基于离散变分原理的耗散动力学模拟方法：模拟三维囊泡形状

将基于离散变分原理的耗散动力学模拟方法应用到三维囊泡体系,通过优化囊泡的弯曲能求解其平衡态形状. 该方法的优点之一是不需要预先假定对称性. 针对特定约化自发曲率的囊泡体系,该方法模拟获得了一系列轴对称形状,模拟结果与文献中预先假定轴对称条件的计算方法所报道的结果一致,这验证了该模拟方法的可靠性及精确性. 此外,使用该方法研究了两个差别巨大的平衡态形状之间的转变动力学,在转变过程中观察到了多个非轴对称的中间形状. 研究结果表明该方法不仅可以模拟囊泡的非轴对称结构,而且具备模拟囊泡在剧烈形变下演化过程的能力. 为研究更复杂的囊泡体系,特别是生物膜的形变提供了一个重要的理论模拟方法. 关键词： 生物膜 离散空间变分法 耗散动力学 三角网格划分     

流场环境下复杂囊泡的动力学行为

采用非平衡态分子动力学方法研究了二维复杂囊泡在剪切流中的动力学行为. 模拟发现了复杂囊泡经典的翻滚(tumbling)、摇摆(trembling)和坦克履(tank-treading)行为, 还观察到由坦克履行为向平动行为(translating)的转变. 囊泡的平动行为与剪切率大小、复杂囊泡的形状密切相关. 当大囊泡均匀嫁接较多数目的小囊泡后, 其平动方式消失. 该研究有益于加深对囊泡在剪切流场中复杂性行为的理解.     

二维情况下两组分带电囊泡形变耦合相分离的理论模拟研究

结合离散空间变分方法和耗散动力学研究了二维两组分带电囊泡的形变耦合相分离,系统地考察了囊泡带电量组分含量、带电组分的电荷密度、两组分间的相容性和温度等因素对形变耦合相分离动力学的影响.模拟结果表明电荷引入可增加不同组分间的表观相溶性.当温度较高时,静电相互作用可直接抑制囊泡相分离,避免了同种组分的团聚;当温度较低时,静电相互作用则可明显增加分相相区数目,使其呈微观相分离,从而避免了同种组分大范围的团聚.     

磷脂跨膜交换机制的研究

膜间磷脂交换是一项重要的生理活动, 其对药物运输及膜功能研究有重要意义. 本文用石英晶体微天平及耗散系数测试仪研究囊泡与囊泡、囊泡与支撑膜间磷脂交换行为, 荧光光谱仪用来测量膜表面电性与膜组分对磷脂交换的影响. 实验结果表明: 磷脂跨膜交换速率与交换时间成反比, 膜表面异电性磷脂的增加会加速膜内相互作用和磷脂跨膜交换速率, 以及改变膜表面组分会对囊泡与支撑膜间的磷脂交换产生影响. 本文研究有助于加深理解磷脂跨膜交换机制, 并对药学研究提供参考.     

立方非线性Schr?dinger方程的动力学行为分析

本文采用辛算法，研究了立方非线性Schr?dinger方程在不同的空间离散下的动力学行为，并讨论了其在不同的非线性参数下的动力学性质。 我们指出，系统的守恒量在长时间的演化下保持不变，但对于强非线性，不同的空间离散精度，将导致不同的动力学行为。     

磷脂跨膜交换机制的研究

膜间磷脂交换是一项重要的生理活动,其对药物运输及膜功能研究有重要意义.本文用石英晶体微天平及耗散系数测试仪研究囊泡与囊泡、囊泡与支撑膜间磷脂交换行为,荧光光谱仪用来测量膜表面电性与膜组分对磷脂交换的影响.实验结果表明:磷脂跨膜交换速率与交换时间成反比,膜表面异电性磷脂的增加会加速膜内相互作用和磷脂跨膜交换速率,以及改变膜表面组分会对囊泡与支撑膜间的磷脂交换产生影响.本文研究有助于加深理解磷脂跨膜交换机制,并对药学研究提供参考.     

