气体分子与纳米粒子碰撞的分子动力学仿真分析

当气体分子与纳米粒子碰撞的时候,纳米粒子传输理论预测到当纳米粒子的直径由小变大时,碰撞会由镜面反射转化为漫反射.文章利用分子动力学仿真研究了气体分子与纳米粒子碰撞的过程.在验证了这种转化存在同时,又探讨了碰撞转化的机理,即漫反射的起因.仿真结果揭示了漫反射的起因是由于纳米粒子对气体分子的吸附作用.这种吸附作用是由于纳米粒子对能量的容纳特性而产生的.     

气体分子与纳米粒子碰撞的分子动力学仿真分析

当气体分子与纳米粒子碰撞的时候,纳米粒子传输理论预测到当纳米粒子的直径由小变大时,碰撞会由镜面反射转化为漫反射.文章利用分子动力学仿真研究了气体分子与纳米粒子碰撞的过程.在验证了这种转化存在同时,又探讨了碰撞转化的机理,即漫反射的起因.仿真结果揭示了漫反射的起因是由于纳米粒子对气体分子的吸附作用.这种吸附作用是由于纳米粒子对能量的容纳特性而产生的.     

Light scattering by Gaussian particles with internal inclusions and roughened surfaces using ray optics

We study light scattering by Gaussian-random-sphere particles that are large compared to the wavelength of the incident light using ray optics that, in addition to Fresnellian reflection and refraction, accounts for diffuse scattering. We consider two types of diffusely scattering media. One type of media constitutes a uniform medium inside the particle, i.e. a diffuse internal medium. The second type of media constitutes a layer on the surface of the particle that is thin compared to the particle dimensions and acts as a diffuse external medium mimicking the particle surface roughness. We illustrate the effects of the diffuse media on the scattering characteristics for both cases and show that incorporating diffuse scatterers allows us to explain the scattering matrices measured experimentally for Saharan sand particles large compared to the wavelength.     

Temperature-sensitive poly(N-Isopropyl-Acrylamide) microgel particles: A light scattering study

We present a light scattering study of aqueous suspensions of microgel particles consisting of poly(N-Isopropyl-Acrylamide) cross-linked gels. The solvent quality for the particles depends on temperature and thus allows tuning of the particle size. The particle synthesis parameters are chosen such that the resulting high surface charge of the particles prevents aggregation even in the maximally collapsed state. We present results on static and dynamic light scattering (SLS/DLS) for a highly diluted sample and for diffuse optical transmission on a more concentrated system. In the maximally collapsed state the scattering properties are well described by Mie theory for homogenous hard spheres. Upon swelling we find that a radially inhomogeneous density profile develops.     

Study on the Relationship between Particle Size and Near Infrared Diffuse Reflectance Spectroscopic Data

Since the particle size distribution is a critical parameter of pharmaceutical excipients used for tablet manufacturing by direct compression, the mean particle size of sieved sorbitol powder was studied by near‐infrared diffuse reflectance spectroscopy (NIRDRS). The aim of this study was to investigate the effect of the particle size (reciprocally proportional to the bulk density) on the reflectance spectrum. The effects of the particle characteristics on the spectral changes were described on the basis of the Kubelka‐Munk theory taking the scattering into consideration. A smaller particle size fraction was associated with a lower spectral value, and at the characteristic wavelength of 1584 nm a linear relationship was established for the particle size range from 125 to 670 μm. The diffuse reflectance measurement was sensitive to the particle characteristics, which offers a fast, non‐destructive alternative test method that can be applied after detailed calibration.     

One-dimensional fluctuating nanodomains in the charge-transfer molecular system TTF-CA and their first-order crystallization

We report on the direct observation by x-ray diffuse scattering measurements of thermally induced one-dimensional nanoscale ordered fluctuations in the mixed-stack charge-transfer molecular system tetrathiafulvalene-p chloranil (TTF-CA), the prototype for the neutral-ionic phase transition. The unusual physical properties of this compound are considered to be driven by such one-dimensional excitations. The results are discussed in relation to previous experimental and theoretical experiments both at thermal equilibrium and under light irradiation.     

小波变换在岩屑样品红外漫反射光谱中的应用

测定了不同粒度烃源岩粉末的红外漫反射光谱。结果表明,样品原始光谱吸光度的测量值随着粒径的减小而减小;利用小波变换对原始光谱进行预处理,消除了由散射引起的背景和基线漂移后,发现光谱的吸光度测量值随着粒径的减小而增加。这说明粉末样品的散射对光谱线形的影响很大,因此在定量分析中应保持被测样品的粒度与建模时样品的粒度一致,以减小由散射差异所带来的误差。小波变换滤掉的光谱低频信息反映了样品的散射强弱,利用该低频信息在特征吸收峰2 820 cm-1附近建立预测样品粒径的模型,并与利用原始光谱信息建立的模型比较,前者相关性远高于后者,并且相关系数达到r=0.999 7,可用于预测和控制样品的粒度分布,从而提高红外光谱定量分析的精确度。同时文章还验证了散射系数S和吸收系数K都与粒径d成反比关系。     

