二分量带电胶体悬浮系统的等效硬球模型

研究由无限稀薄的靶粒子散布于有限浓度(体积分数为)的主粒子悬浮液中而组成的二分量带电胶体系统,计算了靶粒子的短时间平动和转动自扩散系数.当系统中的粒子浓度和电解质浓度都不太高时,只考虑流体力学相互作用对扩散张量的首项两体贡献.为了计算体系的对分布函数,在数值计算的基础上发展了一个等效硬球模型,近似地把主粒子和靶粒子看作等效半径为δEHS的相同硬球粒子.结果表明,靶粒子的自扩散系数随两种粒子尺寸比和主粒子体积分数变化的关系可以很好地用等效硬球模型来解释 关键词： 胶体悬浮系统 自扩散 等效硬球 流体力学作用 关联函数     

二分量胶体悬浮系统的短时间动力学

研究了二分量带电粒子悬浮系统的短时间平动和转动自扩散系数.由于存在静电相互作用和流体力学作用,扩散系数与两种粒子的尺寸比,它们的体积分数,以及所带的有效电荷都有关.计入了流体力学相互作用对扩散张量的二体贡献和首项三体贡献.计算结果表明,流体力学作用对于带电粒子系统的影响要小于它对硬球粒子系统的影响.扩散系数随两种粒子的尺寸比和它们的体积分数变化的关系可以用有效硬球模型来解释,而其定性结果与实验相符合.     

二分量胶体悬浮系统的短时间动力学

研究了二分量带电粒子悬浮系统的短时间平动和转动自扩散系数.由于存在静电相互作用和流体力学作用,扩散系数与两种粒子的尺寸比,它们的体积分数,以及所带的有效电荷都有关.计入了流体力学相互作用对扩散张量的二体贡献和首项三体贡献.计算结果表明,流体力学作用对于带电粒子系统的影响要小于它对硬球粒子系统的影响.扩散系数随两种粒子的尺寸比和它们的体积分数变化的关系可以用有效硬球模型来解释,而其定性结果与实验相符合.     

光电反馈式静电悬浮的机制研究

本文主要讨论光电反馈式静电悬浮的理论问题。研究在静电场作用下导电悬浮体的静电感应机制和绝缘悬浮体的静电极化机理,推导出悬浮体表面的感应电荷或极化电荷的计算公式,以及作用于悬浮体上的静电悬浮力的计算公式,在理论上揭示了静电力与电极电压、电极面积、悬浮间距及悬浮体电学特性等因素之间的关系。结果表明,导电悬浮体表面的感应电荷量比绝缘悬浮体表面的极化电荷量多,因此前者所受的静电悬浮力比后者大,但两者均可实现静电悬浮,这些结论与实验结果完全符合。本文工作为实现静电悬浮的光电反馈控制提供了理论依据。     

相对论自旋流体力学

近年来,随着重离子碰撞实验中超子自旋极化与矢量介子自旋排列现象的发现,关于夸克胶子物质中自旋输运的理论研究也得到蓬勃发展,其中包括相对论自旋流体力学,它是描述自旋输运的流体力学理论.本文对相对论自旋流体力学的近期发展进行了综述,主要包括以下内容:1)相对论自旋流体力学基本方程的推导,包括宏观的唯象学推导、基于有效场论的推导以及基于输运理论的推导; 2)该理论框架的一些特殊性质,包括能动量张量中的反对称结构以及赝规范变换性质等; 3)在Bjorken和Gubser膨胀体系中的解析解及其对于重离子碰撞物理的意义.     

一维单轴式超声悬浮的实验研究

由声波发射端和反射端构成超声谐振系统,其发射端和反射端间形成超声驻波.在波节处声辐射压的合力可以悬浮微重力样品,称为单轴式超声悬浮;是一种有望成为具有应用价值的无容器处理技术.作者在参加物理第二课堂活动中,对此作了初步的实验研究,成功地悬浮起发泡材料等小物体以及蚂蚁等小动物,并观察研究了它们的悬浮姿态.     