具有大范围运动和非线性变形的空间柔性梁的精确动力学建模

研究具有大范围运动和非线性变形的空间柔性梁的有限元动力学建模.首先在精确描述空间柔性梁的非线性变形的基础上,采用有限元方法对梁结构进行离散,导出其动能、势能及外力对应的广义力,然后利用Lagrange方程建立了空间柔性梁的精确动力学方程.该方程在原有一次耦合模型的基础上,增加了新的表征纵向、横向、侧向弯曲变形,以及扭转变形的耦合项,同时包含了变形运动与大范围运动之间的相互耦合项.本建模方法和所得结论可为以后空间柔性梁的动力学特性分析作以参考.     

磷脂在膜结构间的交换:温度和离子强度的影响

磷脂跨膜交换对生物膜功能与药学研究有重要意义.石英电子微天平及耗散系数测量仪被用于研究囊泡与支撑膜间磷脂的交换行为.研究表明:首先,在磷脂跨膜输运过程中,热力学环境和离子强度对支撑膜表面吸附囊泡的形变程度影响较小,囊泡与支撑膜的总接触面积直接取决于囊泡的吸附数量;其次,交换过程中膜结构间最大总接触面积随着温度的升高和离子强度的降低而增大,温度和离子引起的囊泡吸附速率和跨膜交换速率的变化在其中发挥着关键调节作用.本研究有助于加深对磷脂在生理条件下跨膜输运过程的理解,并为基于脂质体的药物载运体系研究提供参考.     

变分法研究一维Bose-Fermi系统的稳定性

利用能量泛函变分法研究了一维Bose-Fermi系统稳定基态的存在条件.根据Bose-Fermi系统的Lagrange量可以得到三维Bose-Fermi体系所满足的非线性动力学方程组.当外势阱的横向囚禁频率远大于轴向囚禁频率时,体系可以当作一维模型来处理.从描述三维体系的动力学方程可以得到描述一维体系的动力学方程,选取适当的无量纲参数,可以对一维动力学方程组进行无量纲处理,得到数值计算和理论分析中常用到的无量纲方程.选择高斯型试探解(简单孤立子解),利用能量泛函变分法得到一维Bose-Fermi体系稳定的 关键词： Bose-Fermi 稳定性 基态 临界条件     

多层膜磁性微泡的非线性声振动特性

超顺磁性氧化铁纳米粒子与造影剂微泡结合形成磁性微泡,用于产生多模态造影剂,以增强医学超声和磁共振成像.将装载有纳米磁性颗粒的微泡包膜层看作由磁流体膜与磷脂膜组合而成的双层膜结构,同时考虑磁性纳米颗粒体积分数a对膜密度及黏度的影响,从气泡动力学基本理论出发,构建多层膜结构磁性微泡非线性动力学方程.数值分析了驱动声压和频率等声场参数、颗粒体积分数、膜层厚度以及表面张力等膜壳参数对微泡声动力学行为的影响.结果表明,当磁性颗粒体积分数较小且a≤0.1时,磁性微泡声响应特性与普通包膜微泡相似,微泡的声频响应与其初始尺寸和驱动压有关;当驱动声场频率f为磁性微泡共振频率f0的2倍(f=2f0)时,微泡振动失稳临界声压最低;磁性颗粒的存在抑制了泡的膨胀和收缩但抑制效果非常有限;磁性微泡外膜层材料的表面张力参数K及膜层厚度d也会影响微泡的振动,当表面张力参数及膜厚取值分别为0.2—0.4 N/m及50—150 nm时,可观察到气泡存在不稳定振动响应区.     

Direct Visualization of Vesicle Maturation and Plasma Membrane Protein Trafficking

Internalization and intracellular trafficking of membrane proteins are now recognized as essential mechanisms that contribute to a number of cellular processes. Current methods lack the ability to specifically label the plasma membrane of a live cell, follow internalization of labeled membrane molecules, and conclusively differentiate newly formed membrane-derived vesicles from pre-existing endocytic or secretory structures in the cytoplasm. Here, we detail a visualization method for surface biotinylation of plasma membrane-derived vesicles that allows us to follow their progress from membrane to cytosol at specific time points. Using the transmembrane receptor RET as a model, we demonstrate how this method can be applied to identify plasma membrane-derived vesicle maturation, determine RET’s presence within these structures, and monitor RET’s recycling to the cell surface. This method improves on static and less discriminatory methods, providing a tool for analysis of real-time vesicle trafficking that is applicable to many systems.     

