Gruhn-Hess两体势修正模型向列相液晶微滴的Monte Carlo模拟

基于Gruhn-Hess两体势修正模型,用Monte Carlo方法模拟向列相液晶微滴.Gruhn-Hess两体势是空间各向异性的,修正模型的势参数依赖于液晶的弹性常数K₁₁、K₂₂、K₃₃及表面弹性常数K₁₃.假设向列相液晶微滴具有自由表面,在低温下计算分子二阶序参数和切向内禀强度在微滴内不同区域的变化,并与Gruhn-Hess两体势模型一作对比分析.结果表明:附加K₁₃项的修正模型在微滴表面产生内禀锚定,其大小决定了内禀易取向方向及内禀锚定强度的大小,且内禀强度的大小与K₃₃/K₁₁值有关;空间不完整的向列相液晶微滴由内层到外层有序度逐渐降低.     

空间各向异性两体势向列相液晶形变研究

基于分子两体势研究向列相液晶的形变.该两体势是空间各向异性的并且依赖于液晶的弹性常数.理论处理中假定具有理想向列序，这意味着分子长轴取向方向与液晶指向矢是重合的，而总自由能等于总相互作用能.以解析形式研究了三种基本的Fréedericksz 转变并对混合排列向列相液晶盒中的指向矢分布进行了数值计算.检查了文献中最近提出的两种从弹性能到两体作用势的映射方案，发现只有一种方案给出的结果与连续体理论一致. 关键词： 空间各向异性两体势 理想向列序 液晶形变 Fréedericksz 转变     

基于修正的Gruhn-Hess两体势模型研究弯曲形变向列相液晶盒

使用修正的Gruhn-Hess空间各向异性两体势模型研究弯曲形变向列相液晶盒.为了易与弹性理论的结果进行比较,做单一弹性常数近似.在理想有序条件下,对模型进行连续化处理,截断到形变的二阶项,给出的自由能密度公式与含k₁₃项的弹性理论公式一致.使用模型直接研究强锚泊弯曲形变向列相盒,没有出现弹性理论给出的边界处不连续.通过Monte Carlo模拟研究发现,两体势中的k₁₃项将加剧弯曲盒中间层的涨落.     

向列相液晶中弱光引致各向异性衍射图样的研究

研究了弱线偏振光(≈0.16 W/cm²)通过垂直排列C₆₀掺杂的向列相液晶(5CB)薄膜的远场衍射图样.基于取向光折变机理,二波耦合使液晶分子进行二次取向之后,强度为高斯分布的光束通过样品时将形成高斯分布的空间电荷场,偏振光束通过样品时将产生偏振衍射图样.衍射图样的轮廓是同心圆环,在垂直于光的偏振方向有对称缺口.改变入射光的偏振方向可以看到衍射图样也随之改变, 有效非线性折射率系数n₂≈0.3cm²/W 关键词： 向列相液晶 取向光折变效应 自相位调制 衍射图样     

体心立方金属W薄膜晶体取向的膜厚尺寸效应及其表面映射

体心立方W膜(110)织构系数T₁₁₀的变化存在非单调的厚度尺寸效应，这依赖于薄膜中晶粒形核和长大时表面能和应变能的相互作用，薄膜表面结构演变反映了两者的竞争过程.应用小波变换结合分形几何描述薄膜表面结构各向异性行为，用此法构建了薄膜织构系数T₁₁₀与表面结构各向异性的关系，表明薄膜晶体取向存在表面映射. 关键词： 金属薄膜 晶体取向 膜厚 表面形貌     

Analytical investigation on mean and turbulent velocity fields of a plane jet

This paper analyses the downstream developments of the mean and the turbulent velocity fields of a plane jet. Based on the conservation of mass and the conservation of momentum, the mean-velocity half width (reflecting the jet spread rate) and the relative mass flow rate (jet entrainment) are related to the decay rate of the centreline mean velocity. These relations are not subject to self-preservation. Both analytical and experimental results suggest that the jet spread rate (K₁) and the entrainment rate (K₃) (and thus the decay rate K₂) can be well estimated from the centreline velocity, i.e., K₁ ≈ 0.6K₂ and K₃ ∝ K_2. The effect of initial mean velocity and RMS velocity profiles on the downstream mean velocity field appears to be embodied in the constants K₁ K₂ and K₃. The analytical relationship for the self-preserving Reynolds shear stress, obtained for the first time, works well.     