具有环形限位夹持力的单轴式超声悬浮系统

基于超声悬浮原理,通过改变单轴式超声悬浮系统的反射和发射端的几何形状,利用凸状发射、反射面在悬浮区域周围产生环形声辐射力势阱作为限位夹持力,提高悬浮样品在驻波声场中的悬浮稳定性。基于时域有限差分法,研究不同谐振腔内的声辐射力分布,以及影响声辐射力大小的因素,优化环形限位夹持力和悬浮力。理论及实验研究结果表明,具有环形限位夹持力的超声悬浮系统可以提高普通单轴式超声悬浮装置对悬浮样品的稳定性。      

基于光纤的三维可调胶体光子晶体

用改进的垂直沉积法在光纤端面制备了高质量的SiO₂胶体光子晶体,经过烧结、固化构成胶体光子晶体-光纤结构. 用扫描电子显微镜确定了样品为面心立方密排结构,其密排面平行于光纤基底表面. 利用全光纤传感网络测试了该胶体光子晶体,反射峰中心位于845 nm处,与Bragg理论计算值符合很好. 将该样品浸入不同折射率的液体中,反射光谱的峰值位置随着液体折射率的改变而发生偏移,近似呈线性关系,实现了峰位可调. 对于不同浓度引起的液体折射率的变化,基于光纤的胶体光子晶体结构也能够很好地分辨出来. 关键词： 可调胶体光子晶体 2微球')" href="#">SiO₂微球 Bragg反射 光纤     

平板电极间胶体晶体在电场作用下的各向同性压缩

用直径 300 nm的聚苯乙烯微球配制不同浓度的胶体晶体溶液, 将其快速注入内表面镀有导电薄膜的玻璃样品池中, 形成 (111) 晶面平行于玻璃表面的面心立方单晶结构. 通过激光衍射Kossel线方法, 研究了不同体积分数的胶体晶体样品 及它们在均匀电场作用下晶体结构的变化. 实验发现, 随着电场强度的增加, 胶体晶体表现为各向同性的压缩. 胶体晶体在恒定电场下始终保持面心立方结构, 晶格常数随着电场强度的增加逐渐减小. 实验结果可用电场力、电流体力学作用力及颗粒间静电斥力共同解释: 电场力使带电微球克服静电斥力并沿电场反方向运动导致晶体压缩, 而由电场力作用引起的电流体力学液流产生的持续推力使垂直于电场平面上的胶体微球相互靠近. 本实验为天宫一号搭载科学实验的地基实验. 关键词： 胶体晶体 Kossel衍射 结构变化     

剪应力作用下流体力学关联对胶体聚团的影响

用计算机模拟的方法，探讨了剪应力下流体力学关联对胶体聚团特性的影响，并与无流体力学关联时的结果作了比较，研究表明，流体力学关联因素对胶体簇团的分形维数和栩位数没有明显的影响，但却使聚团的机理发生改变，从无流体力学关联时的颗粒－颗粒型向簇团－簇团型过渡。这个结果都很好地解释了有关的实验现象。     

MRI diffusion tensor reconstruction with PROPELLER data acquisition

MRI diffusion imaging is effective in measuring the diffusion tensor in brain, cardiac, liver, and spinal tissue. Diffusion tensor tomography MRI (DTT MRI) method is based on reconstructing the diffusion tensor field from measurements of projections of the tensor field. Projections are obtained by appropriate application of rotated diffusion gradients. In the present paper, the potential of a novel data acquisition scheme, PROPELLER (Periodically Rotated Overlapping ParallEL Lines with Enhanced Reconstruction), is examined in combination with DTT MRI for its capability and sufficiency for diffusion imaging. An iterative reconstruction algorithm is used to reconstruct the diffusion tensor field from rotated diffusion weighted blades by appropriate rotated diffusion gradients. DTT MRI with PROPELLER data acquisition shows significant potential to reduce the number of weighted measurements, avoid ambiguity in reconstructing diffusion tensor parameters, increase signal-to-noise ratio, and decrease the influence of signal distortion.     