Morphology and decay kinetics of Co-Ni-Ti alloys

The effects of small-angle diffuse modulated-structure electron scattering in Co-Ni-Ti alloys are studied. It is shown that elastic atomic-plane displacement plays a determining role in satellite formation on electron-diffraction patterns. It is also shown that in the early disintegration stages in Co-Ni-Ti alloys, the new-phase particles are equiaxial and randomly disposed. At later stages, a transition to one-dimensionally periodic modulated structure is observed. The time dependence of modulation period and particle size is described by the Lifshits-Slezov-Vagner equation. The coagulation-activation energy determined from kinetics curves is Q=64 kcal/mole. It is concluded that volume diffusion controls particle coarsening.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 123–130, December, 1972.The authors consider it their pleasant duty to express appreciation to L. S. Bushnev for discussion of the present work.     

Neutron diffuse scattering in Fe3O4 due to molecular polarons

The results of a precise re-examination of neutron diffuse scattering above the Verwey transition of Fe₃O₄ are reported. A model to describe the properties of valence fluctuations at the B-site Fe-ions are proposed, which assumes the existance of “molecular polarons”. The experimental results of anisotropic distribution of diffuse scattering seem to give an evidence for the feasibility of the proposed model.     

Diffuse reflectance of TiO2 pigmented paints: Spectral dependence of the average pathlength parameter and the forward scattering ratio

Diffuse reflectance spectra of paint coatings with different pigment concentrations, normally illuminated with unpolarized radiation, have been measured. A four-flux radiative transfer approach is used to model the diffuse reflectance of TiO₂ (rutile) pigmented coatings through the solar spectral range. The spectral dependence of the average pathlength parameter and of the forward scattering ratio for diffuse radiation, are explicitly incorporated into this four-flux model from two novel approximations. The size distribution of the pigments has been taken into account to obtain the averages of the four-flux parameters: scattering and absorption cross sections, forward scattering ratios for collimated and isotropic diffuse radiation, and coefficients involved in the expansion of the single particle phase function in terms of Legendre polynomials.     

使用扩散模型数值模拟二维湍流气固两相流

本文从描述多维湍流气固两相流的两流体模型出发,导出了计算湍流气固两相流中固体颗粒扩散速度的计算模型,进而基于在一维流场中颗粒的终端速度是重力加速度gi的函数,提出在多维流场中颗粒相处于一个修正的加速度场g'i的作用下,该修正的加速度g'i包含了包括重力在内的各种力的作用,这些力对颗粒的加速作用与重力对颗粒的加速作用没有区别。根据这种观点,提出了用于模拟多维湍流气固两相流的改进的扩散模型。本文使用改进的扩散模型对台阶后方的气固两相流进行了数值模拟,并将数值计算结果与实验结果进行了比较,结果表明,改进的扩散模型的预报结果与实验结果符合得相当好。     

Optimization and design of pigments for heat-insulating coatings

This paper reports that heat insulating property of infrared reflective coatings is obtained through the use of pigments which diffuse near-infrared thermal radiation.Suitable structure and size distribution of pigments would attain maximum diffuse infrared radiation and reduce the pigment volume concentration required.The optimum structure and size range of pigments for reflective infrared coatings are studied by using Kubelka-Munk theory,Mie model and independent scattering approximation.Taking titania particle as the pigment embedded in an inorganic coating,the computational results show that core-shell particles present excellent scattering ability,more so than solid and hollow spherical particles.The optimum radius range of core-shell particles is around 0.3 ～ 1.6 μm.Furthermore,the influence of shell thickness on optical parameters of the coating is also obvious and the optimal thickness of shell is 100-300 nm.     

Positive and negative linear magnetoresistance of graphite

The magnetoresistance (MR) of bulk graphite with different particle sizes is investigated. The MR of the graphite decreases with the particle size decreases. The graphite with micro-sized particles has a positive MR and exhibits positive linear field dependence of MR at about 50 K, whereas the graphite with particle size of 30.2 nm has a negative MR and exhibits negative linear field dependence of MR at about 25 K. The possible mechanism for the MR of graphite can be partially understood using ordinary MR theory, weak localization theory and diffuse scattering theory.     

颗粒溶液颗粒尺寸及浓度的差分偏振弹性散射光谱在线分析方法

偏振弹性散射光谱技术的基本原理为在偏振光入射条件下,根据出射光的偏振特性不同可以筛选出浅表层组织的单次散射光信息和深层组织的漫散射光信息。该研究的创新点在于将这种方法应用于颗粒溶液检测,目的是在颗粒溶液原始状态下实现对颗粒尺寸及浓度的同时检测。设计了一个共轴笼式光学系统,测量了聚苯乙烯微球颗粒溶液某一角度的背向散射信号,通过控制入射端和收集端偏振片的偏振方向获得了颗粒溶液的偏振平行光谱与偏振垂直光谱,两者之差即偏振差分光谱对应颗粒的单次散射信息,将该单次散射信息与Mie散射数据库进行比对获得颗粒的尺寸,然后在颗粒尺寸作为已知的条件下进一步分析偏振垂直光谱,将该垂直光谱对应的颗粒溶液的漫散射信息代入光漫散射下的近似表达式拟合得到颗粒的浓度信息。将实验结果与样品提供值进行了比对,并进一步分析了在获取颗粒数浓度时,颗粒直径的方差分布对结果的影响,最终验证了该实验方法的可行性。该方法的潜在应用包括对标准颗粒制造厂商的产品在线检测以及对牛奶制品中脂肪和蛋白质的浓度检测研究。     