Tb0.3Dy0.7Fe2合金的本构参数辨识方法研究

建立了一种简便的、适用于磁畴模型应用的Tb_0.3Dy_0.7Fe₂ 合金本构参数辨识方法. 针对Tb_0.3Dy_0.7Fe₂合金磁畴模型中本构参数不明确且直接实验测试困难的问题, 提出了一种数值计算与实验测试相结合的参数辨识方法. 采用坐标变换与绘制自由能等势曲线相结合的方法, 简化了载荷作用下Tb_0.3Dy_0.7Fe₂ 合金内磁畴角度偏转的数值计算, 研究了合金磁畴角度偏转模型的参数依赖性. 在此基础上, 结合简单的实验测试, 建立了Tb_0.3Dy_0.7Fe₂合金各向异性常数K₁ 和K₂、能量分布因子ω、晶轴取向分布的辨识及修正方法. 该方法能够简单、快速地完成Tb_0.3Dy_0.7Fe₂ 合金磁畴模型中本构参数的辨识, 对完善磁致伸缩材料磁畴偏转的数值计算模型非常有意义. 理论分析可为类磁致伸缩材料磁机耦合模型的建立、完善, 以及材料本构参数的辨识、获取提供参考.     

Ne原子与HF分子碰撞振转激发分波截面的研究

采用单双取代并加入三重激发项校正的二次组态相互作用QCISD(T)方法, 以及aug-cc-pVTZ基组对Ne-HF分子间相互作用势进行了计算, 并考虑了Boys和Bernardi提出的均衡法, 在计算的基础上消除基组重叠误差. 计算得到了11个方向的Ne-HF碰撞系统的相互作用势能点数据, 使用Huxley势函数对数据点进行非线性最小二乘法拟合, 计算了各向异性势的径向系数V₀, V₁, V₂, V₃等, 其函数形式能够很好地描述Ne原子与HF分子碰撞系统的势能面; 采用密耦近似计算得到了Ne原子与HF分子碰撞系统不同能量下的总截面、 弹性分波截面和非弹性分波截面. 关键词： He-HF 碰撞 相互作用势 密耦近似     

聚酰亚胺膜上液晶表面锚定强度的研究

研究了取向剂聚酰亚胺上摩擦强度与液晶的表面锚定强度的关系,分析了液晶的表面锚定强度与液晶分子表面预倾角的关系,并运用空间相互作用模型及表面沟槽理论进行了对比分析,认为液晶分子的表面排列机理主要是聚合物与液晶分子的空间相互作用力。 关键词：     

表面摩擦处理对全息聚合物分散液晶光栅电光特性的影响

为了提高全息聚合物分散液晶光栅的衍射效率并降低其驱动电压,改善光栅的电光特性,研究了表面平行摩擦取向对全息聚合物分散液晶光栅电光特性的影响.理论分析认为,改善相分离结构和降低液晶微滴之间的有序度差异是优化光栅电光特性的根本所在.由于进行表面取向处理后的液晶和单体之间达到扩散匹配,使得相分离的程度大幅提高,在衍射能力增强的同时驱动电压也实现了大幅下降,而且,表面取向作用也使光栅内的液晶分子均匀排列,降低了液晶微滴之间的有序度差异,从而减少了光栅的散射损失.实验结果表明:进行取向处理后的光栅其衍射效率由传统光 关键词： 全息聚合物分散液晶 衍射效率 驱动电压     

Monte Carlo simulations of biaxial structure in thin hybrid nematic film based upon spatially anisotropic pair potential

Hybrid nematic films have been studied by Monte Carlo simulations using a lattice spin model, in which the pair potential is spatially anisotropic and dependent on elastic constants of liquid crystals. We confirm in the thin hybrid nematic film the existence of a biaxially nonbent structure and the structure transition from the biaxial to the bent-director structure, which is similar to the result obtained using the Lebwohl--Lasher model. However, the step-like director's profile, characteristic for the biaxial structure, is spatially asymmetric in the film because the pair potential leads to K₁ ≠ K₃. We estimate the upper cell thickness to be 69 spin layers, in which the biaxial structure can be found.     

Nematic order in nanoscopic liquid crystal droplets

We have used atomistic molecular-dynamics simulations to model the detailed molecular configuration of 5CB (4-n-pentyl-4'-cyanobiphenyl) molecules in the form of a nanoscopic liquid crystal droplet in a vacuum microgravity environment. We find the equilibrium state of droplets consisting of as few as 26 or 50 molecules to exhibit significant nematic ordering. The shape of the droplets is also anisotropic, but there is little angular correlation between the nematic director and the long axis of the droplet. Some tendency to micelle formation is observed in droplets of 50 molecules.     

Study of intrinsic anchoring in nematic liquid crystals based on modified Gruhn-Hess pair potential

A nematic liquid crystal slab composed of N molecular layers is investigated using a simple cubic lattice model, based upon the molecular pair potential which is spatially anisotropic and dependent on elastic constants of liquid crystals. A perfect nematic order is assumed in the theoretical treatment, which means the orientation of the molecular long axis coincides with the director of liquid crystal and the total free energy equals to the total interaction energy. We present a modified Gruhn-Hess model, which is relative to the splay-bend elastic constant K₁₃. Furthermore, we have studied the free nematic interfacial behavior (intrinsic anchoring) by this model in the assumption of the perfect nematic order. We find that the preferred orientation at the free interface and the intrinsic anchoring strength change with the value of modification, and that the director profile can be determined by the competition of the intrinsic anchoring with external forces present in the system. Also we simulate the intrinsic anchoring at different temperatures using Monte Carlo method and the simulation results show that the intrinsic anchoring favors planar alignment and the free interface is more disordered than the bulk.     