A continuous tensor field approximation of discrete DT-MRI data for extracting microstructural and architectural features of tissue

The effective diffusion tensor of water, D, measured by diffusion tensor MRI (DT-MRI), is inherently a discrete, noisy, voxel-averaged sample of an underlying macroscopic effective diffusion tensor field, D(x). Within fibrous tissues this field is presumed to be continuous and smooth at a gross anatomical length scale. Here a new, general mathematical framework is proposed that uses measured DT-MRI data to produce a continuous approximation to D(x). One essential finding is that the continuous tensor field representation can be constructed by repeatedly performing one-dimensional B-spline transforms of the DT-MRI data. The fidelity and noise-immunity of this approximation are tested using a set of synthetically generated tensor fields to which background noise is added via Monte Carlo methods. Generally, these tensor field templates are reproduced faithfully except at boundaries where diffusion properties change discontinuously or where the tensor field is not microscopically homogeneous. Away from such regions, the tensor field approximation does not introduce bias in useful DT-MRI parameters, such as Trace(D(x)). It also facilitates the calculation of several new parameters, particularly differential quantities obtained from the tensor of spatial gradients of D(x). As an example, we show that they can identify tissue boundaries across which diffusion properties change rapidly using in vivo human brain data. One important application of this methodology is to improve the reliability and robustness of DT-MRI fiber tractography.     

Tensor diffusion level set method for infrared targets contours extraction

A tensor diffusion level set method is presented to extract infrared (IR) targets contour under a sky-mountain-water complex background. The proposed model combines tensor diffusion operator and the eigenvalues of tensor-image into a common energy minimization level set framework. By incorporating the information of image tensor diffusion operator into the external energy term, the level set function can move in a specific way. And eigenvalues of tensor-image are used for the regularization of zero level curves in order to diminish the influence of image ‘clutter’ and noise. An additional benefit of the proposed method is robust to initial conditions. Experimental results show very good performance of the tensor diffusion level set method for IR targets contours extraction.     

Efficient cardiac diffusion tensor MRI by three-dimensional reconstruction of solenoidal tensor fields

Tensor tomography is being investigated as a technique for reconstruction of in vivo diffusion tensor fields that can potentially be used to reduce the number of magnetic resonance imaging (MRI) measurements. Specifically, assessments are being made of the reconstruction of cardiac diffusion tensor fields from 3D Radon planar projections using a filtered backprojection algorithm in order to specify the helical fiber structure of myocardial tissue. Helmholtz type decomposition is proposed for 3D second order tensor fields. Using this decomposition a Fourier projection theorem is formulated in terms of the solenoidal and irrotational components of the tensor field. From the Fourier projection theorem, two sets of Radon directional measurements, one that reconstructs the solenoidal component and one that reconstructs the irrotational component of the tensor field, are prescribed. Based on these observations filtered backprojection reconstruction formulae are given for the reconstruction of a 3D second order tensor field and its solenoidal and irrotational components from Radon projection measurements. Computer simulations demonstrate the validity of the mathematical formulations and demonstrate that a realistic model of the helical fiber structure of the myocardial tissue specifies a diffusion tensor field for which the first principal vector (the vector associated with the maximum eigenvalue) of the solenoidal component accurately approximates the first principal vector of the diffusion tensor. A priori knowledge of this allows the orientation of the myocardial fiber structure to be specified utilizing one half of the number of MRI measurements of a normal diffusion tensor field study.     

描述人体内水分子扩散各向异性特征的新方法

通过核磁共振扩散张量成像(DTI)得到的特定值域的扩散各向异性指数(DAI) 可用于揭示水分子扩散椭球的形态学特征, 定量反映被成像物体内部水分子扩散的优势方向和强度, 间接得到被成像物体内部的组织结构信息. DAI的可靠性直接影响对DTI数据的分析和理解. 本文基于扩散张量椭球的几何学信息, 提出利用扩散椭球几何比(EGR)定量描述水分子扩散的各向异性程度. 通过蒙特卡罗模拟实验和对人脑DTI数据进行分析, 并与当前广泛应用的水分子扩散各向异性分数(FA)和近期文献提出的扩散椭球面积比(EAR)进行对比. 实验发现EGR在不同级别噪声影响下的对比度效果和抗噪性都优于FA及EAR. 而且EGR 加入了体积修正, 增强了盘形扩散张量情况下的敏感性, 能够更好地鉴别神经纤维束交叉情况, 对于各向异性扩散程度较高的白质深层和相对均质的表层都有较好的量化区分结果. 关键词： 扩散系数 各向异性扩散 扩散张量成像 扩散椭球几何比     

