotropy of the reflection coefficients in the isotropic phase proves that the surface interactions favor an excess of orientational order. Ageing of the anchoring energy and gliding of the easy axis are experimentally observed. Both these phenomena suggest the presence of an anisotropic adsorbed layer of nematic molecules on the PTFE film.     

Universal non-monotonic smectic fluctuations of liquid crystal films in a magnetic field

Free-standing liquid crystal films with positive diamagnetic susceptibility can have the smectic ordering enhanced by an external field applied perpendicular to the plane layers. Within a quadratic functional integral approach, we investigate the interplay between the smectic order induced by an external field H and that due to the surface tension γ between the film and the surrounding gas. We find that the average smectic fluctuation is a non-monotonic function of film thickness, with a characteristic thickness scale ξ _H delimiting the predominance of surface tension and magnetic field effects. This characteristic thickness obeys simple asymptotic power-law relations as a function of the ordering terms which allows us to represent the average smectic fluctuations in a universal scaling form. Received 7 January 2003 Published online 1st April 2003 RID="a" ID="a"e-mail: marcelo@ising.fis.ufal.br     

基于BP网络和专家系统的铝电解槽分层故障诊断

针对铝电解槽故障种类繁多和不易诊断的问题,设计了基于BP网络和专家系统的分层故障诊断系统,包括前层分类和后层预报;通过对槽电阻信号的频谱分析,提取了故障特征信息,并对故障进行分类;建立了基于BP网络的前层分类器,用于诊断特征显著的故障;制定了故障诊断和控制规则,完善了专家系统的知识库,根据前层分类结果对余下故障进行诊断;通过制定规则,将前层分类和后层预报相结合,实现了故障诊断系统的整体设计;仿真结果及理论分析表明,该系统可有效预报单一及复合故障,提高故障诊断的准确率,保证铝电解槽工作状况的稳定。