梯度场中荧光关联谱测量的Monte Carlo模拟

荧光关联谱（FCS）利用少量分子的荧光涨落获得分子运动和反应信息,是一种探测活细胞内生物过程的有力工具。本文利用Monte Carlo方法模拟激光梯度场下瑞利粒子的布郎运动,分析梯度场对FCS测量得到的扩散时间、粒子数等参数的影响,模拟与初步实验结果定性吻合。文中还讨论了Monte Carlo模拟预测的一些实验现象。     

饱和激发情况下的荧光关联谱测量

研究了激发光增强时其对荧光关联谱(FCS)测量的影响. 强激发光产生的饱和激发会改 变激光诱导荧光的空间分布函数,从而影响FCS的测量结果. 首先根据饱和吸收的物理模型,推导出强激发光情况下有效探测区域变化的定量公式,获得了FCS测量所得到的粒子数和扩散时间与饱和激发光强和激光光强的函数关系,并用于消除饱和吸收造成的系统误差.采用Monte-Carlo模拟方法和实验,对理论分析结果进行了验证. 这将有助于完善FCS分析方法的理论模型,为高激发光强度下的FCS探测提供依据. 关键词： 荧光关联谱 饱和激发 Monte-Carlo模拟     

太阳耀斑与太阳高能粒子

考虑日冕区加速中相对论性粒子的扩散激波加速,结果表明:(1)粒子随时间的分布,其主要特点是随时问增加,一定能量的粒子分布迅速增加至最大后下降,并且能量越大,其最大就越向着大时间移动;(2)给出了相应的时间积分谱,它正比于具有指数截止的粒子动量的幂律.还讨论了粒子加速到高能区的条件及可能性.     

光谱学 激光光谱学

O433．54 2005031677 饱和激发情况下的荧光关联谱测量=Fluorescence corre- lation spectroscopy with saturated excitation[刊,中]／丁建 英(清华大学物理系,原子分子纳米科学教育部重点实验 室．北京(100084)),陈波…∥物理学报．-2004,53 (8)．-2503-2508 研究了激发光谱增强时其对荧光关联谱(FCS)测量的 影响。强激发光产生的饱和激发会改变激光诱导荧光的 空间分布函数,从而影响FCS的测量结果。首先根据饱和 吸收的物理模型,推导出强激发光情况下有效探测区域变     

强激光场中氢负离子双光子电离能量谱的研究

利用强场近似(Strong field approximation,SFA)方法研究氢负离子(H-)在强激光场中双光子电离的能量谱,所得到的电离谱随角度的变化规律与实验结果符合得很好.进一步的研究表明,H-离子在强激光场中双光子电离的能量谱与有质动力能有关.激光场强度越大,光电子的有质动力能也越大,能量谱向左移动越明显...     

周期信号调制色泵噪音驱动下单模激光光强关联函数的时间演化特性

采用周期信号调制色泵噪音驱动的单模激光模型,运用线性化近似方法研究了单模激光系统光强关联函数C(t)随时间的演化关系,分析了调制信号的振幅B、频率Ω等对光强关联函数随时间演化的影响. 发现在泵噪音自关联时间τ>>1的情形下,随着调制信号频率Ω、振幅B的增加,C(t)随时间的演化为单调衰减;在τ>>1的情形下,随着调制信号频率Ω、振幅B的增加,C(t)随时间的演化均为周期性振荡衰减.     

周期信号调制色泵噪音驱动下单模激光光强关联函数的时间演化特性

采用周期信号调制色泵噪音驱动的单模激光模型,运用线性化近似方法研究了单模激光系统光强关联函数C(t)随时间的演化关系,分析了调制信号的振幅B、频率Ω等对光强关联函数随时间演化的影响.发现在泵噪音自关联时间τ<<1的情形下,随着调制信号频率Ω、振幅B的增加,C(t)随时间的演化为单调衰减;在τ>>1的情形下,随着调制信号频率Ω、振幅B的增加,C(t)随时间的演化均为周期性振荡衰减.     