A boundary integral method for simulating the dynamics of inextensible vesicles suspended in a viscous fluid in 2D

We present a new method for the evolution of inextensible vesicles immersed in a Stokesian fluid. We use a boundary integral formulation for the fluid that results in a set of nonlinear integro-differential equations for the vesicle dynamics. The motion of the vesicles is determined by balancing the non-local hydrodynamic forces with the elastic forces due to bending and tension. Numerical simulations of such vesicle motions are quite challenging. On one hand, explicit time-stepping schemes suffer from a severe stability constraint due to the stiffness related to high-order spatial derivatives and a milder constraint due to a transport-like stability condition. On the other hand, an implicit scheme can be expensive because it requires the solution of a set of nonlinear equations at each time step. We present two semi-implicit schemes that circumvent the severe stability constraints on the time step and whose computational cost per time step is comparable to that of an explicit scheme. We discretize the equations by using a spectral method in space, and a multistep third-order accurate scheme in time. We use the fast multipole method (FMM) to efficiently compute vesicle–vesicle interaction forces in a suspension with a large number of vesicles. We report results from numerical experiments that demonstrate the convergence and algorithmic complexity properties of our scheme.     

Simulating the dynamics of inextensible vesicles by the penalty immersed boundary method

In this paper, we develop an immersed boundary (IB) method to simulate the dynamics of inextensible vesicles interacting with an incompressible fluid. In order to take into account the inextensibility constraint of the vesicle, the penalty immersed boundary (pIB) method is used to virtually decouple the fluid and vesicle dynamics. As numerical tests of our current pIB method, the dynamics of single and multiple inextensible vesicles under shear flows have been extensively explored, and compared with the previous literature. The method is also validated by a series of convergence study, which confirms its consistent first-order accuracy on the velocity field, the vesicle configuration, the vesicle area and the perimeter errors. In addition, the method is also applied to study a binary-component vesicle problem.     

New Results for Directed Vesicles and Chains near an Attractive Wall

In this paper we present new exact results for single fully directed walks and fully directed vesicles near an attractive wall. This involves a novel method of solution for these types of problems. The major advantage of this method is that it, unlike many other single-walker methods, generalizes to an arbitrary number of walkers. The method of solution involves solving a set of partial difference equations with a Bethe Ansatz. The solution is expressed as a “constant-term” formula which evaluates to sums of products of binomial coefficients. The vesicle critical temperature is found at which a binding transition takes place, and the asymptotic forms of the associated partition functions are found to have three different entropic exponents depending on whether the temperature is above, below, or at its critical value. The expected number of monomers adsorbed onto the surface is found to become proportional to the vesicle length at temperatures below critical. Scaling functions near the critical point are determined.     

基于案例和规则融合的民用客机故障诊断系统设计

目前很多工程应用中的飞机故障诊断方法,只考虑单一的基于案例或基于规则的诊断方法,故障诊断的效率有待提高。本文针对民用客机地面运营支持系统的特点和要求,将案例和规则融合的推理机制引入到飞机故障诊断系统的设计中。该方法采用规则推理为前导、案例推理后置补充的结合方式,最大化地利用飞机故障隔离手册知识和从维修记录中提取的案例知识,从而提高首次故障隔离效率、降低无效排故时间。以民用客机某系统的典型故障为例,阐述了该推理机制在故障诊断中的应用。通过历史飞行数据试验,验证了提出的基于案例与规则融合诊断方法的有效性,以此为基础设计的故障诊断方案可为航空公司构建更完善的专家系统提供参考。     

A phase field method for simulating morphological evolution of vesicles in electric fields

A phase field method is developed to investigate the morphological evolution of a vesicle in an electric field, taking into account coupled mechanical and electric effects such as bending, osmotic pressure, surface tension, flexoelectricity, and dielectricity of the membrane. The energy of the system is formulated in terms of a continuous phase field variable and the electric potential. The governing equations of the phase field and the electric field are solved using the Galerkin weighted residual approach. The validation and robustness of the algorithm are verified by direct comparisons of the obtained numerical solutions with relevant experimental results. The morphological evolution of an axisymmetric vesicle under an electric field is studied in detail. The results demonstrate that the present method can simulate complex morphological evolutions of vesicles under coupled mechanical–electrical fields.