Water droplets in a spherically confined nematic solvent: A numerical investigation

Recently, it was observed that water droplets suspended in a nematic liquid crystal form linear chains [Poulin et al., Science 275, 1770 (1997)]. The chaining occurs, e.g., in a large nematic drop with homeotropic boundary conditions at all the surfaces. Between each pair of water droplets a point defect in the liquid crystalline order was found in accordance with topological constraints. This point defect causes a repulsion between the water droplets. In our numerical investigation we limit ourselves to a chain of two droplets. For such a complex geometry we use the method of finite elements to minimize the Frank free energy. We confirm an experimental observation that the distance d of the point defect from the surface of a water droplet scales with the radius r of the droplet like .When the water droplets are moved apart, we find that the point defect does not stay in the middle between the droplets, but rather forms a dipole with one of them. This confirms a theoretical model for the chaining. Analogies to a second order phase transition are drawn. We also find the dipole when one water droplet is suspended in a bipolar nematic drop with two boojums, i.e., surface defects at the outer boundary. Finally, we present a configuration where two droplets repel each other without a defect between them. Received 11 December 1998     

Molecular field theory for nematic liquid crystal film with finite layers

The nematic liquid crystal film composed of n molecular layers is studied based upon a spatially anisotropic pair potential, which reproduces approximately the elastic free energy density. On condition that the system has perfect nematic order, as in the Lebwohl—Lasher model, the director in the film is isotropic. The effect of the temperature is investigated by means of molecular field theory. Some new results are obtained. Firstly, symmetry breaking takes place when taking account of the temperature, and the state with the director along the normal of the film has the lowest free energy. Secondly, the N—I phase transition temperature increases as an effect of finite sizes instead of decreasing as in the Lebwohl—Lasher model. Thirdly, the nematic order is induced in the layers near the surface in the isotropic phase.     

Coherent transmission and angular structure of light scattering by monolayer films of polymer dispersed liquid crystals with inhomogeneous boundary conditions

We consider monolayer polymer films with oriented droplets of a nematic liquid crystal (LC). Relations for the coherent transmission coefficients of a layer of oriented ellipsoidal droplets and for the intensity of light scattered by monolayers of spherical and spheroidal droplets have been obtained. The amplitude-phase screen model and the interference approximation of the theory of multiple wave scattering have been used. To describe light scattering by an individual ellipsoidal droplet with inhomogeneous surface binding, we have developed an anomalous diffraction approximation. For monolayers of spherical LC droplets, the coherent scattering coefficients and the angular scattering structure have been analyzed. The internal structure of nematic droplets have been calculated by the relaxation method based on the solution of the minimization problem of the free energy volume density. We have studied basic regular features of light scattering by a monolayer with homogeneous and inhomogeneous boundary conditions at the LC-polymer interface. We show that, for films that contain droplets with inhomogeneous boundary conditions of the tangentially normal type, the angular structure of the scattered light is asymmetric with respect to the polar scattering angle.     

液态三元Fe-Sn-Si/Ge偏晶合金相分离过程的实验和模拟研究

本文采用自由落体实验技术和格子玻尔兹曼计算方法研究了低重力条件下液态Fe-Sn-Si/Ge合金的相分离过程. 实验发现, 二种合金液滴在自由下落过程中均发生显著的液相分离, 形成了壳核和弥散组织. 当Fe-Sn-Si合金中的Si元素被等量的Ge元素替换后, 壳核组织中富Fe区和富Sn区的分布次序会发生反转. 计算表明, 在液相分离过程中冷却速率、Marangoni对流和表面偏析对壳核构型的选择和弥散组织的形成起决定性作用.     

Highly ordered Ga nanodroplets on a GaAs surface formed by a focused ion beam

The morphological evolution of a GaAs surface induced by a focused ion beam (FIB) has been investigated by in situ electron microscopy. Under off-normal bombardment without sample rotation, Ga droplets with sizes from 70 to 25 nm in diameter on the GaAs surface can self-assemble into a highly ordered hexagonal pattern instead of Ostwald ripening or coalescence. The mechanism relies on a balance between anisotropic loss of atoms on the surface of droplets due to sputtering and an anisotropic supply of atoms on the substrate surface due to preferential sputtering of As. The ratio of wavelength to the droplet diameter predicted by this model is in excellent agreement with experimental observations.