随机双指数记忆耗散系统的非马尔可夫扩散

针对具有双指数耗散记忆核函数的两自由度耦合系统, 本文利用Laplace变换导出了热宽带噪声激励下该系统响应二阶矩的解析表达式. 并观察到位移二阶矩不同于单自由度情形下的反常扩散:<x²(t)> ∝ t^α (0<α<2, α≠1), 而是随时间及噪声等参数变化呈现普遍的振荡扩散现象.分析可得, 阻尼耦合因子B使粒子远离简谐势场的束缚, <x²(t)>随B的增大扩散加剧而摩擦系数增大却使其趋于平稳状态.进一步, 若两热噪声互关联时, 较小的互关联时间对二阶矩的影响较大, 反之作用较小. 伴随互关联强度递增, 位移二阶矩的扩散加剧, 位移间的相关性加强, 与物理直观相符. 关键词： 热噪声 非马尔可夫扩散 广义朗之万方程 关联性     

强激光场中氢负离子双光子电离能量谱的研究

利用强场近似（Strong field approximation,SFA）方法研究氢负离子（H ）在强激光场中双光子电离的能量谱,所得到的电离谱随角度的变化规律与实验结果符合得很好.进一步的研究表明, H 离子在强激光场中双光子电离的能量谱与有质动力能有关.激光场强度越大,光电子的有质动力能也越大,能量谱向左移动越明显.我们的结果表明,使用强场近似是一种研究负离子在强激光场中电离过程的有效方法.     

激光尾波场中光子加速研究

在SILEX-Ⅰ激光装置上,测量了超短超强激光脉冲与稀薄等离子体相互作用之后的透射谱. 实验中发现,激光尾波场产生的密度扰动导致等离子体折射率随时间空间不断变化,导致光子的加速/减速. 透射谱上主要表现为激光频率谱峰的劈裂和随密度变化的展宽,没有发现与前向受激拉曼散射或自调制不稳定性相联系的边频波. 同时,利用LPIC++无碰撞粒子模拟程序模拟了超短超强激光与稀薄等离子体相互作用后的透射谱,模拟结果也发现了明显的光子加速过程. 关键词： 超短超强激光脉冲 透射谱 光子加速/减速     

Two-dimensional Monte Carlo simulations of a suspension comprised of magnetic and nonmagnetic particles in gradient magnetic fields

Distributions of particles in a suspension comprised of magnetic particles (MPs) and nonmagnetic particles (NPs) under gradient magnetic fields are vitally important for the preparation of magnetic-nonmagnetic functionally graded materials (FGMs). In the present study, the effects of magnetic field gradient, magnetic interaction between MPs and concentration of NPs on the distributions of particles in the suspension are investigated using a two-dimensional Monte Carlo simulation. The results show that a gradient distribution of MPs is formed under gradient magnetic fields and increases with increasing the field gradient. However, as the interaction between MPs increases, the distribution gradient decreases, accompanied by the formation of chain-like MP clusters. Moreover, NPs are found to hinder the translation of MPs along the field direction. As the NP concentration increases, the translation of MPs becomes difficult.     

Optimal orientation field to manufacture magnetostrictive composites with high magnetostrictive performance

Magnetostrictive properties have relationship with the applied orientation field during the preparation of giant magnetostrictive composites. To understand the dependence of the optimal orientation field on particle volume fraction, composites with 20%, 30% and 50% particles by volume were fabricated by distributing Terfenol-D particles in an unsaturated polyester resin under various orientation fields. Their magnetostrictive properties were tested without pre-stress at room temperature. The results indicate that as the particle volume fraction increases, the optimal orientation field increases. The main reason for this phenomenon is the packing density for the composites with higher particle volume fraction is larger than that for those with lower particle content.     

相场法模拟第二相颗粒对柱状晶形成的影响

基于定向退火条件下的移动热区模型,采用相场法研究第二相颗粒对多晶材料中柱状晶结构形成的影响.结果表明:第二相颗粒对柱状晶结构的形成具有抑制作用,其抑制作用随第二相颗粒体积分数的增加和第二相颗粒尺寸的减小而增强.定向退火条件下第二相颗粒对晶粒极限尺寸的影响遵循Zener关系.为了获得良好的柱状晶结构,应尽量减少材料中第二相颗粒的体积分数及其在晶界上的弥散分布.     

负径向电场对荷电粒子运动的影响及其数值模拟

从负径向电场产生的电漂移改变荷电粒子运动的极向运动速度着手,推导出在负径向电场存在时安全因子的表达式,分析了安全因子对荷电粒子漂移位移和运动轨迹的影响。建立了在负径向电场条件下,荷电粒子在梯度磁场和曲率磁场中运动数学模型。通过数值模拟,获得了通行粒子、香蕉粒子的漂移位移和运动轨迹所呈现出的新特点和规律:负径向电场改变了荷电粒子的最大漂移位移。当荷电粒子的极向运动速度增加时,最大漂移位移减小,反之增大;改变了荷电粒子的运动轨迹,通行粒子的轨亦可能变为香蕉粒子的轨迹,香蕉粒子的轨迹可能变为通行粒子的轨迹,当电场达到足够的强度时,均成为在极向上顺时针运动的通行粒子轨迹。     