Determination of the rotational diffusion tensor of macromolecules in solution from nmr relaxation data with a combination of exact and approximate methods--application to the determination of interdomain orientation in multidomain proteins

In this paper we present a method for determining the rotational diffusion tensor from NMR relaxation data using a combination of approximate and exact methods. The approximate method, which is computationally less intensive, computes values of the principal components of the diffusion tensor and estimates the Euler angles, which relate the principal axis frame of the diffusion tensor to the molecular frame. The approximate values of the principal components are then used as starting points for an exact calculation by a downhill simplex search for the principal components of the tensor over a grid of the space of Euler angles relating the diffusion tensor frame to the molecular frame. The search space of Euler angles is restricted using the tensor orientations calculated using the approximate method. The utility of this approach is demonstrated using both simulated and experimental relaxation data. A quality factor that determines the extent of the agreement between the measured and predicted relaxation data is provided. This approach is then used to estimate the relative orientation of SH3 and SH2 domains in the SH(32) dual-domain construct of Abelson kinase complexed with a consolidated ligand.     

Efficient anisotropic filtering of diffusion tensor images

To improve the accuracy of structural and architectural characterization of living tissue with diffusion tensor imaging, an efficient smoothing algorithm is presented for reducing noise in diffusion tensor images. The algorithm is based on anisotropic diffusion filtering, which allows both image detail preservation and noise reduction. However, traditional numerical schemes for anisotropic filtering have the drawback of inefficiency and inaccuracy due to their poor stability and first order time accuracy. To address this, an unconditionally stable and second order time accuracy semi-implicit Craig-Sneyd scheme is adapted in our anisotropic filtering. By using large step size, unconditional stability allows this scheme to take much fewer iterations and thus less computation time than the explicit scheme to achieve a certain degree of smoothing. Second-order time accuracy makes the algorithm reduce noise more effectively than a first order scheme with the same total iteration time. Both the efficiency and effectiveness are quantitatively evaluated based on synthetic and in vivo human brain diffusion tensor images, and these tests demonstrate that our algorithm is an efficient and effective tool for denoising diffusion tensor images.     

基于边缘定向扩散的图像增强方法

针对前向后向扩散方程不能较好的保持流线状结构,而相干增强不能锐化边缘的缺点, 提出一种新的基于边缘定向的张量型前向后向扩散模型.该模型将前向-后向扩散方程引进到张量型扩散方程中,在扩散系数的选取上合并了相干增强扩散与前向-后向扩散的长处,又克服了他们各自的缺点.采用类似相干增强扩散的边缘定向算子实现对边缘的定向,然后根据边缘定向的结果设置扩散张量的特征根,使扩散张量沿边缘方向为正向扩散以增强边缘,而垂直于边缘方向为逆扩散以锐化边缘.理论分析和数值计算表明,该方法具有比相干增强扩散及前向-后向扩散更好的增强效果.     

A framework for quality control and parameter optimization in diffusion tensor imaging: theoretical analysis and validation

In this communication, a theoretical framework for quality control and parameter optimization in diffusion tensor imaging (DTI) is presented and validated. The approach is based on the analytical error propagation of the mean diffusivity (D(av)) obtained directly from the diffusion-weighted data acquired using rotationally invariant and uniformly distributed icosahedral encoding schemes. The error propagation of a recently described and validated cylindrical tensor model is further extrapolated to the spherical tensor case (diffusion anisotropy approximately 0) to relate analytically the precision error in fractional tensor anisotropy (FA) with the mean diffusion-to-noise ratio (DNR). The approach provided simple analytical and empirical quality control measures for optimization of diffusion parameter space in an isotropic medium that can be tested using widely available water phantoms.     

Generalized Fick law for anomalous diffusion in the multidimensional comb model

The problem of multidimensional diffusion is considered within the framework of the comb model. It is shown that the diffusion current for the case of anomalous subdiffusion random walks is described by the generalized Fick law containing the diffusion tensor instead of the usual coefficient. The form of the diffusion tensor components is an unusual form of operator as fractional time derivatives. The orders of the fractional exponents are different for different directions.