混浊介质中后向单次漫散射米勒矩阵特征

采用斯托克斯(Stokes)矢量形式,推导出当无限窄的连续光束垂直入射到混浊介质表面时,后向单次漫散射米勒(Mueller)矩阵的解析表达式。基于米氏(Mie)散射模式,详细分析了单次散射米勒矩阵元素的分布模式,以及与介质粒子数密度,粒子尺寸参量之间的关系。研究表明:单次散射米勒矩阵的方位变化随粒子数密度的增加,逐渐消失,而矩阵元素m22,m33,m23,m32随粒子数密度的变化,具有更显著的方位变化特征。矩阵元素m22,m33在方位角=45°时的值随尺寸参量的变化有一定的规律性,当尺寸参量小于某一特征参量时,其值呈下降趋势,反之则呈波动上升趋势。当介质粒子数密度以及粒子尺寸参量改变时,米勒矩阵元素强度的径向分布模式不变,即在任何方位,强度随径向距离都近似成指数规律衰减,方位变化呈周期性。     

气粒两相流传热问题的光滑离散颗粒流体动力学方法数值模拟

在气粒两相流动问题中,颗粒间以及气体与颗粒间的传热问题不可忽略.光滑离散颗粒流体动力学(SDPH)模型作为一种新的求解气粒两相流动问题的方法,已经成功应用于模拟风沙运动等问题.在此基础上,提出了SDPH方法的热传导模型,模拟了气粒两相流动问题中的热传导过程以及颗粒蒸发过程.首先引入各相的能量方程,利用有限差分与光滑粒子流体动力学一阶导数相结合的方法,处理各相内部热传导项中的二阶导数问题,基于气粒两相间温度差及对流换热系数计算颗粒与气体间的热传导量,推导得到了含热传导模型的气粒两相流SDPH计算方程组,模拟计算了圆盘形颗粒团算例及鼓泡流化床内部热传导算例,并与双流体模型计算结果进行对比,结果基本符合;其次利用离散液滴模型中的颗粒蒸发传质传热定律计算颗粒的蒸发过程,数值模拟了颗粒射流蒸发过程,并与离散颗粒模型结果进行对比,两者符合得较好,验证了该方法的准确性及实用性.     

Light scattering by slightly nonspherical particles on surfaces

We investigate the shape dependence of the scattering by dielectric and metallic particles on surfaces by considering particles whose free-space scattering properties are nearly identical. The scattering by metallic particles is strongly dependent on the shape of the particle in the region near where the particle touches the surface. The scattering by dielectric particles displays a weaker, but nonetheless significant, dependence on particle shape. These results have a significant effect on the use of light scattering to size and identify particles on surfaces.     

循环流化床收集器内颗粒团聚流动特性的研究

基于气固两相流理论和气溶胶动力学原理,建立流化床收集器(CFBA)内气体细颗粒聚团气固两相双流体模型。对不同入口气体速度、初始颗粒尺寸分布和不同颗粒团聚形成机理下收集器内颗粒聚团流动的流体动力特性进行数值模拟。研究结果表明湍流运动和剪切作用对颗粒聚团的形成起主要作用,布朗运动对颗粒团聚形成的影响可忽略不计。吸收颗粒可有效提高捕获细颗粒和颗粒聚团形成的能力。     

惯性颗粒所见被动标量统计特性的直接数值模拟研究

本文采用直接数值模拟方法,对惯性颗粒所见均匀各向同性湍流中具有平均标量梯度的被动标量场统计特性进行了研究。模拟结果表明:与颗粒所见流体速度的自相关特性不同,颗粒所见标量的自相关特性随颗粒惯性的增加而单调减少;颗粒所见标量脉动能随颗粒惯性的增加先减少再增大,在St≈1.0的临界颗粒附近达到最小值,而颗粒所见标量耗散率随颗粒惯性的变化行为与颗粒所见标量脉动能的变化行为相反;数值模拟的结果进一步揭示,在St≈1.0时颗粒所见流体标量脉动能和耗散率的极值是因为St≈1.0的临界颗粒聚集于低涡量、高应变区域和标量场在高应变区域形成强耗散的片状结构所致。     

Adiabatic approximation on the impact-parameter model

In this paper we study the impact-parameter model for the scattering of a light particle by two heavy ones in the case when the coupling constants of the potentials acting on the light particle due to the presence of the two heavy ones are the same. We study the asymptotic behavior of the transition probability when the relative velocity of the heavy particles goes to zero. We show that the probability of a transition can be arbitrarily close to the one of no transition.