梯度磁场下磁流体三维结构的格子-Boltzmann方法模拟

考虑磁性颗粒受到的各种内力与外力包括重力、布朗力、van der、Waaks力、磁偶极-偶极作用力以及外磁场作用力,建立了描述磁流体结构的两相格子-Boltzmann三维模型,对外加梯度磁场条件下磁流体的介观结构进行了模拟.模拟结果表明:外加梯度磁场时磁流体粒子沿梯度方向聚集并出现分层现象,且随时间推移和外加磁场增大,分层现象越来越明显.     

A 3D numerical simulation of mixed convection of a magnetic nanofluid in the presence of non-uniform magnetic field in a vertical tube using two phase mixture model

In this paper, results of applying a non-uniform magnetic field on a ferrofluid (kerosene and 4 vol% Fe₃O₄ ) flow in a vertical tube have been reported. The hydrodynamics and thermal behavior of the flow are investigated numerically using the two phase mixture model and the control volume technique. Two positive and negative magnetic field gradients have been examined. Based on the obtained results the Nusselt number can be controlled externally using the magnetic field with different intensity and gradients. It is concluded that the magnetic field with negative gradient acts similar to Buoyancy force and augments the Nusselt number, while the magnetic field with positive gradient decreases it. Also with the negative gradient of the magnetic field, pumping power increases and vice versa for the positive gradient case.     

Modified gradients for motion suppression: variable echo time and variable bandwidth

A linear algebra based deprivation is presented to demonstrate that linearly time scaling an entire gradient waveform by a factor "R" exponentially increases its sensitivity to time derivatives of position by R(i + 1), where i refers to the i-th derivative of position (e.g., i = 1 is velocity). Thus, time scaling will preserve zero valued refocussing moments associated with artifact reduction techniques designed for motion occurring between excitation and detection. Typically, gradient waveforms for artifact reduction techniques are derived for use only at specific echo times. The time scaling described here allows for simple modification of refocussing gradient waveforms for use at variable echo times. Motion sensitivity associated with non-zero moment gradient waveforms can be easily predicted and modified using this technique, with consideration for field of view, resolution, and bandwidth. A clinical example is presented showing the predicted changes in sensitivity to nonrefocussed derivatives of position as the imaging gradients are time scaled. Further, trade-offs and alternatives in sensitivity to motion, slice thickness, image bandwidth, field of view and resolution will be discussed in conjunction with time scaling. This technique will have applicability in many situations involving MRI of moving tissue and a clinical example in cardiac imaging is presented.     

Some aspects concerning the design of an immunomagnetic separation process

We used a stochastic model and have performed a series of numerical simulations to optimise the step of microorganisms collection on magnetic carrier particles. We obtained that the time of capture depends on concentration of carrier particles in suspension, their size, the volume fraction of magnetite in the composite material, the applied magnetic field intensity and its gradient, the size and density of micro-organisms to be collected and the density and viscosity of the liquid in which the particles are suspended.     

Numerical study of magnetic nanoparticles concentration in biofluid (blood) under influence of high gradient magnetic field

Ferrofluids are widely used in pharmaceutical industries as magnetic separation tools, anti-cancer drug carriers and micro-valve applications. The purpose of the current study is to investigate the effect of a magnetic field on the volume concentration of magnetic nanoparticles of a non-Newtonian biofluid (blood) as a drug carrier. The effect of particles on the flow field is considered. The governing non-linear differential equations, concentration and Naviar-stokes are coupled with the magnetic field. To solve these equations, a finite volume based code is developed and utilized. The results show accumulation of magnetic nanoparticles near the magnetic source until it looks like a solid object. The accumulation of nanoparticles is due to the magnetic force that overcomes the fluid drag force. As the magnetic strength and size of the magnetic particles increase, the accumulation of nanoparticles increases, as well. The magnetic susceptibility of particles also affects the flow field and the contour of the concentration considerably.     

离心力场作用下三维流动和传热的场协同理论探讨

通过对能量方程进行体积分得到了三维流场中场协同理论的表达式。该式不仅含速度矢量与温度梯度矢量的点积项,还含有速度矢量与压力梯度矢量的点积项。在此基础上讨论了速度场、压力场、温度场之间的协同关系,探讨了通过政变边界条件调节诸场形态,实现传热强化的方法。并通过三个盘腔算例,对此方法进行了验证。